diff --git a/linq4j/build.gradle.kts b/linq4j/build.gradle.kts
index 7a9db175f24e..05f7e1965dd3 100644
--- a/linq4j/build.gradle.kts
+++ b/linq4j/build.gradle.kts
@@ -19,4 +19,5 @@ dependencies {
 
     implementation("com.google.guava:guava")
     implementation("org.apache.calcite.avatica:avatica-core")
+    implementation("org.checkerframework:checker-qual")
 }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/BaseQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/BaseQueryable.java
index fe4a63506224..6099ce7dd1e7 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/BaseQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/BaseQueryable.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.tree.Expression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Iterator;
 
@@ -34,10 +36,10 @@ public abstract class BaseQueryable<TSource>
     extends AbstractQueryable<TSource> {
   protected final QueryProvider provider;
   protected final Type elementType;
-  protected final Expression expression;
+  protected final @Nullable Expression expression;
 
   public BaseQueryable(QueryProvider provider, Type elementType,
-      Expression expression) {
+      @Nullable Expression expression) {
     this.provider = provider;
     this.elementType = elementType;
     this.expression = expression;
@@ -51,7 +53,7 @@ public Type getElementType() {
     return elementType;
   }
 
-  public Expression getExpression() {
+  public @Nullable Expression getExpression() {
     return expression;
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultEnumerable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultEnumerable.java
index 3cc4c9ed0c8a..42a46962dba6 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultEnumerable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultEnumerable.java
@@ -33,6 +33,9 @@
 import org.apache.calcite.linq4j.function.Predicate1;
 import org.apache.calcite.linq4j.function.Predicate2;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+
 import java.math.BigDecimal;
 import java.util.Collection;
 import java.util.Comparator;
@@ -69,7 +72,7 @@ protected OrderedEnumerable<T> getThisOrdered() {
     return this;
   }
 
-  public <R> R foreach(Function1<T, R> func) {
+  public <R> @Nullable R foreach(Function1<T, R> func) {
     R result = null;
     try (Enumerator<T> enumerator = enumerator()) {
       while (enumerator.moveNext()) {
@@ -89,12 +92,12 @@ protected OrderedQueryable<T> asOrderedQueryable() {
     return EnumerableDefaults.asOrderedQueryable(this);
   }
 
-  public T aggregate(Function2<T, T, T> func) {
+  public @Nullable T aggregate(Function2<@Nullable T, T, T> func) {
     return EnumerableDefaults.aggregate(getThis(), func);
   }
 
-  public <TAccumulate> TAccumulate aggregate(TAccumulate seed,
-      Function2<TAccumulate, T, TAccumulate> func) {
+  public <TAccumulate> @PolyNull TAccumulate aggregate(@PolyNull TAccumulate seed,
+      Function2<@PolyNull TAccumulate, T, @PolyNull TAccumulate> func) {
     return EnumerableDefaults.aggregate(getThis(), seed, func);
   }
 
@@ -191,11 +194,11 @@ public <TKey> OrderedEnumerable<T> createOrderedEnumerable(
         keySelector, comparator, descending);
   }
 
-  public Enumerable<T> defaultIfEmpty() {
+  public Enumerable<@Nullable T> defaultIfEmpty() {
     return EnumerableDefaults.defaultIfEmpty(getThis());
   }
 
-  public Enumerable<T>  defaultIfEmpty(T value) {
+  public Enumerable<@PolyNull T> defaultIfEmpty(@PolyNull T value) {
     return EnumerableDefaults.defaultIfEmpty(getThis(), value);
   }
 
@@ -211,7 +214,7 @@ public T elementAt(int index) {
     return EnumerableDefaults.elementAt(getThis(), index);
   }
 
-  public T elementAtOrDefault(int index) {
+  public @Nullable T elementAtOrDefault(int index) {
     return EnumerableDefaults.elementAtOrDefault(getThis(), index);
   }
 
@@ -241,11 +244,11 @@ public T first(Predicate1<T> predicate) {
     return EnumerableDefaults.first(getThis(), predicate);
   }
 
-  public T firstOrDefault() {
+  public @Nullable T firstOrDefault() {
     return EnumerableDefaults.firstOrDefault(getThis());
   }
 
-  public T firstOrDefault(Predicate1<T> predicate) {
+  public @Nullable T firstOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.firstOrDefault(getThis(), predicate);
   }
 
@@ -428,11 +431,11 @@ public T last(Predicate1<T> predicate) {
     return EnumerableDefaults.last(getThis(), predicate);
   }
 
-  public T lastOrDefault() {
+  public @Nullable T lastOrDefault() {
     return EnumerableDefaults.lastOrDefault(getThis());
   }
 
-  public T lastOrDefault(Predicate1<T> predicate) {
+  public @Nullable T lastOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.lastOrDefault(getThis(), predicate);
   }
 
@@ -621,11 +624,11 @@ public T single(Predicate1<T> predicate) {
     return EnumerableDefaults.single(getThis(), predicate);
   }
 
-  public T singleOrDefault() {
+  public @Nullable T singleOrDefault() {
     return EnumerableDefaults.singleOrDefault(getThis());
   }
 
-  public T singleOrDefault(Predicate1<T> predicate) {
+  public @Nullable T singleOrDefault(Predicate1<T> predicate) {
     return EnumerableDefaults.singleOrDefault(getThis(), predicate);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultQueryable.java
index 4d09d433ad4d..0e474c8e5d4c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/DefaultQueryable.java
@@ -33,6 +33,8 @@
 import org.apache.calcite.linq4j.function.Predicate2;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.math.BigDecimal;
 import java.util.Comparator;
 
@@ -148,7 +150,7 @@ public Enumerator<T> enumerator() {
     return factory.ofType(getThis(), clazz);
   }
 
-  @Override public Queryable<T> defaultIfEmpty() {
+  @Override public Queryable<@Nullable T> defaultIfEmpty() {
     return factory.defaultIfEmpty(getThis());
   }
 
@@ -162,7 +164,7 @@ public Enumerator<T> enumerator() {
 
   // End disambiguate
 
-  public T aggregate(FunctionExpression<Function2<T, T, T>> selector) {
+  public @Nullable T aggregate(FunctionExpression<Function2<@Nullable T, T, T>> selector) {
     return factory.aggregate(getThis(), selector);
   }
 
@@ -243,7 +245,7 @@ public T first(FunctionExpression<Predicate1<T>> predicate) {
     return factory.first(getThis(), predicate);
   }
 
-  public T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return factory.firstOrDefault(getThis(), predicate);
   }
 
@@ -421,7 +423,7 @@ public T single(FunctionExpression<Predicate1<T>> predicate) {
     return factory.single(getThis(), predicate);
   }
 
-  public T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return factory.singleOrDefault(getThis(), predicate);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerable.java
index 181d0358ee6c..19d90242b0d5 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerable.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Exposes the enumerator, which supports a simple iteration over a collection.
  *
@@ -26,6 +28,7 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface Enumerable<T>
     extends RawEnumerable<T>, Iterable<T>, ExtendedEnumerable<T> {
   /**
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
index 512130d1b75b..4c811aed5631 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableDefaults.java
@@ -41,6 +41,11 @@
 import com.google.common.collect.Sets;
 
 import org.apiguardian.api.API;
+import org.checkerframework.checker.nullness.qual.KeyFor;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+import org.checkerframework.dataflow.qual.Pure;
+import org.checkerframework.framework.qual.HasQualifierParameter;
 
 import java.math.BigDecimal;
 import java.util.AbstractList;
@@ -67,6 +72,8 @@
 import static org.apache.calcite.linq4j.Linq4j.ListEnumerable;
 import static org.apache.calcite.linq4j.function.Functions.adapt;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Default implementations of methods in the {@link Enumerable} interface.
  */
@@ -75,10 +82,13 @@ public abstract class EnumerableDefaults {
   /**
    * Applies an accumulator function over a sequence.
    */
-  public static <TSource> TSource aggregate(Enumerable<TSource> source,
-      Function2<TSource, TSource, TSource> func) {
-    TSource result = null;
+  public static <TSource> @Nullable TSource aggregate(Enumerable<TSource> source,
+      Function2<@Nullable TSource, TSource, TSource> func) {
     try (Enumerator<TSource> os = source.enumerator()) {
+      if (!os.moveNext()) {
+        return null;
+      }
+      TSource result = os.current();
       while (os.moveNext()) {
         TSource o = os.current();
         result = func.apply(result, o);
@@ -347,7 +357,7 @@ public static <TSource> boolean contains(Enumerable<TSource> enumerable,
     try (Enumerator<TSource> os = enumerable.enumerator()) {
       while (os.moveNext()) {
         TSource o = os.current();
-        if (o.equals(element)) {
+        if (Objects.equals(o, element)) {
           return true;
         }
       }
@@ -391,7 +401,7 @@ public static <TSource> int count(Enumerable<TSource> enumerable,
    * the type parameter's default value in a singleton collection if
    * the sequence is empty.
    */
-  public static <TSource> Enumerable<TSource> defaultIfEmpty(
+  public static <TSource> Enumerable<@Nullable TSource> defaultIfEmpty(
       Enumerable<TSource> enumerable) {
     return defaultIfEmpty(enumerable, null);
   }
@@ -401,24 +411,25 @@ public static <TSource> Enumerable<TSource> defaultIfEmpty(
    * the specified value in a singleton collection if the sequence
    * is empty.
    */
-  public static <TSource> Enumerable<TSource> defaultIfEmpty(
+  @SuppressWarnings("return.type.incompatible")
+  public static <TSource> Enumerable<@PolyNull TSource> defaultIfEmpty(
       Enumerable<TSource> enumerable,
-      TSource value) {
+      @PolyNull TSource value) {
     try (Enumerator<TSource> os = enumerable.enumerator()) {
       if (os.moveNext()) {
-        return Linq4j.asEnumerable(() -> new Iterator<TSource>() {
+        return Linq4j.<TSource>asEnumerable(() -> new Iterator<TSource>() {
 
           private boolean nonFirst;
 
-          private Iterator<TSource> rest;
+          private @Nullable Iterator<TSource> rest;
 
           public boolean hasNext() {
-            return !nonFirst || rest.hasNext();
+            return !nonFirst || castNonNull(rest).hasNext();
           }
 
           public TSource next() {
             if (nonFirst) {
-              return rest.next();
+              return castNonNull(rest).next();
             } else {
               final TSource first = os.current();
               nonFirst = true;
@@ -501,7 +512,7 @@ public static <TSource> TSource elementAt(Enumerable<TSource> enumerable,
    * sequence or a default value if the index is out of
    * range.
    */
-  public static <TSource> TSource elementAtOrDefault(
+  public static <TSource> @Nullable TSource elementAtOrDefault(
       Enumerable<TSource> enumerable, int index) {
     final ListEnumerable<TSource> list = enumerable instanceof ListEnumerable
         ? ((ListEnumerable<TSource>) enumerable)
@@ -552,7 +563,8 @@ public static <TSource> Enumerable<TSource> except(
     try (Enumerator<TSource> os = source1.enumerator()) {
       while (os.moveNext()) {
         TSource o = os.current();
-        collection.remove(o);
+        @SuppressWarnings("argument.type.incompatible")
+        boolean unused = collection.remove(o);
       }
       return Linq4j.asEnumerable(collection);
     }
@@ -625,7 +637,7 @@ public static <TSource> TSource first(Enumerable<TSource> enumerable,
    * Returns the first element of a sequence, or a
    * default value if the sequence contains no elements.
    */
-  public static <TSource> TSource firstOrDefault(
+  public static <TSource> @Nullable TSource firstOrDefault(
         Enumerable<TSource> enumerable) {
     try (Enumerator<TSource> os = enumerable.enumerator()) {
       if (os.moveNext()) {
@@ -640,7 +652,7 @@ public static <TSource> TSource firstOrDefault(
    * satisfies a condition or a default value if no such element is
    * found.
    */
-  public static <TSource> TSource firstOrDefault(Enumerable<TSource> enumerable,
+  public static <TSource> @Nullable TSource firstOrDefault(Enumerable<TSource> enumerable,
       Predicate1<TSource> predicate) {
     for (TSource o : enumerable) {
       if (predicate.apply(o)) {
@@ -860,9 +872,9 @@ private static class SortedAggregateEnumerator<TSource, TKey, TAccumulate, TResu
     private final Comparator<TKey> comparator;
     private boolean isInitialized;
     private boolean isLastMoveNextFalse;
-    private TAccumulate curAccumulator;
+    private @Nullable TAccumulate curAccumulator;
     private Enumerator<TSource> enumerator;
-    private TResult curResult;
+    private @Nullable TResult curResult;
 
     SortedAggregateEnumerator(
         Enumerable<TSource> enumerable,
@@ -888,7 +900,7 @@ private static class SortedAggregateEnumerator<TSource, TKey, TAccumulate, TResu
       if (isLastMoveNextFalse) {
         throw new NoSuchElementException();
       }
-      return curResult;
+      return castNonNull(curResult);
     }
 
     @Override public boolean moveNext() {
@@ -913,24 +925,24 @@ private static class SortedAggregateEnumerator<TSource, TKey, TAccumulate, TResu
       curResult = null;
       TSource o = enumerator.current();
       TKey prevKey = keySelector.apply(o);
-      curAccumulator = accumulatorAdder.apply(curAccumulator, o);
+      curAccumulator = accumulatorAdder.apply(castNonNull(curAccumulator), o);
       while (enumerator.moveNext()) {
         o = enumerator.current();
         TKey curKey = keySelector.apply(o);
         if (comparator.compare(prevKey, curKey) != 0) {
           // current key is different from previous key, get accumulated results and re-create
           // accumulator for current key.
-          curResult = resultSelector.apply(prevKey, curAccumulator);
+          curResult = resultSelector.apply(prevKey, castNonNull(curAccumulator));
           curAccumulator = accumulatorInitializer.apply();
           break;
         }
-        curAccumulator = accumulatorAdder.apply(curAccumulator, o);
+        curAccumulator = accumulatorAdder.apply(castNonNull(curAccumulator), o);
         prevKey = curKey;
       }
 
       if (curResult == null) {
         // current key is the last key.
-        curResult = resultSelector.apply(prevKey, curAccumulator);
+        curResult = resultSelector.apply(prevKey, castNonNull(curAccumulator));
         // no need to keep accumulator for the last key.
         curAccumulator = null;
       }
@@ -961,6 +973,7 @@ private static <TSource, TKey, TAccumulate, TResult> Enumerable<TResult> groupBy
       while (os.moveNext()) {
         TSource o = os.current();
         TKey key = keySelector.apply(o);
+        @SuppressWarnings("argument.type.incompatible")
         TAccumulate accumulator = map.get(key);
         if (accumulator == null) {
           accumulator = accumulatorInitializer.apply();
@@ -989,6 +1002,7 @@ private static <TSource, TKey, TAccumulate, TResult> Enumerable<TResult> groupBy
         for (Function1<TSource, TKey> keySelector : keySelectors) {
           TSource o = os.current();
           TKey key = keySelector.apply(o);
+          @SuppressWarnings("argument.type.incompatible")
           TAccumulate accumulator = map.get(key);
           if (accumulator == null) {
             accumulator = accumulatorInitializer.apply();
@@ -1041,6 +1055,7 @@ public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
           public TResult current() {
             final Map.Entry<TKey, TSource> entry = entries.current();
+            @SuppressWarnings("argument.type.incompatible")
             final Enumerable<TInner> inners = innerLookup.get(entry.getKey());
             return resultSelector.apply(entry.getValue(),
                 inners == null ? Linq4j.emptyEnumerable() : inners);
@@ -1082,6 +1097,7 @@ public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
           public TResult current() {
             final Map.Entry<TKey, TSource> entry = entries.current();
+            @SuppressWarnings("argument.type.incompatible")
             final Enumerable<TInner> inners = innerLookup.get(entry.getKey());
             return resultSelector.apply(entry.getValue(),
                 inners == null ? Linq4j.emptyEnumerable() : inners);
@@ -1126,7 +1142,9 @@ public static <TSource> Enumerable<TSource> intersect(
     try (Enumerator<TSource> os = source0.enumerator()) {
       while (os.moveNext()) {
         TSource o = os.current();
-        if (set1.remove(o)) {
+        @SuppressWarnings("argument.type.incompatible")
+        boolean removed = set1.remove(o);
+        if (removed) {
           resultCollection.add(o);
         }
       }
@@ -1225,7 +1243,7 @@ public static <TSource, TInner, TKey, TResult> Enumerable<TResult> hashJoin(
       Function1<TSource, TKey> outerKeySelector,
       Function1<TInner, TKey> innerKeySelector,
       Function2<TSource, TInner, TResult> resultSelector,
-      EqualityComparer<TKey> comparer, boolean generateNullsOnLeft,
+      @Nullable EqualityComparer<TKey> comparer, boolean generateNullsOnLeft,
       boolean generateNullsOnRight) {
     return hashEquiJoin_(
         outer,
@@ -1248,7 +1266,7 @@ public static <TSource, TInner, TKey, TResult> Enumerable<TResult> hashJoin(
       Function1<TSource, TKey> outerKeySelector,
       Function1<TInner, TKey> innerKeySelector,
       Function2<TSource, TInner, TResult> resultSelector,
-      EqualityComparer<TKey> comparer, boolean generateNullsOnLeft,
+      @Nullable EqualityComparer<TKey> comparer, boolean generateNullsOnLeft,
       boolean generateNullsOnRight,
       Predicate2<TSource, TInner> predicate) {
     if (predicate == null) {
@@ -1281,7 +1299,8 @@ private static <TSource, TInner, TKey, TResult> Enumerable<TResult> hashEquiJoin
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
       final Function2<TSource, TInner, TResult> resultSelector,
-      final EqualityComparer<TKey> comparer, final boolean generateNullsOnLeft,
+      final @Nullable EqualityComparer<TKey> comparer,
+      final boolean generateNullsOnLeft,
       final boolean generateNullsOnRight) {
     return new AbstractEnumerable<TResult>() {
       public Enumerator<TResult> enumerator() {
@@ -1293,7 +1312,7 @@ public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
           Enumerator<TSource> outers = outer.enumerator();
           Enumerator<TInner> inners = Linq4j.emptyEnumerator();
-          Set<TKey> unmatchedKeys =
+          @Nullable Set<TKey> unmatchedKeys =
               generateNullsOnLeft
                   ? new HashSet<>(innerLookup.keySet())
                   : null;
@@ -1314,7 +1333,9 @@ public boolean moveNext() {
                   // not the left.
                   List<TInner> list = new ArrayList<>();
                   for (TKey key : unmatchedKeys) {
-                    for (TInner tInner : innerLookup.get(key)) {
+                    @SuppressWarnings("argument.type.incompatible")
+                    Enumerable<TInner> innerValues = castNonNull(innerLookup.get(key));
+                    for (TInner tInner : innerValues) {
                       list.add(tInner);
                     }
                   }
@@ -1374,7 +1395,8 @@ private static <TSource, TInner, TKey, TResult> Enumerable<TResult> hashJoinWith
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
       final Function2<TSource, TInner, TResult> resultSelector,
-      final EqualityComparer<TKey> comparer, final boolean generateNullsOnLeft,
+      final @Nullable EqualityComparer<TKey> comparer,
+      final boolean generateNullsOnLeft,
       final boolean generateNullsOnRight, final Predicate2<TSource, TInner> predicate) {
 
     return new AbstractEnumerable<TResult>() {
@@ -1397,7 +1419,7 @@ public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
           Enumerator<TSource> outers = outer.enumerator();
           Enumerator<TInner> inners = Linq4j.emptyEnumerator();
-          List<TInner> innersUnmatched =
+          @Nullable List<TInner> innersUnmatched =
               generateNullsOnLeft
                   ? new ArrayList<>(innerToLookUp.toList())
                   : null;
@@ -1483,7 +1505,7 @@ public void close() {
   public static <TSource, TInner, TResult> Enumerable<TResult> correlateJoin(
       final JoinType joinType, final Enumerable<TSource> outer,
       final Function1<TSource, Enumerable<TInner>> inner,
-      final Function2<TSource, TInner, TResult> resultSelector) {
+      final Function2<TSource, ? super @Nullable TInner, TResult> resultSelector) {
     if (joinType == JoinType.RIGHT || joinType == JoinType.FULL) {
       throw new IllegalArgumentException("JoinType " + joinType + " is not valid for correlation");
     }
@@ -1491,14 +1513,14 @@ public static <TSource, TInner, TResult> Enumerable<TResult> correlateJoin(
     return new AbstractEnumerable<TResult>() {
       public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
-          private Enumerator<TSource> outerEnumerator = outer.enumerator();
-          private Enumerator<TInner> innerEnumerator;
-          TSource outerValue;
-          TInner innerValue;
+          private final Enumerator<TSource> outerEnumerator = outer.enumerator();
+          private @Nullable Enumerator<TInner> innerEnumerator;
+          @Nullable TSource outerValue;
+          @Nullable TInner innerValue;
           int state = 0; // 0 -- moving outer, 1 moving inner;
 
           public TResult current() {
-            return resultSelector.apply(outerValue, innerValue);
+            return resultSelector.apply(castNonNull(outerValue), innerValue);
           }
 
           public boolean moveNext() {
@@ -1544,6 +1566,7 @@ public boolean moveNext() {
                 continue;
               case 1:
                 // subsequent move inner
+                Enumerator<TInner> innerEnumerator = castNonNull(this.innerEnumerator);
                 if (innerEnumerator.moveNext()) {
                   innerValue = innerEnumerator.current();
                   return true;
@@ -1635,18 +1658,19 @@ public static <TSource, TInner, TResult> Enumerable<TResult> correlateBatchJoin(
     return new AbstractEnumerable<TResult>() {
       @Override public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
-          Enumerator<TSource> outerEnumerator = outer.enumerator();
-          List<TSource> outerValues = new ArrayList<>(batchSize);
-          List<TInner> innerValues = new ArrayList<>();
-          TSource outerValue;
-          TInner innerValue;
-          Enumerable<TInner> innerEnumerable;
-          Enumerator<TInner> innerEnumerator;
+          final Enumerator<TSource> outerEnumerator = outer.enumerator();
+          final List<TSource> outerValues = new ArrayList<>(batchSize);
+          final List<TInner> innerValues = new ArrayList<>();
+          @Nullable TSource outerValue;
+          @Nullable TInner innerValue;
+          @Nullable Enumerable<TInner> innerEnumerable;
+          @Nullable Enumerator<TInner> innerEnumerator;
           boolean innerEnumHasNext = false;
           boolean atLeastOneResult = false;
           int i = -1; // outer position
           int j = -1; // inner position
 
+          @SuppressWarnings("argument.type.incompatible")
           @Override public TResult current() {
             return resultSelector.apply(outerValue, innerValue);
           }
@@ -1694,7 +1718,7 @@ public static <TSource, TInner, TResult> Enumerable<TResult> correlateBatchJoin(
                 outerValue = outerValues.get(i); // get current outer value
                 nextInnerValue();
                 // Compare current block row to current inner value
-                if (predicate.apply(outerValue, innerValue)) {
+                if (predicate.apply(castNonNull(outerValue), castNonNull(innerValue))) {
                   atLeastOneResult = true;
                   // Skip the rest of inner values in case of
                   // ANTI and SEMI when a match is found
@@ -1702,6 +1726,7 @@ public static <TSource, TInner, TResult> Enumerable<TResult> correlateBatchJoin(
                     // Two ways of skipping inner values,
                     // enumerator way and ArrayList way
                     if (i == 0) {
+                      Enumerator<TInner> innerEnumerator = castNonNull(this.innerEnumerator);
                       while (innerEnumHasNext) {
                         innerValues.add(innerEnumerator.current());
                         innerEnumHasNext = innerEnumerator.moveNext();
@@ -1737,6 +1762,7 @@ public void nextOuterValue() {
 
           private void nextInnerValue() {
             if (i == 0) {
+              Enumerator<TInner> innerEnumerator = castNonNull(this.innerEnumerator);
               innerValue = innerEnumerator.current();
               innerValues.add(innerValue);
               innerEnumHasNext = innerEnumerator.moveNext(); // next enumerator inner value
@@ -1883,10 +1909,11 @@ public Enumerator<TSource> enumerator() {
 
         final Predicate1<TSource> predicate = v0 -> {
           TKey key = outerKeySelector.apply(v0);
-          if (!innerLookup.get().containsKey(key)) {
+          @SuppressWarnings("argument.type.incompatible")
+          Enumerable<TInner> innersOfKey = innerLookup.get().get(key);
+          if (innersOfKey == null) {
             return anti;
           }
-          Enumerable<TInner> innersOfKey = innerLookup.get().get(key);
           try (Enumerator<TInner> os = innersOfKey.enumerator()) {
             while (os.moveNext()) {
               TInner v1 = os.current();
@@ -1911,7 +1938,7 @@ private static <TSource, TInner, TKey> Enumerable<TSource> semiEquiJoin_(
       final Enumerable<TSource> outer, final Enumerable<TInner> inner,
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
-      final EqualityComparer<TKey> comparer,
+      final @Nullable EqualityComparer<TKey> comparer,
       final boolean anti) {
     return new AbstractEnumerable<TSource>() {
       public Enumerator<TSource> enumerator() {
@@ -1937,7 +1964,7 @@ public Enumerator<TSource> enumerator() {
   public static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoin(
       final Enumerable<TSource> outer, final Enumerable<TInner> inner,
       final Predicate2<TSource, TInner> predicate,
-      Function2<TSource, TInner, TResult> resultSelector,
+      Function2<? super @Nullable TSource, ? super @Nullable TInner, TResult> resultSelector,
       final JoinType joinType) {
     if (!joinType.generatesNullsOnLeft()) {
       return nestedLoopJoinOptimized(outer, inner, predicate, resultSelector, joinType);
@@ -1952,7 +1979,7 @@ public static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoin(
   private static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoinAsList(
       final Enumerable<TSource> outer, final Enumerable<TInner> inner,
       final Predicate2<TSource, TInner> predicate,
-      Function2<TSource, TInner, TResult> resultSelector,
+      Function2<? super @Nullable TSource, ? super @Nullable TInner, TResult> resultSelector,
       final JoinType joinType) {
     final boolean generateNullsOnLeft = joinType.generatesNullsOnLeft();
     final boolean generateNullsOnRight = joinType.generatesNullsOnRight();
@@ -1976,7 +2003,8 @@ private static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoinAsLi
               break;
             } else {
               if (rightUnmatched != null) {
-                rightUnmatched.remove(right);
+                @SuppressWarnings("argument.type.incompatible")
+                boolean unused = rightUnmatched.remove(right);
               }
               result.add(resultSelector.apply(left, right));
               if (joinType == JoinType.SEMI) {
@@ -2007,7 +2035,7 @@ private static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoinAsLi
   private static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoinOptimized(
       final Enumerable<TSource> outer, final Enumerable<TInner> inner,
       final Predicate2<TSource, TInner> predicate,
-      Function2<TSource, TInner, TResult> resultSelector,
+      Function2<? super TSource, ? super TInner, TResult> resultSelector,
       final JoinType joinType) {
     if (joinType == JoinType.RIGHT || joinType == JoinType.FULL) {
       throw new IllegalArgumentException("JoinType " + joinType + " is unsupported");
@@ -2017,14 +2045,14 @@ private static <TSource, TInner, TResult> Enumerable<TResult> nestedLoopJoinOpti
       public Enumerator<TResult> enumerator() {
         return new Enumerator<TResult>() {
           private Enumerator<TSource> outerEnumerator = outer.enumerator();
-          private Enumerator<TInner> innerEnumerator = null;
+          private @Nullable Enumerator<TInner> innerEnumerator = null;
           private boolean outerMatch = false; // whether the outerValue has matched an innerValue
-          private TSource outerValue;
-          private TInner innerValue;
+          private @Nullable TSource outerValue;
+          private @Nullable TInner innerValue;
           private int state = 0; // 0 moving outer, 1 moving inner
 
           @Override public TResult current() {
-            return resultSelector.apply(outerValue, innerValue);
+            return resultSelector.apply(castNonNull(outerValue), castNonNull(innerValue));
           }
 
           @Override public boolean moveNext() {
@@ -2043,9 +2071,11 @@ public Enumerator<TResult> enumerator() {
                 continue;
               case 1:
                 // move inner
+                Enumerator<TInner> innerEnumerator = castNonNull(this.innerEnumerator);
                 if (innerEnumerator.moveNext()) {
-                  innerValue = innerEnumerator.current();
-                  if (predicate.apply(outerValue, innerValue)) {
+                  TInner innerValue = innerEnumerator.current();
+                  this.innerValue = innerValue;
+                  if (predicate.apply(castNonNull(outerValue), innerValue)) {
                     outerMatch = true;
                     switch (joinType) {
                     case ANTI: // try next outer row
@@ -2106,7 +2136,7 @@ private void closeInner() {
       final Enumerable<TInner> inner,
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
-      final Function2<TSource, TInner, TResult> resultSelector,
+      final Function2<TSource, @Nullable TInner, TResult> resultSelector,
       boolean generateNullsOnLeft,
       boolean generateNullsOnRight) {
     if (generateNullsOnLeft) {
@@ -2146,7 +2176,7 @@ public static boolean isMergeJoinSupported(JoinType joinType) {
       final Enumerable<TInner> inner,
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
-      final Function2<TSource, TInner, TResult> resultSelector,
+      final Function2<TSource, @Nullable TInner, TResult> resultSelector,
       final JoinType joinType,
       final Comparator<TKey> comparator) {
     return mergeJoin(outer, inner, outerKeySelector, innerKeySelector, null, resultSelector,
@@ -2175,10 +2205,10 @@ public static boolean isMergeJoinSupported(JoinType joinType) {
       final Enumerable<TInner> inner,
       final Function1<TSource, TKey> outerKeySelector,
       final Function1<TInner, TKey> innerKeySelector,
-      final Predicate2<TSource, TInner> extraPredicate,
-      final Function2<TSource, TInner, TResult> resultSelector,
+      final @Nullable Predicate2<TSource, TInner> extraPredicate,
+      final Function2<TSource, @Nullable TInner, TResult> resultSelector,
       final JoinType joinType,
-      final Comparator<TKey> comparator) {
+      final @Nullable Comparator<TKey> comparator) {
     if (!isMergeJoinSupported(joinType)) {
       throw new UnsupportedOperationException("MergeJoin unsupported for join type " + joinType);
     }
@@ -2231,7 +2261,7 @@ public static <TSource> TSource last(Enumerable<TSource> enumerable,
    * Returns the last element of a sequence, or a
    * default value if the sequence contains no elements.
    */
-  public static <TSource> TSource lastOrDefault(
+  public static <TSource> @Nullable TSource lastOrDefault(
       Enumerable<TSource> enumerable) {
     final ListEnumerable<TSource> list = enumerable instanceof ListEnumerable
         ? ((ListEnumerable<TSource>) enumerable)
@@ -2261,7 +2291,7 @@ public static <TSource> TSource lastOrDefault(
    * satisfies a condition or a default value if no such element is
    * found.
    */
-  public static <TSource> TSource lastOrDefault(Enumerable<TSource> enumerable,
+  public static <TSource> @Nullable TSource lastOrDefault(Enumerable<TSource> enumerable,
       Predicate1<TSource> predicate) {
     final ListEnumerable<TSource> list = enumerable instanceof ListEnumerable
         ? ((ListEnumerable<TSource>) enumerable)
@@ -2330,8 +2360,7 @@ public static <TSource> long longCount(Enumerable<TSource> enumerable,
    */
   public static <TSource extends Comparable<TSource>> TSource max(
       Enumerable<TSource> source) {
-    Function2<TSource, TSource, TSource> max = maxFunction();
-    return aggregate(source, null, max);
+    return aggregate(source, maxFunction());
   }
 
   /**
@@ -2340,8 +2369,7 @@ public static <TSource extends Comparable<TSource>> TSource max(
    */
   public static <TSource> BigDecimal max(Enumerable<TSource> source,
       BigDecimalFunction1<TSource> selector) {
-    Function2<BigDecimal, BigDecimal, BigDecimal> max = maxFunction();
-    return aggregate(source.select(selector), null, max);
+    return aggregate(source.select(selector), maxFunction());
   }
 
   /**
@@ -2351,8 +2379,7 @@ public static <TSource> BigDecimal max(Enumerable<TSource> source,
    */
   public static <TSource> BigDecimal max(Enumerable<TSource> source,
       NullableBigDecimalFunction1<TSource> selector) {
-    Function2<BigDecimal, BigDecimal, BigDecimal> max = maxFunction();
-    return aggregate(source.select(selector), null, max);
+    return aggregate(source.select(selector), maxFunction());
   }
 
   /**
@@ -2361,8 +2388,7 @@ public static <TSource> BigDecimal max(Enumerable<TSource> source,
    */
   public static <TSource> double max(Enumerable<TSource> source,
       DoubleFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null,
-        Extensions.DOUBLE_MAX);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.DOUBLE_MAX));
   }
 
   /**
@@ -2372,7 +2398,7 @@ public static <TSource> double max(Enumerable<TSource> source,
    */
   public static <TSource> Double max(Enumerable<TSource> source,
       NullableDoubleFunction1<TSource> selector) {
-    return aggregate(source.select(selector), null, Extensions.DOUBLE_MAX);
+    return aggregate(source.select(selector), Extensions.DOUBLE_MAX);
   }
 
   /**
@@ -2381,8 +2407,7 @@ public static <TSource> Double max(Enumerable<TSource> source,
    */
   public static <TSource> int max(Enumerable<TSource> source,
       IntegerFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null,
-        Extensions.INTEGER_MAX);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.INTEGER_MAX));
   }
 
   /**
@@ -2392,7 +2417,7 @@ public static <TSource> int max(Enumerable<TSource> source,
    */
   public static <TSource> Integer max(Enumerable<TSource> source,
       NullableIntegerFunction1<TSource> selector) {
-    return aggregate(source.select(selector), null, Extensions.INTEGER_MAX);
+    return aggregate(source.select(selector), Extensions.INTEGER_MAX);
   }
 
   /**
@@ -2401,7 +2426,7 @@ public static <TSource> Integer max(Enumerable<TSource> source,
    */
   public static <TSource> long max(Enumerable<TSource> source,
       LongFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null, Extensions.LONG_MAX);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.LONG_MAX));
   }
 
   /**
@@ -2411,7 +2436,7 @@ public static <TSource> long max(Enumerable<TSource> source,
    */
   public static <TSource> Long max(Enumerable<TSource> source,
       NullableLongFunction1<TSource> selector) {
-    return aggregate(source.select(selector), null, Extensions.LONG_MAX);
+    return aggregate(source.select(selector), Extensions.LONG_MAX);
   }
 
   /**
@@ -2420,8 +2445,7 @@ public static <TSource> Long max(Enumerable<TSource> source,
    */
   public static <TSource> float max(Enumerable<TSource> source,
       FloatFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null,
-        Extensions.FLOAT_MAX);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.FLOAT_MAX));
   }
 
   /**
@@ -2431,7 +2455,7 @@ public static <TSource> float max(Enumerable<TSource> source,
    */
   public static <TSource> Float max(Enumerable<TSource> source,
       NullableFloatFunction1<TSource> selector) {
-    return aggregate(source.select(selector), null, Extensions.FLOAT_MAX);
+    return aggregate(source.select(selector), Extensions.FLOAT_MAX);
   }
 
   /**
@@ -2441,8 +2465,7 @@ public static <TSource> Float max(Enumerable<TSource> source,
    */
   public static <TSource, TResult extends Comparable<TResult>> TResult max(
       Enumerable<TSource> source, Function1<TSource, TResult> selector) {
-    Function2<TResult, TResult, TResult> max = maxFunction();
-    return aggregate(source.select(selector), null, max);
+    return aggregate(source.select(selector), maxFunction());
   }
 
   /**
@@ -2451,8 +2474,7 @@ public static <TSource, TResult extends Comparable<TResult>> TResult max(
    */
   public static <TSource extends Comparable<TSource>> TSource min(
       Enumerable<TSource> source) {
-    Function2<TSource, TSource, TSource> min = minFunction();
-    return aggregate(source, null, min);
+    return aggregate(source, minFunction());
   }
 
   @SuppressWarnings("unchecked")
@@ -2484,8 +2506,7 @@ public static <TSource> BigDecimal min(Enumerable<TSource> source,
    */
   public static <TSource> BigDecimal min(Enumerable<TSource> source,
       NullableBigDecimalFunction1<TSource> selector) {
-    Function2<BigDecimal, BigDecimal, BigDecimal> min = minFunction();
-    return aggregate(source.select(selector), null, min);
+    return aggregate(source.select(selector), minFunction());
   }
 
   /**
@@ -2494,8 +2515,7 @@ public static <TSource> BigDecimal min(Enumerable<TSource> source,
    */
   public static <TSource> double min(Enumerable<TSource> source,
       DoubleFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null,
-        Extensions.DOUBLE_MIN);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.DOUBLE_MIN));
   }
 
   /**
@@ -2505,7 +2525,7 @@ public static <TSource> double min(Enumerable<TSource> source,
    */
   public static <TSource> Double min(Enumerable<TSource> source,
       NullableDoubleFunction1<TSource> selector) {
-    return aggregate(source.select(selector), null, Extensions.DOUBLE_MIN);
+    return aggregate(source.select(selector), Extensions.DOUBLE_MIN);
   }
 
   /**
@@ -2514,8 +2534,7 @@ public static <TSource> Double min(Enumerable<TSource> source,
    */
   public static <TSource> int min(Enumerable<TSource> source,
       IntegerFunction1<TSource> selector) {
-    return aggregate(source.select(adapt(selector)), null,
-        Extensions.INTEGER_MIN);
+    return castNonNull(aggregate(source.select(adapt(selector)), Extensions.INTEGER_MIN));
   }
 
   /**
@@ -2611,7 +2630,7 @@ public static <TSource, TKey extends Comparable> Enumerable<TSource> orderBy(
    */
   public static <TSource, TKey> Enumerable<TSource> orderBy(
       Enumerable<TSource> source, Function1<TSource, TKey> keySelector,
-      Comparator<TKey> comparator) {
+      @Nullable Comparator<TKey> comparator) {
     return new AbstractEnumerable<TSource>() {
       @Override public Enumerator<TSource> enumerator() {
         // NOTE: TreeMap allows null comparator. But the caller of this method
@@ -2960,7 +2979,7 @@ public static <TSource> boolean sequenceEqual(Enumerable<TSource> first,
    * {@code EqualityComparer<TSource>}.
    */
   public static <TSource> boolean sequenceEqual(Enumerable<TSource> first,
-      Enumerable<TSource> second, EqualityComparer<TSource> comparer) {
+      Enumerable<TSource> second, @Nullable EqualityComparer<TSource> comparer) {
     Objects.requireNonNull(first);
     Objects.requireNonNull(second);
     if (comparer == null) {
@@ -3052,7 +3071,7 @@ public static <TSource> TSource single(Enumerable<TSource> source,
    * exception if there is more than one element in the
    * sequence.
    */
-  public static <TSource> TSource singleOrDefault(Enumerable<TSource> source) {
+  public static <TSource> @Nullable TSource singleOrDefault(Enumerable<TSource> source) {
     TSource toRet = null;
     try (Enumerator<TSource> os = source.enumerator()) {
       if (os.moveNext()) {
@@ -3073,7 +3092,7 @@ public static <TSource> TSource singleOrDefault(Enumerable<TSource> source) {
    * element exists; this method throws an exception if more than
    * one element satisfies the condition.
    */
-  public static <TSource> TSource singleOrDefault(Enumerable<TSource> source,
+  public static <TSource> @Nullable TSource singleOrDefault(Enumerable<TSource> source,
       Predicate1<TSource> predicate) {
     TSource toRet = null;
     for (TSource s : source) {
@@ -3469,6 +3488,7 @@ static <TSource, TKey, TElement> LookupImpl<TKey, TElement> toLookup_(
       while (os.moveNext()) {
         TSource o = os.current();
         final TKey key = keySelector.apply(o);
+        @SuppressWarnings("nullness")
         List<TElement> list = map.get(key);
         if (list == null) {
           // for first entry, use a singleton list to save space
@@ -3667,7 +3687,7 @@ public static <T> OrderedQueryable<T> asOrderedQueryable(
     return source instanceof OrderedQueryable
         ? ((OrderedQueryable<T>) source)
         : new EnumerableOrderedQueryable<>(
-            source, (Class) Object.class, null, null);
+            source, (Class) Object.class, castNonNull(null), null);
   }
 
   /** Default implementation of {@link ExtendedEnumerable#into(Collection)}. */
@@ -3825,12 +3845,15 @@ public void close() {
 
   /** Enumerator that casts each value.
    *
-   * @param <T> element type */
-  static class CastingEnumerator<T> implements Enumerator<T> {
-    private final Enumerator<?> enumerator;
+   * @param <F> source element type
+   * @param <T> element type*/
+  @HasQualifierParameter(Nullable.class)
+  static class CastingEnumerator<F extends @PolyNull Object, @PolyNull T extends @PolyNull Object>
+      implements Enumerator<T> {
+    private final Enumerator<F> enumerator;
     private final Class<T> clazz;
 
-    CastingEnumerator(Enumerator<?> enumerator, Class<T> clazz) {
+    CastingEnumerator(Enumerator<F> enumerator, Class<T> clazz) {
       this.enumerator = enumerator;
       this.clazz = clazz;
     }
@@ -3872,7 +3895,7 @@ static <T> Wrapped<T> upAs(EqualityComparer<T> comparer, T element) {
       return comparer.hashCode(element);
     }
 
-    @Override public boolean equals(Object obj) {
+    @Override public boolean equals(@Nullable Object obj) {
       //noinspection unchecked
       return obj == this || obj instanceof Wrapped && comparer.equal(element,
           ((Wrapped<T>) obj).element);
@@ -3896,8 +3919,9 @@ protected WrapMap(Function0<Map<Wrapped<K>, V>> mapProvider, EqualityComparer<K>
       this.comparer = comparer;
     }
 
-    @Override public Set<Entry<K, V>> entrySet() {
-      return new AbstractSet<Entry<K, V>>() {
+    @Override public Set<Entry<@KeyFor("this") K, V>> entrySet() {
+      return new AbstractSet<Entry<@KeyFor("this") K, V>>() {
+        @SuppressWarnings("override.return.invalid")
         @Override public Iterator<Entry<K, V>> iterator() {
           final Iterator<Entry<Wrapped<K>, V>> iterator =
               map.entrySet().iterator();
@@ -3924,23 +3948,26 @@ public void remove() {
       };
     }
 
+    @SuppressWarnings("contracts.conditional.postcondition.not.satisfied")
     @Override public boolean containsKey(Object key) {
       return map.containsKey(wrap((K) key));
     }
 
+    @Pure
     private Wrapped<K> wrap(K key) {
       return Wrapped.upAs(comparer, key);
     }
 
-    @Override public V get(Object key) {
+    @Override public @Nullable V get(Object key) {
       return map.get(wrap((K) key));
     }
 
-    @Override public V put(K key, V value) {
+    @SuppressWarnings("contracts.postcondition.not.satisfied")
+    @Override public @Nullable V put(K key, V value) {
       return map.put(wrap(key), value);
     }
 
-    @Override public V remove(Object key) {
+    @Override public @Nullable V remove(Object key) {
       return map.remove(wrap((K) key));
     }
 
@@ -4003,30 +4030,31 @@ private static class MergeJoinEnumerator<TResult, TSource, TInner, TKey extends
     private final List<TInner> rights = new ArrayList<>();
     private final Enumerable<TSource> leftEnumerable;
     private final Enumerable<TInner> rightEnumerable;
-    private Enumerator<TSource> leftEnumerator = null;
-    private Enumerator<TInner> rightEnumerator = null;
+    private @Nullable Enumerator<TSource> leftEnumerator = null;
+    private @Nullable Enumerator<TInner> rightEnumerator = null;
     private final Function1<TSource, TKey> outerKeySelector;
     private final Function1<TInner, TKey> innerKeySelector;
     // extra predicate in case of non equi-join, in case of equi-join it will be null
-    private final Predicate2<TSource, TInner> extraPredicate;
-    private final Function2<TSource, TInner, TResult> resultSelector;
+    private final @Nullable Predicate2<TSource, TInner> extraPredicate;
+    private final Function2<TSource, @Nullable TInner, TResult> resultSelector;
     private final JoinType joinType;
     // key comparator, possibly null (Comparable#compareTo to be used in that case)
-    private final Comparator<TKey> comparator;
+    private final @Nullable Comparator<TKey> comparator;
     private boolean done;
-    private Enumerator<TResult> results = null;
+    private @Nullable Enumerator<TResult> results = null;
     // used for LEFT/ANTI join: if right input is over, all remaining elements from left are results
     private boolean remainingLeft;
     private TResult current = (TResult) DUMMY;
 
+    @SuppressWarnings("method.invocation.invalid")
     MergeJoinEnumerator(Enumerable<TSource> leftEnumerable,
         Enumerable<TInner> rightEnumerable,
         Function1<TSource, TKey> outerKeySelector,
         Function1<TInner, TKey> innerKeySelector,
-        Predicate2<TSource, TInner> extraPredicate,
-        Function2<TSource, TInner, TResult> resultSelector,
+        @Nullable Predicate2<TSource, TInner> extraPredicate,
+        Function2<TSource, @Nullable TInner, TResult> resultSelector,
         JoinType joinType,
-        Comparator<TKey> comparator) {
+        @Nullable Comparator<TKey> comparator) {
       this.leftEnumerable = leftEnumerable;
       this.rightEnumerable = rightEnumerable;
       this.outerKeySelector = outerKeySelector;
@@ -4119,9 +4147,9 @@ private int compareNullsLast(TKey v0, TKey v1) {
      * enumerator. */
     private boolean advance() {
       for (;;) {
-        TSource left = leftEnumerator.current();
+        TSource left = castNonNull(leftEnumerator).current();
         TKey leftKey = outerKeySelector.apply(left);
-        TInner right = rightEnumerator.current();
+        TInner right = castNonNull(rightEnumerator).current();
         TKey rightKey = innerKeySelector.apply(right);
         // iterate until finding matching keys (or ANTI join results)
         for (;;) {
@@ -4315,7 +4343,9 @@ public void reset() {
       results = null;
       current = (TResult) DUMMY;
       remainingLeft = false;
-      leftEnumerator.reset();
+      if (leftEnumerator != null) {
+        leftEnumerator.reset();
+      }
       if (rightEnumerator != null) {
         rightEnumerator.reset();
       }
@@ -4323,7 +4353,9 @@ public void reset() {
     }
 
     public void close() {
-      leftEnumerator.close();
+      if (leftEnumerator != null) {
+        leftEnumerator.close();
+      }
       if (rightEnumerator != null) {
         rightEnumerator.close();
       }
@@ -4337,10 +4369,10 @@ public void close() {
    * @param <TInner> right input record type */
   private static class CartesianProductJoinEnumerator<TResult, TOuter, TInner>
       extends CartesianProductEnumerator<Object, TResult> {
-    private final Function2<TOuter, TInner, TResult> resultSelector;
+    private final Function2<TOuter, @Nullable TInner, TResult> resultSelector;
 
     @SuppressWarnings("unchecked")
-    CartesianProductJoinEnumerator(Function2<TOuter, TInner, TResult> resultSelector,
+    CartesianProductJoinEnumerator(Function2<TOuter, @Nullable TInner, TResult> resultSelector,
         Enumerator<TOuter> outer, Enumerator<TInner> inner) {
       super(ImmutableList.of((Enumerator<Object>) outer, (Enumerator<Object>) inner));
       this.resultSelector = resultSelector;
@@ -4384,7 +4416,7 @@ public static <TSource> Enumerable<TSource> repeatUnion(
           private boolean seedProcessed = false;
           private int currentIteration = 0;
           private final Enumerator<TSource> seedEnumerator = seed.enumerator();
-          private Enumerator<TSource> iterativeEnumerator = null;
+          private @Nullable Enumerator<TSource> iterativeEnumerator = null;
 
           // Set to control duplicates, only used if "all" is false
           private final Set<Wrapped<TSource>> processed = new HashSet<>();
@@ -4434,8 +4466,9 @@ private boolean checkValue(TSource value) {
                 return false;
               }
 
+              Enumerator<TSource> iterativeEnumerator = this.iterativeEnumerator;
               if (iterativeEnumerator == null) {
-                iterativeEnumerator = iteration.enumerator();
+                this.iterativeEnumerator = iterativeEnumerator = iteration.enumerator();
               }
 
               while (iterativeEnumerator.moveNext()) {
@@ -4454,7 +4487,7 @@ private boolean checkValue(TSource value) {
               // current iteration level (which returned some values) is finished, go to next one
               current = (TSource) DUMMY;
               iterativeEnumerator.close();
-              iterativeEnumerator = null;
+              this.iterativeEnumerator = null;
               currentIteration++;
             }
           }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableOrderedQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableOrderedQueryable.java
index 0fe30bfeba86..675fd24f7e35 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableOrderedQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableOrderedQueryable.java
@@ -20,6 +20,8 @@
 import org.apache.calcite.linq4j.tree.Expression;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Comparator;
 
 /**
@@ -31,7 +33,7 @@
 class EnumerableOrderedQueryable<T> extends EnumerableQueryable<T>
     implements OrderedQueryable<T> {
   EnumerableOrderedQueryable(Enumerable<T> enumerable, Class<T> rowType,
-      QueryProvider provider, Expression expression) {
+      QueryProvider provider, @Nullable Expression expression) {
     super(provider, rowType, expression, enumerable);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableQueryable.java
index 77f1ba12540a..02c6ed075e5c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/EnumerableQueryable.java
@@ -34,6 +34,8 @@
 import org.apache.calcite.linq4j.tree.Expression;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.math.BigDecimal;
 import java.util.Comparator;
@@ -49,10 +51,10 @@ class EnumerableQueryable<T> extends DefaultEnumerable<T>
   private final QueryProvider provider;
   private final Class<T> elementType;
   private final Enumerable<T> enumerable;
-  private final Expression expression;
+  private final @Nullable Expression expression;
 
   EnumerableQueryable(QueryProvider provider, Class<T> elementType,
-      Expression expression, Enumerable<T> enumerable) {
+      @Nullable Expression expression, Enumerable<T> enumerable) {
     this.enumerable = enumerable;
     this.elementType = elementType;
     this.provider = provider;
@@ -153,7 +155,7 @@ public Queryable<T> reverse() {
     return EnumerableDefaults.ofType(getThis(), clazz).asQueryable();
   }
 
-  @Override public Queryable<T> defaultIfEmpty() {
+  @Override public Queryable<@Nullable T> defaultIfEmpty() {
     return EnumerableDefaults.defaultIfEmpty(getThis()).asQueryable();
   }
 
@@ -167,7 +169,7 @@ public Type getElementType() {
     return elementType;
   }
 
-  public Expression getExpression() {
+  public @Nullable Expression getExpression() {
     return expression;
   }
 
@@ -177,7 +179,7 @@ public QueryProvider getProvider() {
 
   // .............
 
-  public T aggregate(FunctionExpression<Function2<T, T, T>> selector) {
+  public @Nullable T aggregate(FunctionExpression<Function2<@Nullable T, T, T>> selector) {
     return EnumerableDefaults.aggregate(getThis(), selector.getFunction());
   }
 
@@ -260,7 +262,7 @@ public T first(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.first(getThis(), predicate.getFunction());
   }
 
-  public T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T firstOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.firstOrDefault(getThis(),
         predicate.getFunction());
   }
@@ -373,7 +375,7 @@ public T last(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.last(getThis(), predicate.getFunction());
   }
 
-  public T lastOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T lastOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.lastOrDefault(getThis(), predicate.getFunction());
   }
 
@@ -466,7 +468,7 @@ public T single(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.single(getThis(), predicate.getFunction());
   }
 
-  public T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
+  public @Nullable T singleOrDefault(FunctionExpression<Predicate1<T>> predicate) {
     return EnumerableDefaults.singleOrDefault(getThis(),
         predicate.getFunction());
   }
@@ -557,7 +559,7 @@ public <T1, TResult> Queryable<TResult> zip(Enumerable<T1> source1,
         resultSelector.getFunction()).asQueryable();
   }
 
-  public T aggregate(Function2<T, T, T> func) {
+  public @Nullable T aggregate(Function2<@Nullable T, T, T> func) {
     return EnumerableDefaults.aggregate(getThis(), func);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerator.java b/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerator.java
index 90bc29ce039d..df879fb16989 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerator.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/Enumerator.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Supports a simple iteration over a collection.
  *
@@ -26,6 +28,7 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface Enumerator<T> extends AutoCloseable {
   /**
    * Gets the current element in the collection.
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedEnumerable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedEnumerable.java
index 5ed792424254..d29083ed9778 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedEnumerable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedEnumerable.java
@@ -33,6 +33,10 @@
 import org.apache.calcite.linq4j.function.Predicate1;
 import org.apache.calcite.linq4j.function.Predicate2;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+import org.checkerframework.framework.qual.Covariant;
+
 import java.math.BigDecimal;
 import java.util.Collection;
 import java.util.Comparator;
@@ -44,6 +48,7 @@
  *
  * @param <TSource> Element type
  */
+@Covariant(0)
 public interface ExtendedEnumerable<TSource> {
 
   /**
@@ -55,21 +60,21 @@ public interface ExtendedEnumerable<TSource> {
    * @param func Operation
    * @param <R> Return type
    */
-  <R> R foreach(Function1<TSource, R> func);
+  <R> @Nullable R foreach(Function1<TSource, R> func);
 
   /**
    * Applies an accumulator function over a
    * sequence.
    */
-  TSource aggregate(Function2<TSource, TSource, TSource> func);
+  @Nullable TSource aggregate(Function2<@Nullable TSource, TSource, TSource> func);
 
   /**
    * Applies an accumulator function over a
    * sequence. The specified seed value is used as the initial
    * accumulator value.
    */
-  <TAccumulate> TAccumulate aggregate(TAccumulate seed,
-      Function2<TAccumulate, TSource, TAccumulate> func);
+  <TAccumulate> @PolyNull TAccumulate aggregate(@PolyNull TAccumulate seed,
+      Function2<@PolyNull TAccumulate, TSource, @PolyNull TAccumulate> func);
 
   /**
    * Applies an accumulator function over a
@@ -263,14 +268,14 @@ <TAccumulate, TResult> TResult aggregate(TAccumulate seed,
    * the type parameter's default value in a singleton collection if
    * the sequence is empty.
    */
-  Enumerable<TSource> defaultIfEmpty();
+  Enumerable<@Nullable TSource> defaultIfEmpty();
 
   /**
    * Returns the elements of the specified sequence or
    * the specified value in a singleton collection if the sequence
    * is empty.
    */
-  Enumerable<TSource> defaultIfEmpty(TSource value);
+  Enumerable<@PolyNull TSource> defaultIfEmpty(@PolyNull TSource value);
 
   /**
    * Returns distinct elements from a sequence by using
@@ -295,7 +300,7 @@ <TAccumulate, TResult> TResult aggregate(TAccumulate seed,
    * sequence or a default value if the index is out of
    * range.
    */
-  TSource elementAtOrDefault(int index);
+  @Nullable TSource elementAtOrDefault(int index);
 
   /**
    * Produces the set difference of two sequences by
@@ -344,14 +349,14 @@ Enumerable<TSource> except(Enumerable<TSource> enumerable1,
    * Returns the first element of a sequence, or a
    * default value if the sequence contains no elements.
    */
-  TSource firstOrDefault();
+  @Nullable TSource firstOrDefault();
 
   /**
    * Returns the first element of the sequence that
    * satisfies a condition or a default value if no such element is
    * found.
    */
-  TSource firstOrDefault(Predicate1<TSource> predicate);
+  @Nullable TSource firstOrDefault(Predicate1<TSource> predicate);
 
   /**
    * Groups the elements of a sequence according to a
@@ -644,14 +649,14 @@ <TInner, TResult> Enumerable<TResult> correlateJoin(
    * Returns the last element of a sequence, or a
    * default value if the sequence contains no elements.
    */
-  TSource lastOrDefault();
+  @Nullable TSource lastOrDefault();
 
   /**
    * Returns the last element of a sequence that
    * satisfies a condition or a default value if no such element is
    * found.
    */
-  TSource lastOrDefault(Predicate1<TSource> predicate);
+  @Nullable TSource lastOrDefault(Predicate1<TSource> predicate);
 
   /**
    * Returns an long that represents the total number
@@ -956,7 +961,7 @@ boolean sequenceEqual(Enumerable<TSource> enumerable1,
    * exception if there is more than one element in the
    * sequence.
    */
-  TSource singleOrDefault();
+  @Nullable TSource singleOrDefault();
 
   /**
    * Returns the only element of a sequence that
@@ -964,7 +969,7 @@ boolean sequenceEqual(Enumerable<TSource> enumerable1,
    * element exists; this method throws an exception if more than
    * one element satisfies the condition.
    */
-  TSource singleOrDefault(Predicate1<TSource> predicate);
+  @Nullable TSource singleOrDefault(Predicate1<TSource> predicate);
 
   /**
    * Bypasses a specified number of elements in a
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedQueryable.java
index 573dcb908492..8af18f3424ab 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/ExtendedQueryable.java
@@ -33,6 +33,9 @@
 import org.apache.calcite.linq4j.function.Predicate2;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.framework.qual.Covariant;
+
 import java.math.BigDecimal;
 import java.util.Comparator;
 
@@ -41,13 +44,14 @@
  *
  * @param <TSource> Element type
  */
+@Covariant(0)
 interface ExtendedQueryable<TSource> extends ExtendedEnumerable<TSource> {
 
   /**
    * Applies an accumulator function over a sequence.
    */
-  TSource aggregate(
-      FunctionExpression<Function2<TSource, TSource, TSource>> selector);
+  @Nullable TSource aggregate(
+      FunctionExpression<Function2<@Nullable TSource, TSource, TSource>> selector);
 
   /**
    * Applies an accumulator function over a
@@ -172,7 +176,7 @@ Long averageNullableLong(
    * the type parameter's default value in a singleton collection if
    * the sequence is empty.
    */
-  Queryable<TSource> defaultIfEmpty();
+  Queryable<@Nullable TSource> defaultIfEmpty();
 
   /**
    * Returns distinct elements from a sequence by using
@@ -227,7 +231,7 @@ Queryable<TSource> except(Enumerable<TSource> enumerable,
    * satisfies a specified condition or a default value if no such
    * element is found.
    */
-  TSource firstOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
+  @Nullable TSource firstOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
 
   /**
    * Groups the elements of a sequence according to a
@@ -400,7 +404,7 @@ <TInner, TKey, TResult> Queryable<TResult> join(Enumerable<TInner> inner,
    * satisfies a condition or a default value if no such element is
    * found.
    */
-  TSource lastOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
+  @Nullable TSource lastOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
 
   /**
    * Returns an long that represents the number of
@@ -564,7 +568,7 @@ <TCollection, TResult> Queryable<TResult> selectManyN(
    * exception if there is more than one element in the
    * sequence.
    */
-  TSource singleOrDefault();
+  @Nullable TSource singleOrDefault();
 
   /**
    * Returns the only element of a sequence that
@@ -572,7 +576,7 @@ <TCollection, TResult> Queryable<TResult> selectManyN(
    * element exists; this method throws an exception if more than
    * one element satisfies the condition.
    */
-  TSource singleOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
+  @Nullable TSource singleOrDefault(FunctionExpression<Predicate1<TSource>> predicate);
 
   /**
    * Bypasses a specified number of elements in a
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/Grouping.java b/linq4j/src/main/java/org/apache/calcite/linq4j/Grouping.java
index f3091afc27dc..aede1d0254f9 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/Grouping.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/Grouping.java
@@ -16,12 +16,15 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Represents a collection of objects that have a common key.
  *
  * @param <K> Key type
  * @param <V> Element type
  */
+@Covariant(0)
 public interface Grouping<K, V> extends Enumerable<V> {
   /**
    * Gets the key of this Grouping.
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/GroupingImpl.java b/linq4j/src/main/java/org/apache/calcite/linq4j/GroupingImpl.java
index 9444dc540cfd..49c1bfe4b427 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/GroupingImpl.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/GroupingImpl.java
@@ -16,6 +16,9 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Collection;
 import java.util.Map;
 import java.util.Objects;
@@ -26,7 +29,8 @@
  * @param <K> Key type
  * @param <V> Value type
  */
-class GroupingImpl<K, V> extends AbstractEnumerable<V>
+@SuppressWarnings("type.argument.type.incompatible")
+class GroupingImpl<K extends @NonNull Object, V> extends AbstractEnumerable<V>
     implements Grouping<K, V>, Map.Entry<K, Enumerable<V>> {
   private final K key;
   private final Collection<V> values;
@@ -48,7 +52,7 @@ class GroupingImpl<K, V> extends AbstractEnumerable<V>
     return key.hashCode() ^ values.hashCode();
   }
 
-  @Override public boolean equals(Object obj) {
+  @Override public boolean equals(@Nullable Object obj) {
     return obj instanceof GroupingImpl
            && key.equals(((GroupingImpl) obj).key)
            && values.equals(((GroupingImpl) obj).values);
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/Linq4j.java b/linq4j/src/main/java/org/apache/calcite/linq4j/Linq4j.java
index b1251e898b02..4c4f94227e7c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/Linq4j.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/Linq4j.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.function.Function1;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -28,6 +30,8 @@
 import java.util.Objects;
 import java.util.RandomAccess;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Utility and factory methods for Linq4j.
  */
@@ -36,7 +40,7 @@ private Linq4j() {}
 
   private static final Object DUMMY = new Object();
 
-  public static Method getMethod(String className, String methodName,
+  public static @Nullable Method getMethod(String className, String methodName,
       Class... parameterTypes) {
     try {
       return Class.forName(className).getMethod(methodName, parameterTypes);
@@ -423,7 +427,7 @@ public static <T> T requireNonNull(T o) {
   }
 
   /** Closes an iterator, if it can be closed. */
-  private static <T> void closeIterator(Iterator<T> iterator) {
+  private static <T> void closeIterator(@Nullable Iterator<? extends T> iterator) {
     if (iterator instanceof AutoCloseable) {
       try {
         ((AutoCloseable) iterator).close();
@@ -441,7 +445,7 @@ private static <T> void closeIterator(Iterator<T> iterator) {
   @SuppressWarnings("unchecked")
   static class IterableEnumerator<T> implements Enumerator<T> {
     private final Iterable<? extends T> iterable;
-    Iterator<? extends T> iterator;
+    @Nullable Iterator<? extends T> iterator;
     T current;
 
     IterableEnumerator(Iterable<? extends T> iterable) {
@@ -458,7 +462,7 @@ public T current() {
     }
 
     public boolean moveNext() {
-      if (iterator.hasNext()) {
+      if (castNonNull(iterator).hasNext()) {
         current = iterator.next();
         return true;
       }
@@ -564,6 +568,7 @@ protected Collection<T> getCollection() {
       return getCollection().size();
     }
 
+    @SuppressWarnings("argument.type.incompatible")
     @Override public boolean contains(T element) {
       return getCollection().contains(element);
     }
@@ -644,7 +649,7 @@ public void close() {
   /** Enumerator that returns one null element.
    *
    * @param <E> element type */
-  private static class SingletonNullEnumerator<E> implements Enumerator<E> {
+  private static class SingletonNullEnumerator<@Nullable E> implements Enumerator<E> {
     int i = 0;
 
     public E current() {
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/LookupImpl.java b/linq4j/src/main/java/org/apache/calcite/linq4j/LookupImpl.java
index def63c4fad20..ab210c8eef88 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/LookupImpl.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/LookupImpl.java
@@ -18,6 +18,9 @@
 
 import org.apache.calcite.linq4j.function.Function2;
 
+import org.checkerframework.checker.nullness.qual.KeyFor;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.AbstractCollection;
 import java.util.AbstractMap;
 import java.util.AbstractSet;
@@ -81,6 +84,7 @@ public boolean isEmpty() {
     return map.isEmpty();
   }
 
+  @SuppressWarnings("contracts.conditional.postcondition.not.satisfied")
   public boolean containsKey(Object key) {
     return map.containsKey(key);
   }
@@ -91,17 +95,18 @@ public boolean containsValue(Object value) {
     return map.containsValue(list);
   }
 
-  public Enumerable<V> get(Object key) {
+  public @Nullable Enumerable<V> get(Object key) {
     final List<V> list = map.get(key);
     return list == null ? null : Linq4j.asEnumerable(list);
   }
 
-  public Enumerable<V> put(K key, Enumerable<V> value) {
+  @SuppressWarnings("contracts.postcondition.not.satisfied")
+  public @Nullable Enumerable<V> put(K key, Enumerable<V> value) {
     final List<V> list = map.put(key, value.toList());
     return list == null ? null : Linq4j.asEnumerable(list);
   }
 
-  public Enumerable<V> remove(Object key) {
+  public @Nullable Enumerable<V> remove(Object key) {
     final List<V> list = map.remove(key);
     return list == null ? null : Linq4j.asEnumerable(list);
   }
@@ -116,7 +121,8 @@ public void clear() {
     map.clear();
   }
 
-  public Set<K> keySet() {
+  @SuppressWarnings("return.type.incompatible")
+  public Set<@KeyFor("this") K> keySet() {
     return map.keySet();
   }
 
@@ -147,8 +153,9 @@ public int size() {
     };
   }
 
-  public Set<Entry<K, Enumerable<V>>> entrySet() {
-    final Set<Entry<K, List<V>>> entries = map.entrySet();
+  @SuppressWarnings("return.type.incompatible")
+  public Set<Entry<@KeyFor({"this"}) K, Enumerable<V>>> entrySet() {
+    final Set<Entry<@KeyFor("map") K, List<V>>> entries = map.entrySet();
     return new AbstractSet<Entry<K, Enumerable<V>>>() {
       public Iterator<Entry<K, Enumerable<V>>> iterator() {
         final Iterator<Entry<K, List<V>>> iterator = entries.iterator();
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryEnumerator.java b/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryEnumerator.java
index 13167b122844..c09661cde38b 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryEnumerator.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryEnumerator.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.concurrent.atomic.AtomicInteger;
 
 /**
@@ -23,7 +25,7 @@
  *
  * @param <E> Row value
  */
-public class MemoryEnumerator<E> implements Enumerator<MemoryFactory.Memory<E>> {
+public class MemoryEnumerator<@Nullable E> implements Enumerator<MemoryFactory.Memory<E>> {
   private final Enumerator<E> enumerator;
   private final MemoryFactory<E> memoryFactory;
   private final AtomicInteger prevCounter;
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryFactory.java b/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryFactory.java
index 45d641a6c051..f68fa91777e7 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryFactory.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/MemoryFactory.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Arrays;
 
 /**
@@ -30,7 +32,7 @@ public class MemoryFactory<E> {
   // Index:      0   1   2   3   4
   // Idea       -2  -1   0  +1  +2
   ModularInteger offset;
-  private Object[] values;
+  private final @Nullable Object[] values;
 
   public MemoryFactory(int history, int future) {
     this.history = history;
@@ -62,10 +64,10 @@ public static class Memory<E> {
     private final int history;
     private final int future;
     private final ModularInteger offset;
-    private final Object[] values;
+    private final @Nullable Object[] values;
 
     public Memory(int history, int future,
-        ModularInteger offset, Object[] values) {
+        ModularInteger offset, @Nullable Object[] values) {
       this.history = history;
       this.future = future;
       this.offset = offset;
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/ModularInteger.java b/linq4j/src/main/java/org/apache/calcite/linq4j/ModularInteger.java
index e6b7695eefe7..12cc470c468d 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/ModularInteger.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/ModularInteger.java
@@ -18,6 +18,8 @@
 
 import com.google.common.base.Preconditions;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * Represents an integer in modular arithmetic.
  * Its {@code value} is between 0 and {@code m - 1} for some modulus {@code m}.
@@ -35,7 +37,7 @@ class ModularInteger {
     this.modulus = modulus;
   }
 
-  @Override public boolean equals(Object obj) {
+  @Override public boolean equals(@Nullable Object obj) {
     return obj == this
         || obj instanceof ModularInteger
         && value == ((ModularInteger) obj).value
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/NullnessUtil.java b/linq4j/src/main/java/org/apache/calcite/linq4j/NullnessUtil.java
new file mode 100644
index 000000000000..afd8127834a1
--- /dev/null
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/NullnessUtil.java
@@ -0,0 +1,65 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.calcite.linq4j;
+
+import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.dataflow.qual.Pure;
+
+/**
+ * The methods in this class allow to cast nullable reference to a non-nullable one.
+ * This is an internal class, and it is not meant to be used as a public API.
+ * <p>The class enables to remove checker-qual runtime dependency, and helps IDEs to see
+ * the resulting types of {@code castNonNull} better</p>
+ */
+@SuppressWarnings({"cast.unsafe", "NullableProblems", "contracts.postcondition.not.satisfied"})
+public class NullnessUtil {
+  private NullnessUtil() {
+  }
+
+  /**
+   * Casts nullable reference to a non-null type.
+   *
+   * @param <T> the type of the reference
+   * @param ref a reference of @Nullable type, that is non-null at run time
+   * @return the argument, casted to have the type qualifier @NonNull
+   */
+  @Pure
+  public static @EnsuresNonNull("#1")
+  <T extends @Nullable Object> @NonNull T castNonNull(
+      @Nullable T ref) {
+    assert ref != null : "Misuse of castNonNull: called with a null argument";
+    return (@NonNull T) ref;
+  }
+
+  /**
+   * Casts nullable reference to a non-null type.
+   *
+   * @param <T>     the type of the reference
+   * @param ref     a reference of @Nullable type, that is non-null at run time
+   * @param message extra message in case the argument is null
+   * @return the argument, casted to have the type qualifier @NonNull
+   */
+  @Pure
+  public static @EnsuresNonNull("#1")
+  <T extends @Nullable Object> @NonNull T castNonNull(
+      @Nullable T ref, String message) {
+    assert ref != null : "Misuse of castNonNull: called with a null argument " + message;
+    return (@NonNull T) ref;
+  }
+}
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/Queryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/Queryable.java
index afdda6a16640..9f9f049744ee 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/Queryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/Queryable.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Provides functionality to evaluate queries against a specific data source
  * wherein the type of the data is known.
@@ -24,5 +26,6 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface Queryable<T> extends RawQueryable<T>, ExtendedQueryable<T> {
 }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableDefaults.java b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableDefaults.java
index 8200c17470d1..37f0813b7992 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableDefaults.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableDefaults.java
@@ -36,11 +36,15 @@
 import org.apache.calcite.linq4j.tree.Expressions;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.math.BigDecimal;
 import java.util.Comparator;
 import java.util.Iterator;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Default implementations for methods in the {@link Queryable} interface.
  */
@@ -741,13 +745,13 @@ public static <T> Queryable<T> reverse(Queryable<T> source) {
   public static <T, TResult> Queryable<TResult> select(Queryable<T> source,
       FunctionExpression<Function1<T, TResult>> selector) {
     return source.getProvider().createQuery(
-        Expressions.call(source.getExpression(), "select", selector),
+        Expressions.call(castNonNull(source.getExpression()), "select", selector),
         functionResultType(selector));
   }
 
   private static <P0, R> Type functionResultType(
       FunctionExpression<Function1<P0, R>> selector) {
-    return selector.body.getType();
+    return castNonNull(selector.body).getType();
   }
 
   /**
@@ -1202,7 +1206,7 @@ public Type getElementType() {
       return original.getElementType();
     }
 
-    public Expression getExpression() {
+    public @Nullable Expression getExpression() {
       return original.getExpression();
     }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableFactory.java b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableFactory.java
index 9db00aefed06..2fb884735830 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableFactory.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableFactory.java
@@ -33,6 +33,10 @@
 import org.apache.calcite.linq4j.function.Predicate2;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+import org.checkerframework.framework.qual.Covariant;
+
 import java.math.BigDecimal;
 import java.util.Comparator;
 
@@ -41,13 +45,14 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface QueryableFactory<T> {
 
   /**
    * Applies an accumulator function over a sequence.
    */
-  T aggregate(Queryable<T> source,
-      FunctionExpression<Function2<T, T, T>> selector);
+  @Nullable T aggregate(Queryable<T> source,
+      FunctionExpression<Function2<@Nullable T, T, T>> selector);
 
   /**
    * Applies an accumulator function over a
@@ -201,14 +206,14 @@ boolean contains(Queryable<T> source, T element,
    * the type parameter's default value in a singleton collection if
    * the sequence is empty.
    */
-  Queryable<T> defaultIfEmpty(Queryable<T> source);
+  Queryable<@Nullable T> defaultIfEmpty(Queryable<T> source);
 
   /**
    * Returns the elements of the specified sequence or
    * the specified value in a singleton collection if the sequence
    * is empty.
    */
-  Queryable<T> defaultIfEmpty(Queryable<T> source, T value);
+  Queryable<@PolyNull T> defaultIfEmpty(Queryable<T> source, @PolyNull T value);
 
   /**
    * Returns distinct elements from a sequence by using
@@ -282,14 +287,14 @@ Queryable<T> except(Queryable<T> source, Enumerable<T> enumerable,
    * Returns the first element of a sequence, or a
    * default value if the sequence contains no elements.
    */
-  T firstOrDefault(Queryable<T> source);
+  @Nullable T firstOrDefault(Queryable<T> source);
 
   /**
    * Returns the first element of a sequence that
    * satisfies a specified condition or a default value if no such
    * element is found.
    */
-  T firstOrDefault(Queryable<T> source,
+  @Nullable T firstOrDefault(Queryable<T> source,
       FunctionExpression<Predicate1<T>> predicate);
 
   /**
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableRecorder.java b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableRecorder.java
index 17d399d58f70..f480ee31b91c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableRecorder.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/QueryableRecorder.java
@@ -33,12 +33,18 @@
 import org.apache.calcite.linq4j.function.Predicate2;
 import org.apache.calcite.linq4j.tree.FunctionExpression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+import org.checkerframework.framework.qual.Covariant;
+
 import java.lang.reflect.Type;
 import java.math.BigDecimal;
 import java.util.Comparator;
 
 import static org.apache.calcite.linq4j.QueryableDefaults.NonLeafReplayableQueryable;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Implementation of {@link QueryableFactory} that records each event
  * and returns an object that can replay the event when you call its
@@ -47,6 +53,7 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public class QueryableRecorder<T> implements QueryableFactory<T> {
   private static final QueryableRecorder INSTANCE = new QueryableRecorder();
 
@@ -55,8 +62,8 @@ public static <T> QueryableRecorder<T> instance() {
     return INSTANCE;
   }
 
-  public T aggregate(final Queryable<T> source,
-      final FunctionExpression<Function2<T, T, T>> func) {
+  public @Nullable T aggregate(final Queryable<T> source,
+      final FunctionExpression<Function2<@Nullable T, T, T>> func) {
     return new QueryableDefaults.NonLeafReplayableQueryable<T>(source) {
       public void replay(QueryableFactory<T> factory) {
         factory.aggregate(source, func);
@@ -253,7 +260,7 @@ public void replay(QueryableFactory<T> factory) {
     }.<Integer>castSingle(); // CHECKSTYLE: IGNORE 0
   }
 
-  public Queryable<T> defaultIfEmpty(final Queryable<T> source) {
+  public Queryable<@Nullable T> defaultIfEmpty(final Queryable<T> source) {
     return new NonLeafReplayableQueryable<T>(source) {
       public void replay(QueryableFactory<T> factory) {
         factory.defaultIfEmpty(source);
@@ -261,7 +268,8 @@ public void replay(QueryableFactory<T> factory) {
     };
   }
 
-  public Queryable<T> defaultIfEmpty(final Queryable<T> source, final T value) {
+  @SuppressWarnings("return.type.incompatible")
+  public Queryable<@PolyNull T> defaultIfEmpty(final Queryable<T> source, final @PolyNull T value) {
     return new NonLeafReplayableQueryable<T>(source) {
       public void replay(QueryableFactory<T> factory) {
         factory.defaultIfEmpty(source, value);
@@ -347,7 +355,7 @@ public void replay(QueryableFactory<T> factory) {
     }.single(); // CHECKSTYLE: IGNORE 0
   }
 
-  public T firstOrDefault(final Queryable<T> source) {
+  public @Nullable T firstOrDefault(final Queryable<T> source) {
     return new NonLeafReplayableQueryable<T>(source) {
       public void replay(QueryableFactory<T> factory) {
         factory.firstOrDefault(source);
@@ -355,7 +363,7 @@ public void replay(QueryableFactory<T> factory) {
     }.single(); // CHECKSTYLE: IGNORE 0
   }
 
-  public T firstOrDefault(final Queryable<T> source,
+  public @Nullable T firstOrDefault(final Queryable<T> source,
       final FunctionExpression<Predicate1<T>> predicate) {
     return new NonLeafReplayableQueryable<T>(source) {
       public void replay(QueryableFactory<T> factory) {
@@ -688,7 +696,7 @@ public void replay(QueryableFactory<T> factory) {
       }
 
       @Override public Type getElementType() {
-        return selector.body.type;
+        return castNonNull(selector.body).type;
       }
     }.castQueryable(); // CHECKSTYLE: IGNORE 0
   }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/RawEnumerable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/RawEnumerable.java
index 4dc6538f6bc8..35d3ac1db50e 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/RawEnumerable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/RawEnumerable.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j;
 
+import org.checkerframework.framework.qual.Covariant;
+
 /**
  * Exposes the enumerator, which supports a simple iteration over a collection,
  * without the extension methods.
@@ -29,6 +31,7 @@
  * @param <T> Element type
  * @see Enumerable
  */
+@Covariant(0)
 public interface RawEnumerable<T> {
   /**
    * Returns an enumerator that iterates through a collection.
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/RawQueryable.java b/linq4j/src/main/java/org/apache/calcite/linq4j/RawQueryable.java
index 7a2bacfe3867..953f6a07411c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/RawQueryable.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/RawQueryable.java
@@ -18,6 +18,9 @@
 
 import org.apache.calcite.linq4j.tree.Expression;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.framework.qual.Covariant;
+
 import java.lang.reflect.Type;
 
 /**
@@ -29,6 +32,7 @@
  *
  * @param <T> Element type
  */
+@Covariant(0)
 public interface RawQueryable<T> extends Enumerable<T> {
   /**
    * Gets the type of the element(s) that are returned when the expression
@@ -38,8 +42,9 @@ public interface RawQueryable<T> extends Enumerable<T> {
 
   /**
    * Gets the expression tree that is associated with this Queryable.
+   * @return null if the expression is not available
    */
-  Expression getExpression();
+  @Nullable Expression getExpression();
 
   /**
    * Gets the query provider that is associated with this data source.
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/function/Functions.java b/linq4j/src/main/java/org/apache/calcite/linq4j/function/Functions.java
index 539ea5016d31..15c57038793b 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/function/Functions.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/function/Functions.java
@@ -16,6 +16,10 @@
  */
 package org.apache.calcite.linq4j.function;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
+
 import java.io.Serializable;
 import java.lang.reflect.Type;
 import java.math.BigDecimal;
@@ -75,7 +79,8 @@ private Functions() {}
   private static final EqualityComparer<Object[]> ARRAY_COMPARER =
       new ArrayEqualityComparer();
 
-  private static final Function1 CONSTANT_NULL_FUNCTION1 = s -> null;
+  private static final Function1 CONSTANT_NULL_FUNCTION1 =
+      (Function1<Object, @Nullable Object>) s -> null;
 
   private static final Function1 TO_STRING_FUNCTION1 =
       (Function1<Object, String>) Object::toString;
@@ -532,7 +537,7 @@ public boolean equal(T v1, T v2) {
     }
 
     public int hashCode(T t) {
-      return t == null ? 0x789d : selector.apply(t).hashCode();
+      return t == null ? 0x789d : Objects.hashCode(selector.apply(t));
     }
   }
 
@@ -613,7 +618,7 @@ public int compare(Comparable o1, Comparable o2) {
    * @param <R> result type
    * @param <T0> first argument type
    * @param <T1> second argument type */
-  private static final class Ignore<R, T0, T1>
+  private static final class Ignore<@Nullable R, T0, T1>
       implements Function0<R>, Function1<T0, R>, Function2<T0, T1, R> {
     public R apply() {
       return null;
@@ -627,7 +632,13 @@ public R apply(T0 p0, T1 p1) {
       return null;
     }
 
-    static final Ignore INSTANCE = new Ignore();
+    @DefaultQualifier(
+        value = Nullable.class,
+        locations = {
+        TypeUseLocation.LOWER_BOUND,
+        TypeUseLocation.UPPER_BOUND,
+    })
+    static final Ignore INSTANCE = new Ignore<>();
   }
 
   /** List that generates each element using a function.
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/package-info.java b/linq4j/src/main/java/org/apache/calcite/linq4j/package-info.java
index cf57933faeaa..726df9815271 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/package-info.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/package-info.java
@@ -18,4 +18,11 @@
 /**
  * Language-integrated query for Java (linq4j) main package.
  */
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.FIELD)
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.PARAMETER)
+@DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.RETURN)
 package org.apache.calcite.linq4j;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.framework.qual.DefaultQualifier;
+import org.checkerframework.framework.qual.TypeUseLocation;
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/AbstractNode.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/AbstractNode.java
index 014db9045ccb..0a2622564d9d 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/AbstractNode.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/AbstractNode.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Objects;
 
@@ -70,12 +72,12 @@ public Node accept(Shuttle shuttle) {
         "visit not supported: " + getClass() + ":" + nodeType);
   }
 
-  public Object evaluate(Evaluator evaluator) {
+  public @Nullable Object evaluate(Evaluator evaluator) {
     throw new RuntimeException(
         "evaluation not supported: " + getClass() + ":" + nodeType);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ArrayLengthRecordField.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ArrayLengthRecordField.java
index d6a6e2061e83..f04fb444d066 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ArrayLengthRecordField.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ArrayLengthRecordField.java
@@ -16,10 +16,14 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Array;
 import java.lang.reflect.Type;
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Length field of a RecordType.
  */
@@ -50,15 +54,15 @@ public int getModifiers() {
     return 0;
   }
 
-  public Object get(Object o) throws IllegalAccessException {
-    return Array.getLength(o);
+  @Override public Object get(@Nullable Object o) throws IllegalAccessException {
+    return Array.getLength(castNonNull(o));
   }
 
-  public Type getDeclaringClass() {
+  @Override public Type getDeclaringClass() {
     return clazz;
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BinaryExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BinaryExpression.java
index a18025c2c9f0..83278f5c6342 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BinaryExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BinaryExpression.java
@@ -16,16 +16,20 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Represents an expression that has a binary operator.
  */
 public class BinaryExpression extends Expression {
   public final Expression expression0;
   public final Expression expression1;
-  private final Primitive primitive;
+  private final @Nullable Primitive primitive;
 
   BinaryExpression(ExpressionType nodeType, Type type, Expression expression0,
       Expression expression1) {
@@ -51,9 +55,12 @@ public <R> R accept(Visitor<R> visitor) {
   public Object evaluate(Evaluator evaluator) {
     switch (nodeType) {
     case AndAlso:
-      return (Boolean) expression0.evaluate(evaluator)
-             && (Boolean) expression1.evaluate(evaluator);
+      return evaluateBoolean(evaluator, expression0)
+             && evaluateBoolean(evaluator, expression1);
     case Add:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) + evaluateInt(expression1, evaluator);
@@ -71,6 +78,9 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case Divide:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) / evaluateInt(expression1, evaluator);
@@ -88,9 +98,11 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case Equal:
-      return expression0.evaluate(evaluator)
-          .equals(expression1.evaluate(evaluator));
+      return Objects.equals(expression0.evaluate(evaluator), expression1.evaluate(evaluator));
     case GreaterThan:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) > evaluateInt(expression1, evaluator);
@@ -108,6 +120,9 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case GreaterThanOrEqual:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) >= evaluateInt(expression1, evaluator);
@@ -125,6 +140,9 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case LessThan:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) < evaluateInt(expression1, evaluator);
@@ -142,6 +160,9 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case LessThanOrEqual:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) <= evaluateInt(expression1, evaluator);
@@ -159,6 +180,9 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case Multiply:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) * evaluateInt(expression1, evaluator);
@@ -176,12 +200,14 @@ public Object evaluate(Evaluator evaluator) {
         throw cannotEvaluate();
       }
     case NotEqual:
-      return !expression0.evaluate(evaluator)
-          .equals(expression1.evaluate(evaluator));
+      return !Objects.equals(expression0.evaluate(evaluator), expression1.evaluate(evaluator));
     case OrElse:
-      return (Boolean) expression0.evaluate(evaluator)
-             || (Boolean) expression1.evaluate(evaluator);
+      return evaluateBoolean(evaluator, expression0)
+             || evaluateBoolean(evaluator, expression1);
     case Subtract:
+      if (primitive == null) {
+        throw cannotEvaluate();
+      }
       switch (primitive) {
       case INT:
         return evaluateInt(expression0, evaluator) - evaluateInt(expression1, evaluator);
@@ -217,31 +243,39 @@ private RuntimeException cannotEvaluate() {
       + nodeType + ", primitive=" + primitive);
   }
 
+  private boolean evaluateBoolean(Evaluator evaluator, Expression expression) {
+    return (Boolean) castNonNull(expression.evaluate(evaluator));
+  }
+
+  private Number evaluateNumber(Expression expression, Evaluator evaluator) {
+    return (Number) castNonNull(expression.evaluate(evaluator));
+  }
+
   private int evaluateInt(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).intValue();
+    return evaluateNumber(expression, evaluator).intValue();
   }
 
   private short evaluateShort(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).shortValue();
+    return evaluateNumber(expression, evaluator).shortValue();
   }
 
   private long evaluateLong(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).longValue();
+    return evaluateNumber(expression, evaluator).longValue();
   }
 
   private byte evaluateByte(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).byteValue();
+    return evaluateNumber(expression, evaluator).byteValue();
   }
 
   private float evaluateFloat(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).floatValue();
+    return evaluateNumber(expression, evaluator).floatValue();
   }
 
   private double evaluateDouble(Expression expression, Evaluator evaluator) {
-    return ((Number) expression.evaluate(evaluator)).doubleValue();
+    return evaluateNumber(expression, evaluator).doubleValue();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockBuilder.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockBuilder.java
index f09b03ff4ef2..923a6bc18e56 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockBuilder.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockBuilder.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
@@ -26,6 +28,8 @@
 import java.util.Map;
 import java.util.Set;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Builder for {@link BlockStatement}.
  *
@@ -41,7 +45,7 @@ public class BlockBuilder {
       new HashMap<>();
 
   private final boolean optimizing;
-  private final BlockBuilder parent;
+  private final @Nullable BlockBuilder parent;
 
   private static final Shuttle OPTIMIZE_SHUTTLE = new OptimizeShuttle();
 
@@ -66,7 +70,7 @@ public BlockBuilder(boolean optimizing) {
    *
    * @param optimizing Whether to eliminate common sub-expressions
    */
-  public BlockBuilder(boolean optimizing, BlockBuilder parent) {
+  public BlockBuilder(boolean optimizing, @Nullable BlockBuilder parent) {
     this.optimizing = optimizing;
     this.parent = parent;
   }
@@ -85,7 +89,7 @@ public void clear() {
    * (possibly a variable) that represents the result of the newly added
    * block.
    */
-  public Expression append(String name, BlockStatement block) {
+  public @Nullable Expression append(String name, BlockStatement block) {
     return append(name, block, true);
   }
 
@@ -99,7 +103,7 @@ public Expression append(String name, BlockStatement block) {
    * a variable. Do not do this if the expression has
    * side-effects or a time-dependent value.
    */
-  public Expression append(String name, BlockStatement block,
+  public @Nullable Expression append(String name, BlockStatement block,
       boolean optimize) {
     if (statements.size() > 0) {
       Statement lastStatement = statements.get(statements.size() - 1);
@@ -153,7 +157,7 @@ public Expression append(String name, BlockStatement block,
           result = ((DeclarationStatement) statement).parameter;
         } else if (statement instanceof GotoStatement) {
           statements.remove(statements.size() - 1);
-          result = append_(name, ((GotoStatement) statement).expression,
+          result = append_(name, castNonNull(((GotoStatement) statement).expression),
               optimize);
           if (isSimpleExpression(result)) {
             // already simple; no need to declare a variable or
@@ -184,7 +188,7 @@ public Expression append(String name, Expression expression) {
   /**
    * Appends an expression to a list of statements, if it is not null.
    */
-  public Expression appendIfNotNull(String name, Expression expression) {
+  public @Nullable Expression appendIfNotNull(String name, @Nullable Expression expression) {
     if (expression == null) {
       return null;
     }
@@ -233,7 +237,7 @@ private Expression append_(String name, Expression expression,
    * @param expr expression to test
    * @return true when given expression is safe to always inline
    */
-  protected boolean isSimpleExpression(Expression expr) {
+  protected boolean isSimpleExpression(@Nullable Expression expr) {
     if (expr instanceof ParameterExpression
         || expr instanceof ConstantExpression) {
       return true;
@@ -285,7 +289,7 @@ private Expression normalizeDeclaration(DeclarationStatement decl) {
    * @param expr expression to test
    * @return existing ParameterExpression or null
    */
-  public DeclarationStatement getComputedExpression(Expression expr) {
+  public @Nullable DeclarationStatement getComputedExpression(Expression expr) {
     if (parent != null) {
       DeclarationStatement decl = parent.getComputedExpression(expr);
       if (decl != null) {
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockStatement.java
index 2aaa40bbf9da..a5ee64eedb19 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/BlockStatement.java
@@ -16,6 +16,9 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.initialization.qual.UnderInitialization;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.HashSet;
 import java.util.List;
@@ -38,7 +41,10 @@ public class BlockStatement extends Statement {
     assert distinctVariables(true);
   }
 
-  private boolean distinctVariables(boolean fail) {
+  private boolean distinctVariables(
+      @UnderInitialization(BlockStatement.class) BlockStatement this,
+      boolean fail
+  ) {
     Set<String> names = new HashSet<>();
     for (Statement statement : statements) {
       if (statement instanceof DeclarationStatement) {
@@ -75,7 +81,7 @@ public <R> R accept(Visitor<R> visitor) {
     writer.end("}\n");
   }
 
-  @Override public Object evaluate(Evaluator evaluator) {
+  @Override public @Nullable Object evaluate(Evaluator evaluator) {
     Object o = null;
     for (Statement statement : statements) {
       o = statement.evaluate(evaluator);
@@ -83,7 +89,7 @@ public <R> R accept(Visitor<R> visitor) {
     return o;
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Blocks.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Blocks.java
index 05e3201e661e..b34a1bbad0a4 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Blocks.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Blocks.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * <p>Helper methods concerning {@link BlockStatement}s.</p>
  *
@@ -71,7 +73,7 @@ public static Expression simple(BlockStatement block) {
     if (block.statements.size() == 1) {
       Statement statement = block.statements.get(0);
       if (statement instanceof GotoStatement) {
-        return ((GotoStatement) statement).expression;
+        return castNonNull(((GotoStatement) statement).expression);
       }
     }
     throw new AssertionError("not a simple block: " + block);
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/CatchBlock.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/CatchBlock.java
index 732d63b6b88b..21aff57d96f7 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/CatchBlock.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/CatchBlock.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -31,7 +33,7 @@ public CatchBlock(ParameterExpression parameter,
     this.body = body;
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclaration.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclaration.java
index f4e0c9a266c7..8baaabad88da 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclaration.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclaration.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.List;
@@ -70,7 +72,7 @@ public <R> R accept(Visitor<R> visitor) {
     return visitor.visit(this);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclarationFinder.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclarationFinder.java
index 6373931df3da..fe1a4554d3d6 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclarationFinder.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ClassDeclarationFinder.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.function.Function1;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
 import java.util.ArrayList;
@@ -30,7 +32,7 @@
  * created for optimizing a new expression tree.
  */
 public class ClassDeclarationFinder extends Shuttle {
-  protected final ClassDeclarationFinder parent;
+  protected final @Nullable ClassDeclarationFinder parent;
 
   /**
    * The list of new final static fields to be added to the current class.
@@ -152,7 +154,7 @@ protected ClassDeclarationFinder(ClassDeclarationFinder parent) {
   }
 
   @Override public Expression visit(NewExpression newExpression,
-      List<Expression> arguments, List<MemberDeclaration> memberDeclarations) {
+      List<Expression> arguments, @Nullable List<MemberDeclaration> memberDeclarations) {
     if (parent == null) {
       // Unable to optimize since no wrapper class exists to put fields to.
       arguments = newExpression.arguments;
@@ -254,7 +256,7 @@ protected boolean isConstant(Iterable<? extends Expression> list) {
    * @param expression input expression
    * @return always returns null
    */
-  protected ParameterExpression findDeclaredExpression(Expression expression) {
+  protected @Nullable ParameterExpression findDeclaredExpression(Expression expression) {
     return null;
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalExpression.java
index 55c00cefba48..92875e737934 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Objects;
@@ -59,7 +61,7 @@ public <R> R accept(Visitor<R> visitor) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalStatement.java
index 9d15a458bed9..fe3f7203324a 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConditionalStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.List;
 import java.util.Objects;
 
@@ -72,7 +74,7 @@ private static <E> E last(List<E> collection) {
     return collection.get(collection.size() - 1);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java
index 38b75879bc5c..e05f7f887c43 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
 import java.lang.reflect.Type;
@@ -28,22 +30,21 @@
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Represents an expression that has a constant value.
  */
 public class ConstantExpression extends Expression {
-  public final Object value;
+  public final @Nullable Object value;
 
-  public ConstantExpression(Type type, Object value) {
+  public ConstantExpression(Type type, @Nullable Object value) {
     super(ExpressionType.Constant, type);
     this.value = value;
     if (value != null) {
       if (type instanceof Class) {
         Class clazz = (Class) type;
-        Primitive primitive = Primitive.of(clazz);
-        if (primitive != null) {
-          clazz = primitive.boxClass;
-        }
+        clazz = Primitive.box(clazz);
         if (!clazz.isInstance(value)
             && !((clazz == Float.class || clazz == Double.class)
                 && value instanceof BigDecimal)) {
@@ -54,7 +55,7 @@ public ConstantExpression(Type type, Object value) {
     }
   }
 
-  public Object evaluate(Evaluator evaluator) {
+  public @Nullable Object evaluate(Evaluator evaluator) {
     return value;
   }
 
@@ -77,15 +78,13 @@ public <R> R accept(Visitor<R> visitor) {
   }
 
   private static ExpressionWriter write(ExpressionWriter writer,
-      final Object value, Type type) {
+      final Object value, @Nullable Type type) {
     if (value == null) {
       return writer.append("null");
     }
     if (type == null) {
       type = value.getClass();
-      if (Primitive.isBox(type)) {
-        type = Primitive.ofBox(type).primitiveClass;
-      }
+      type = Primitive.unbox(type);
     }
     if (value instanceof String) {
       escapeString(writer.getBuf(), (String) value);
@@ -171,7 +170,7 @@ private static ExpressionWriter write(ExpressionWriter writer,
       return writer.append(recordType.getName()).append(".class");
     }
     if (value.getClass().isArray()) {
-      writer.append("new ").append(value.getClass().getComponentType());
+      writer.append("new ").append(castNonNull(value.getClass().getComponentType()));
       list(writer, Primitive.asList(value), "[] {\n", ",\n", "}");
       return writer;
     }
@@ -195,7 +194,8 @@ private static ExpressionWriter write(ExpressionWriter writer,
       writer.append("new ").append(value.getClass());
       list(writer,
           Arrays.stream(value.getClass().getFields())
-              .map(field -> {
+              // <@Nullable Object> is needed for CheckerFramework
+              .<@Nullable Object>map(field -> {
                 try {
                   return field.get(value);
                 } catch (IllegalAccessException e) {
@@ -275,7 +275,7 @@ private static ExpressionWriter set(ExpressionWriter writer, Set set,
     return writer.append(end);
   }
 
-  private static Constructor matchingConstructor(Object value) {
+  private static @Nullable Constructor matchingConstructor(Object value) {
     final Field[] fields = value.getClass().getFields();
     for (Constructor<?> constructor : value.getClass().getConstructors()) {
       if (argsMatchFields(fields, constructor.getParameterTypes())) {
@@ -328,7 +328,7 @@ private static void escapeString(StringBuilder buf, String s) {
     buf.append('"');
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     // REVIEW: Should constants with the same value and different type
     // (e.g. 3L and 3) be considered equal.
     if (this == o) {
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantUntypedNull.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantUntypedNull.java
index 48ab4ee8452b..204f82d8e901 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantUntypedNull.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstantUntypedNull.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * Represents a constant null of unknown type
  * Java allows type inference for such nulls, thus "null" cannot always be
@@ -36,7 +38,7 @@ private ConstantUntypedNull() {
     writer.append("null");
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     return o == INSTANCE;
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstructorDeclaration.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstructorDeclaration.java
index ebc244531ec2..47677dd6fe61 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstructorDeclaration.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ConstructorDeclaration.java
@@ -16,12 +16,14 @@
  */
 package org.apache.calcite.linq4j.tree;
 
-import com.google.common.collect.Lists;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Objects;
+import java.util.stream.Collectors;
 
 /**
  * Declaration of a constructor.
@@ -29,13 +31,13 @@
 public class ConstructorDeclaration extends MemberDeclaration {
   public final int modifier;
   public final Type resultType;
-  public final List<ParameterExpression> parameters;
+  public final List<@NonNull ParameterExpression> parameters;
   public final BlockStatement body;
   /** Cached hash code for the expression. */
   private int hash;
 
   public ConstructorDeclaration(int modifier, Type declaredAgainst,
-      List<ParameterExpression> parameters, BlockStatement body) {
+      List<@NonNull ParameterExpression> parameters, BlockStatement body) {
     assert parameters != null : "parameters should not be null";
     assert body != null : "body should not be null";
     assert declaredAgainst != null : "declaredAgainst should not be null";
@@ -65,18 +67,18 @@ public void accept(ExpressionWriter writer) {
     writer
         .append(resultType)
         .list("(", ", ", ")",
-            Lists.transform(parameters, parameter -> {
+            parameters.stream().map(parameter -> {
               final String modifiers1 =
                   Modifier.toString(parameter.modifier);
               return modifiers1 + (modifiers1.isEmpty() ? "" : " ")
                   + Types.className(parameter.getType()) + " "
                   + parameter.name;
-            }))
+            }).collect(Collectors.toList()))
         .append(' ').append(body);
     writer.newlineAndIndent();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeclarationStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeclarationStatement.java
index 31a529dcecce..d4fe2ccdc1ff 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeclarationStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeclarationStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Modifier;
 import java.util.Objects;
 
@@ -25,10 +27,10 @@
 public class DeclarationStatement extends Statement {
   public final int modifiers;
   public final ParameterExpression parameter;
-  public final Expression initializer;
+  public final @Nullable Expression initializer;
 
   public DeclarationStatement(int modifiers, ParameterExpression parameter,
-      Expression initializer) {
+      @Nullable Expression initializer) {
     super(ExpressionType.Declaration, Void.TYPE);
     assert parameter != null : "parameter should not be null";
     this.modifiers = modifiers;
@@ -78,7 +80,7 @@ public void accept2(ExpressionWriter writer, boolean withType) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeterministicCodeOptimizer.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeterministicCodeOptimizer.java
index e9168071c704..e20bd255dc71 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeterministicCodeOptimizer.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/DeterministicCodeOptimizer.java
@@ -21,6 +21,8 @@
 
 import com.google.common.collect.ImmutableSet;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
@@ -167,7 +169,7 @@ && isMethodDeterministic(methodCallExpression.method)) {
   }
 
   @Override public Expression visit(MethodCallExpression methodCallExpression,
-      Expression targetExpression, List<Expression> expressions) {
+      @Nullable Expression targetExpression, List<Expression> expressions) {
     Expression result =
         super.visit(methodCallExpression, targetExpression, expressions);
 
@@ -176,7 +178,7 @@ && isMethodDeterministic(methodCallExpression.method)) {
   }
 
   @Override public Expression visit(MemberExpression memberExpression,
-      Expression expression) {
+      @Nullable Expression expression) {
     Expression result = super.visit(memberExpression, expression);
 
     if (isConstant(expression)
@@ -187,7 +189,7 @@ && isMethodDeterministic(methodCallExpression.method)) {
   }
 
   @Override public MemberDeclaration visit(FieldDeclaration fieldDeclaration,
-      Expression initializer) {
+      @Nullable Expression initializer) {
     if (Modifier.isStatic(fieldDeclaration.modifier)) {
       // Avoid optimization of static fields, since we'll have to track order
       // of static declarations.
@@ -224,7 +226,7 @@ && isMethodDeterministic(methodCallExpression.method)) {
    * @param expression input expression
    * @return parameter of the already existing declaration, or null
    */
-  protected ParameterExpression findDeclaredExpression(Expression expression) {
+  protected @Nullable ParameterExpression findDeclaredExpression(Expression expression) {
     if (!dedup.isEmpty()) {
       ParameterExpression pe = dedup.get(expression);
       if (pe != null) {
@@ -294,7 +296,7 @@ protected String inventFieldName(Expression expression) {
    * @param expression expression to test
    * @return true when the expression is known to be constant
    */
-  @Override protected boolean isConstant(Expression expression) {
+  @Override protected boolean isConstant(@Nullable Expression expression) {
     return expression == null
         || expression instanceof ConstantExpression
         || !constants.isEmpty() && constants.containsKey(expression)
@@ -330,7 +332,7 @@ protected boolean isConstructorDeterministic(NewExpression newExpression) {
         && constructor.isAnnotationPresent(Deterministic.class);
   }
 
-  private <C> Constructor<C> getConstructor(Class<C> klass) {
+  private <C> @Nullable Constructor<C> getConstructor(Class<C> klass) {
     try {
       return klass.getConstructor();
     } catch (NoSuchMethodException e) {
@@ -347,7 +349,7 @@ private <C> Constructor<C> getConstructor(Class<C> klass) {
    */
   protected boolean allMethodsDeterministic(Class klass) {
     return DETERMINISTIC_CLASSES.contains(klass)
-        || klass.getCanonicalName().equals("org.apache.calcite.avatica.util.DateTimeUtils")
+        || "org.apache.calcite.avatica.util.DateTimeUtils".equals(klass.getCanonicalName())
         || klass.isAnnotationPresent(Deterministic.class);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Evaluator.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Evaluator.java
index a7fc34d5a042..f6cc41b7dd2c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Evaluator.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Evaluator.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.ArrayList;
 import java.util.List;
 
@@ -24,12 +26,12 @@
  */
 class Evaluator {
   final List<ParameterExpression> parameters = new ArrayList<>();
-  final List<Object> values = new ArrayList<>();
+  final List<@Nullable Object> values = new ArrayList<>();
 
   Evaluator() {
   }
 
-  void push(ParameterExpression parameter, Object value) {
+  void push(ParameterExpression parameter, @Nullable Object value) {
     parameters.add(parameter);
     values.add(value);
   }
@@ -42,7 +44,7 @@ void pop(int n) {
     }
   }
 
-  Object peek(ParameterExpression param) {
+  @Nullable Object peek(ParameterExpression param) {
     for (int i = parameters.size() - 1; i >= 0; i--) {
       if (parameters.get(i) == param) {
         return values.get(i);
@@ -51,7 +53,7 @@ Object peek(ParameterExpression param) {
     throw new RuntimeException("parameter " + param + " not on stack");
   }
 
-  Object evaluate(Node expression) {
+  @Nullable Object evaluate(Node expression) {
     return ((AbstractNode) expression).evaluate(this);
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionType.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionType.java
index 636620af0318..5f20ae169d37 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionType.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionType.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 /**
  * <p>Analogous to LINQ's System.Linq.Expressions.ExpressionType.</p>
  */
@@ -596,8 +598,8 @@ public enum ExpressionType {
    */
   While;
 
-  final String op;
-  final String op2;
+  final @Nullable String op;
+  final @Nullable String op2;
   final boolean postfix;
   final int lprec;
   final int rprec;
@@ -607,16 +609,16 @@ public enum ExpressionType {
     this(null, false, 0, false);
   }
 
-  ExpressionType(String op, boolean postfix, int prec, boolean right) {
+  ExpressionType(@Nullable String op, boolean postfix, int prec, boolean right) {
     this(op, null, postfix, prec, right);
   }
 
-  ExpressionType(String op, String op2, boolean postfix, int prec,
+  ExpressionType(@Nullable String op, @Nullable String op2, boolean postfix, int prec,
       boolean right) {
     this(op, op2, postfix, prec, right, false);
   }
 
-  ExpressionType(String op, String op2, boolean postfix, int prec,
+  ExpressionType(@Nullable String op, @Nullable String op2, boolean postfix, int prec,
       boolean right, boolean modifiesLvalue) {
     this.op = op;
     this.op2 = op2;
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionWriter.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionWriter.java
index cb7c9113d90e..8bc640bac6e4 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionWriter.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ExpressionWriter.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.avatica.util.Spacer;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Iterator;
 
@@ -130,13 +132,13 @@ public ExpressionWriter append(AbstractNode o) {
     return this;
   }
 
-  public ExpressionWriter append(Object o) {
+  public ExpressionWriter append(@Nullable Object o) {
     checkIndent();
     buf.append(o);
     return this;
   }
 
-  public ExpressionWriter append(String s) {
+  public ExpressionWriter append(@Nullable String s) {
     checkIndent();
     buf.append(s);
     return this;
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Expressions.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Expressions.java
index 8260055542d9..9f7bac24db55 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Expressions.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Expressions.java
@@ -26,6 +26,10 @@
 
 import com.google.common.collect.ImmutableList;
 
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.checker.nullness.qual.PolyNull;
+
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
 import java.lang.reflect.Member;
@@ -41,6 +45,8 @@
 import java.util.Objects;
 import java.util.UUID;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Utility methods for expressions, including a lot of factory methods.
  */
@@ -283,7 +289,7 @@ public static BlockStatement block(Statement... statements) {
    * Creates a BlockExpression that contains the given expressions,
    * has no variables and has specific result type.
    */
-  public static BlockStatement block(Type type,
+  public static BlockStatement block(@Nullable Type type,
       Iterable<? extends Statement> expressions) {
     List<Statement> list = toList(expressions);
     if (type == null) {
@@ -343,7 +349,7 @@ public static GotoStatement break_(LabelTarget labelTarget,
    * static method that has arguments.
    */
   public static MethodCallExpression call(Method method,
-      Iterable<? extends Expression> arguments) {
+      Iterable<? extends @NonNull Expression> arguments) {
     return new MethodCallExpression(method, null, toList(arguments));
   }
 
@@ -360,8 +366,8 @@ public static MethodCallExpression call(Method method,
    * Creates a MethodCallExpression that represents a call to a
    * method that takes arguments.
    */
-  public static MethodCallExpression call(Expression expression, Method method,
-      Iterable<? extends Expression> arguments) {
+  public static MethodCallExpression call(@Nullable Expression expression, Method method,
+      Iterable<? extends @NonNull Expression> arguments) {
     return new MethodCallExpression(method, expression, toList(arguments));
   }
 
@@ -501,14 +507,6 @@ public static Expression condition(Expression test, Expression ifTrue,
     return makeTernary(ExpressionType.Conditional, test, ifTrue, ifFalse);
   }
 
-  private static Type box(Type type) {
-    Primitive primitive = Primitive.of(type);
-    if (primitive != null) {
-      return primitive.boxClass;
-    }
-    return type;
-  }
-
   /** Returns whether an expression always evaluates to null. */
   public static boolean isConstantNull(Expression e) {
     return e instanceof ConstantExpression
@@ -541,19 +539,11 @@ public static ConditionalExpression condition(Expression test,
    * classes that have a constructor with a parameter for each field, and
    * arrays.</p>
    */
-  public static ConstantExpression constant(Object value) {
-    Class type;
+  public static ConstantExpression constant(@Nullable Object value) {
     if (value == null) {
       return ConstantUntypedNull.INSTANCE;
-    } else {
-      final Class clazz = value.getClass();
-      final Primitive primitive = Primitive.ofBox(clazz);
-      if (primitive != null) {
-        type = primitive.primitiveClass;
-      } else {
-        type = clazz;
-      }
     }
+    Class<?> type = Primitive.unbox(value.getClass());
     return new ConstantExpression(type, value);
   }
 
@@ -561,13 +551,13 @@ public static ConstantExpression constant(Object value) {
    * Creates a ConstantExpression that has the Value and Type
    * properties set to the specified values.
    */
-  public static ConstantExpression constant(Object value, Type type) {
+  public static ConstantExpression constant(@Nullable Object value, Type type) {
     if (value != null && type instanceof Class) {
       // Fix up value so that it matches type.
-      Class clazz = (Class) type;
+      Class<?> clazz = (Class<?>) type;
       Primitive primitive = Primitive.ofBoxOr(clazz);
       if (primitive != null) {
-        clazz = primitive.boxClass;
+        clazz = castNonNull(primitive.boxClass);
       }
       if ((clazz == Float.class || clazz == Double.class)
           && value instanceof BigDecimal) {
@@ -839,7 +829,7 @@ public static MemberExpression field(Expression expression, Field field) {
   /**
    * Creates a MemberExpression that represents accessing a field.
    */
-  public static MemberExpression field(Expression expression,
+  public static MemberExpression field(@Nullable Expression expression,
       PseudoField field) {
     return makeMemberAccess(expression, field);
   }
@@ -857,7 +847,7 @@ public static MemberExpression field(Expression expression,
   /**
    * Creates a MemberExpression that represents accessing a field.
    */
-  public static MemberExpression field(Expression expression, Type type,
+  public static MemberExpression field(@Nullable Expression expression, Type type,
       String fieldName) {
     PseudoField field = Types.getField(fieldName, type);
     return makeMemberAccess(expression, field);
@@ -1384,7 +1374,7 @@ public static ListInitExpression listInit(NewExpression newExpression,
    */
   public static ForStatement for_(
       Iterable<? extends DeclarationStatement> declarations,
-      Expression condition, Expression post, Statement body) {
+      @Nullable Expression condition, @Nullable Expression post, Statement body) {
     return new ForStatement(toList(declarations), condition, post, body);
   }
 
@@ -1393,7 +1383,7 @@ public static ForStatement for_(
    */
   public static ForStatement for_(
       DeclarationStatement declaration,
-      Expression condition, Expression post, Statement body) {
+      @Nullable Expression condition, @Nullable Expression post, Statement body) {
     return new ForStatement(Collections.singletonList(declaration), condition,
         post, body);
   }
@@ -1434,7 +1424,7 @@ public static BinaryExpression makeBinary(ExpressionType binaryType,
   /** Returns an expression to box the value of a primitive expression.
    * E.g. {@code box(e, Primitive.INT)} returns {@code Integer.valueOf(e)}. */
   public static Expression box(Expression expression, Primitive primitive) {
-    return call(primitive.boxClass, "valueOf", expression);
+    return call(castNonNull(primitive.boxClass), "valueOf", expression);
   }
 
   /** Converts e.g. "anInteger" to "Integer.valueOf(anInteger)". */
@@ -1450,7 +1440,7 @@ public static Expression box(Expression expression) {
    * E.g. {@code unbox(e, Primitive.INT)} returns {@code e.intValue()}.
    * It is assumed that e is of the right box type (or {@link Number})."Value */
   public static Expression unbox(Expression expression, Primitive primitive) {
-    return call(expression, primitive.primitiveName + "Value");
+    return call(expression, castNonNull(primitive.primitiveName) + "Value");
   }
 
   /** Converts e.g. "anInteger" to "anInteger.intValue()". */
@@ -1526,12 +1516,12 @@ public static TernaryExpression makeTernary(ExpressionType ternaryType,
     switch (ternaryType) {
     case Conditional:
       if (e1 instanceof ConstantUntypedNull) {
-        type = box(e2.getType());
+        type = Primitive.box(e2.getType());
         if (e1.getType() != type) {
           e1 = constant(null, type);
         }
       } else if (e2 instanceof ConstantUntypedNull) {
-        type = box(e1.getType());
+        type = Primitive.box(e1.getType());
         if (e2.getType() != type) {
           e2 = constant(null, type);
         }
@@ -1585,7 +1575,7 @@ public static GotoStatement makeGoto(GotoExpressionKind kind,
   /**
    * Creates a MemberExpression that represents accessing a field.
    */
-  public static MemberExpression makeMemberAccess(Expression expression,
+  public static MemberExpression makeMemberAccess(@Nullable Expression expression,
       PseudoField member) {
     return new MemberExpression(expression, member);
   }
@@ -1631,7 +1621,7 @@ public static UnaryExpression makeUnary(ExpressionType expressionType,
    * method, by calling the appropriate factory method.
    */
   public static UnaryExpression makeUnary(ExpressionType expressionType,
-      Expression expression, Type type, Method method) {
+      Expression expression, Type type, @Nullable Method method) {
     assert type != null;
     return new UnaryExpression(expressionType, type, expression);
   }
@@ -1716,7 +1706,7 @@ public static ConstructorDeclaration constructorDecl(int modifier,
    * Declares a field with an initializer.
    */
   public static FieldDeclaration fieldDecl(int modifier,
-      ParameterExpression parameter, Expression initializer) {
+      ParameterExpression parameter, @Nullable Expression initializer) {
     return new FieldDeclaration(modifier, parameter, initializer);
   }
 
@@ -1899,7 +1889,8 @@ public static UnaryExpression negate(Expression expression) {
    * negation operation.
    */
   public static UnaryExpression negate(Expression expression, Method method) {
-    return makeUnary(ExpressionType.Negate, expression, null, method);
+    // TODO: use method
+    return negate(expression);
   }
 
   /**
@@ -1917,7 +1908,8 @@ public static UnaryExpression negateChecked(Expression expression) {
    */
   public static UnaryExpression negateChecked(Expression expression,
       Method method) {
-    return makeUnary(ExpressionType.NegateChecked, expression, null, method);
+    throw new UnsupportedOperationException("not implemented");
+    //return makeUnary(ExpressionType.NegateChecked, expression, null, method);
   }
 
   /**
@@ -1966,7 +1958,7 @@ public static NewExpression new_(Type type, Expression... arguments) {
    */
   public static NewExpression new_(Type type,
       Iterable<? extends Expression> arguments,
-      Iterable<? extends MemberDeclaration> memberDeclarations) {
+      @Nullable Iterable<? extends MemberDeclaration> memberDeclarations) {
     return new NewExpression(type, toList(arguments),
         toList(memberDeclarations));
   }
@@ -2038,7 +2030,7 @@ public static NewExpression new_(Constructor constructor,
    * that has a specified rank.
    */
   public static NewArrayExpression newArrayBounds(Type type, int dimension,
-      Expression bound) {
+      @Nullable Expression bound) {
     return new NewArrayExpression(type, dimension, bound, null);
   }
 
@@ -2103,7 +2095,8 @@ public static UnaryExpression not(Expression expression) {
    * operation. The implementing method can be specified.
    */
   public static UnaryExpression not(Expression expression, Method method) {
-    return makeUnary(ExpressionType.Not, expression, null, method);
+    // TODO: use method
+    return not(expression);
   }
 
   /**
@@ -2509,13 +2502,13 @@ public static GotoStatement return_(LabelTarget labelTarget) {
    * Creates a GotoExpression representing a return statement. The
    * value passed to the label upon jumping can be specified.
    */
-  public static GotoStatement return_(LabelTarget labelTarget,
-      Expression expression) {
+  public static GotoStatement return_(@Nullable LabelTarget labelTarget,
+      @Nullable Expression expression) {
     return makeGoto(GotoExpressionKind.Return, labelTarget, expression);
   }
 
   public static GotoStatement makeGoto(GotoExpressionKind kind,
-      LabelTarget labelTarget, Expression expression) {
+      @Nullable LabelTarget labelTarget, @Nullable Expression expression) {
     return new GotoStatement(kind, labelTarget, expression);
   }
 
@@ -2698,6 +2691,7 @@ public static BinaryExpression subtractChecked(Expression left,
    * Creates a SwitchExpression that represents a switch statement
    * without a default case.
    */
+  @SuppressWarnings("nullness")
   public static SwitchStatement switch_(Expression switchValue,
       SwitchCase... cases) {
     return switch_(switchValue, null, null, toList(cases));
@@ -2707,6 +2701,7 @@ public static SwitchStatement switch_(Expression switchValue,
    * Creates a SwitchExpression that represents a switch statement
    * that has a default case.
    */
+  @SuppressWarnings("nullness")
   public static SwitchStatement switch_(Expression switchValue,
       Expression defaultBody, SwitchCase... cases) {
     return switch_(switchValue, defaultBody, null, toList(cases));
@@ -2937,7 +2932,7 @@ public static WhileStatement while_(Expression condition, Statement body) {
    * Creates a statement that declares a variable.
    */
   public static DeclarationStatement declare(int modifiers,
-      ParameterExpression parameter, Expression initializer) {
+      ParameterExpression parameter, @Nullable Expression initializer) {
     return new DeclarationStatement(modifiers, parameter, initializer);
   }
 
@@ -2959,7 +2954,7 @@ public static DeclarationStatement declare(int modifiers, String name,
   /**
    * Creates a statement that executes an expression.
    */
-  public static Statement statement(Expression expression) {
+  public static Statement statement(@Nullable Expression expression) {
     return new GotoStatement(GotoExpressionKind.Sequence, null, expression);
   }
 
@@ -3055,7 +3050,7 @@ public static <T> FluentList<T> list(Iterable<T> ts) {
   /**
    * Evaluates an expression and returns the result.
    */
-  public static Object evaluate(Node node) {
+  public static @Nullable Object evaluate(Node node) {
     Objects.requireNonNull(node);
     final Evaluator evaluator = new Evaluator();
     return ((AbstractNode) node).evaluate(evaluator);
@@ -3083,7 +3078,7 @@ private static Class deduceType(List<ParameterExpression> parameterList,
     }
   }
 
-  private static <T> List<T> toList(Iterable<? extends T> iterable) {
+  private static <T> @PolyNull List<T> toList(@PolyNull Iterable<? extends T> iterable) {
     if (iterable == null) {
       return null;
     }
@@ -3112,20 +3107,16 @@ private static <T> Collection<T> toCollection(Iterable<T> iterable) {
     return toList(iterable);
   }
 
-  private static <T> T[] toArray(Iterable<T> iterable, T[] a) {
-    return toCollection(iterable).toArray(a);
-  }
-
-  static <T extends Expression> Expression accept(T node, Shuttle shuttle) {
+  static <T extends @PolyNull Expression> @PolyNull Expression accept(T node, Shuttle shuttle) {
     if (node == null) {
-      return null;
+      return node;
     }
     return node.accept(shuttle);
   }
 
-  static <T extends Statement> Statement accept(T node, Shuttle shuttle) {
+  static <T extends @PolyNull Statement> @PolyNull Statement accept(T node, Shuttle shuttle) {
     if (node == null) {
-      return null;
+      return node;
     }
     return node.accept(shuttle);
   }
@@ -3186,8 +3177,8 @@ static List<DeclarationStatement> acceptDeclarations(
     return declarations1;
   }
 
-  static List<MemberDeclaration> acceptMemberDeclarations(
-      List<MemberDeclaration> memberDeclarations, Shuttle shuttle) {
+  static @PolyNull List<MemberDeclaration> acceptMemberDeclarations(
+      @PolyNull List<MemberDeclaration> memberDeclarations, Shuttle shuttle) {
     if (memberDeclarations == null || memberDeclarations.isEmpty()) {
       return memberDeclarations; // short cut
     }
@@ -3210,7 +3201,8 @@ static List<Expression> acceptExpressions(List<Expression> expressions,
     return expressions1;
   }
 
-  static <R> R acceptNodes(List<? extends Node> nodes, Visitor<R> visitor) {
+  static <R> @Nullable R acceptNodes(@Nullable List<? extends Node> nodes,
+      Visitor<R> visitor) {
     R r = null;
     if (nodes != null) {
       for (Node node : nodes) {
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FieldDeclaration.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FieldDeclaration.java
index 7780f8c4dfb4..9d754cbe6cdc 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FieldDeclaration.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FieldDeclaration.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Modifier;
 import java.util.Objects;
 
@@ -25,10 +27,10 @@
 public class FieldDeclaration extends MemberDeclaration {
   public final int modifier;
   public final ParameterExpression parameter;
-  public final Expression initializer;
+  public final @Nullable Expression initializer;
 
   public FieldDeclaration(int modifier, ParameterExpression parameter,
-      Expression initializer) {
+      @Nullable Expression initializer) {
     assert parameter != null : "parameter should not be null";
     this.modifier = modifier;
     this.parameter = parameter;
@@ -61,7 +63,7 @@ public void accept(ExpressionWriter writer) {
     writer.newlineAndIndent();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForEachStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForEachStatement.java
index d2cc3eff9dd0..eec7cd623ebf 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForEachStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForEachStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -59,7 +61,7 @@ public <R> R accept(Visitor<R> visitor) {
         .append(Blocks.toBlock(body));
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     return this == o
         || o instanceof ForEachStatement
         && parameter.equals(((ForEachStatement) o).parameter)
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForStatement.java
index feb2b6164516..b9ccf47b244b 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ForStatement.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.Ord;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.List;
 import java.util.Objects;
 
@@ -26,14 +28,14 @@
  */
 public class ForStatement extends Statement {
   public final List<DeclarationStatement> declarations;
-  public final Expression condition;
-  public final Expression post;
+  public final @Nullable Expression condition;
+  public final @Nullable Expression post;
   public final Statement body;
   /** Cached hash code for the expression. */
   private int hash;
 
   public ForStatement(List<DeclarationStatement> declarations,
-      Expression condition, Expression post, Statement body) {
+      @Nullable Expression condition, @Nullable Expression post, Statement body) {
     super(ExpressionType.For, Void.TYPE);
     assert declarations != null;
     assert body != null;
@@ -74,7 +76,7 @@ public <R> R accept(Visitor<R> visitor) {
     writer.append(") ").append(Blocks.toBlock(body));
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FunctionExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FunctionExpression.java
index 6ebf8b60a7c1..231369c79fa3 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FunctionExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/FunctionExpression.java
@@ -22,6 +22,9 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Lists;
 
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Method;
 import java.lang.reflect.Proxy;
 import java.lang.reflect.Type;
@@ -30,6 +33,10 @@
 import java.util.List;
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
+import static java.util.Objects.requireNonNull;
+
 /**
  * Represents a strongly typed lambda expression as a data structure in the form
  * of an expression tree. This class cannot be inherited.
@@ -38,14 +45,14 @@
  */
 public final class FunctionExpression<F extends Function<?>>
     extends LambdaExpression {
-  public final F function;
-  public final BlockStatement body;
+  public final @Nullable F function;
+  public final @Nullable BlockStatement body;
   public final List<ParameterExpression> parameterList;
-  private F dynamicFunction;
+  private @Nullable F dynamicFunction;
   /** Cached hash code for the expression. */
   private int hash;
 
-  private FunctionExpression(Class<F> type, F function, BlockStatement body,
+  private FunctionExpression(Class<F> type, @Nullable F function, @Nullable BlockStatement body,
       List<ParameterExpression> parameterList) {
     super(ExpressionType.Lambda, type);
     assert type != null : "type should not be null";
@@ -57,7 +64,7 @@ private FunctionExpression(Class<F> type, F function, BlockStatement body,
     this.parameterList = parameterList;
   }
 
-  public FunctionExpression(F function) {
+  public FunctionExpression(@NonNull F function) {
     this((Class) function.getClass(), function, null, ImmutableList.of());
   }
 
@@ -68,7 +75,7 @@ public FunctionExpression(Class<F> type, BlockStatement body,
 
   @Override public Expression accept(Shuttle shuttle) {
     shuttle = shuttle.preVisit(this);
-    BlockStatement body = this.body.accept(shuttle);
+    BlockStatement body = this.body == null ? null : this.body.accept(shuttle);
     return shuttle.visit(this, body);
   }
 
@@ -82,7 +89,7 @@ public Invokable compile() {
       for (int i = 0; i < args.length; i++) {
         evaluator.push(parameterList.get(i), args[i]);
       }
-      return evaluator.evaluate(body);
+      return evaluator.evaluate(castNonNull(body));
     };
   }
 
@@ -93,8 +100,9 @@ public F getFunction() {
     if (dynamicFunction == null) {
       final Invokable x = compile();
 
+      ClassLoader classLoader = requireNonNull(castNonNull(getClass().getClassLoader()));
       //noinspection unchecked
-      dynamicFunction = (F) Proxy.newProxyInstance(getClass().getClassLoader(),
+      dynamicFunction = (F) Proxy.newProxyInstance(classLoader,
           new Class[]{Types.toClass(type)}, (proxy, method, args) -> x.dynamicInvoke(args));
     }
     return dynamicFunction;
@@ -142,9 +150,10 @@ public F getFunction() {
       boxBridgeParams.add(parameterExpression.declString(parameterBoxType));
       boxBridgeArgs.add(parameterExpression.name
           + (Primitive.is(parameterType)
-          ? "." + Primitive.of(parameterType).primitiveName + "Value()"
+          ? "." + castNonNull(Primitive.of(parameterType)).primitiveName + "Value()"
           : ""));
     }
+    castNonNull(body);
     Type bridgeResultType = Functions.FUNCTION_RESULT_TYPES.get(this.type);
     if (bridgeResultType == null) {
       bridgeResultType = body.getType();
@@ -202,13 +211,11 @@ public F getFunction() {
 
   private boolean isAbstractMethodPrimitive() {
     Method method = getAbstractMethod();
-    assert method != null;
     return Primitive.is(method.getReturnType());
   }
 
   private String getAbstractMethodName() {
     final Method abstractMethod = getAbstractMethod();
-    assert abstractMethod != null;
     return abstractMethod.getName();
   }
 
@@ -222,10 +229,10 @@ private Method getAbstractMethod() {
         return declaredMethods.get(0);
       }
     }
-    return null;
+    throw new IllegalStateException("Method not found, type = " + type);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
@@ -266,6 +273,6 @@ private Method getAbstractMethod() {
 
   /** Function that can be invoked with a variable number of arguments. */
   public interface Invokable {
-    Object dynamicInvoke(Object... args);
+    @Nullable Object dynamicInvoke(@Nullable Object... args);
   }
 }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/GotoStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/GotoStatement.java
index 4c00e62f70cc..be366b60a05f 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/GotoStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/GotoStatement.java
@@ -16,19 +16,23 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Represents an unconditional jump. This includes return statements, break and
  * continue statements, and other jumps.
  */
 public class GotoStatement extends Statement {
   public final GotoExpressionKind kind;
-  public final LabelTarget labelTarget;
-  public final Expression expression;
+  public final @Nullable LabelTarget labelTarget;
+  public final @Nullable Expression expression;
 
-  GotoStatement(GotoExpressionKind kind, LabelTarget labelTarget,
-      Expression expression) {
+  GotoStatement(GotoExpressionKind kind, @Nullable LabelTarget labelTarget,
+      @Nullable Expression expression) {
     super(ExpressionType.Goto,
         expression == null ? Void.TYPE : expression.getType());
     assert kind != null : "kind should not be null";
@@ -88,19 +92,19 @@ public <R> R accept(Visitor<R> visitor) {
     writer.append(';').newlineAndIndent();
   }
 
-  @Override public Object evaluate(Evaluator evaluator) {
+  @Override public @Nullable Object evaluate(Evaluator evaluator) {
     switch (kind) {
     case Return:
     case Sequence:
       // NOTE: We ignore control flow. This is only correct if "return"
       // is the last statement in the block.
-      return expression.evaluate(evaluator);
+      return castNonNull(expression).evaluate(evaluator);
     default:
       throw new AssertionError("evaluate not implemented");
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/IndexExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/IndexExpression.java
index 0f1c909f7220..3fbedb940643 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/IndexExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/IndexExpression.java
@@ -16,9 +16,13 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.List;
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Represents indexing a property or array.
  */
@@ -27,8 +31,7 @@ public class IndexExpression extends Expression {
   public final List<Expression> indexExpressions;
 
   public IndexExpression(Expression array, List<Expression> indexExpressions) {
-    super(ExpressionType.ArrayIndex, Types.getComponentType(array.getType()));
-    assert array != null : "array should not be null";
+    super(ExpressionType.ArrayIndex, castNonNull(Types.getComponentType(array.getType())));
     assert indexExpressions != null : "indexExpressions should not be null";
     assert !indexExpressions.isEmpty() : "indexExpressions should not be empty";
     this.array = array;
@@ -52,7 +55,7 @@ public <R> R accept(Visitor<R> visitor) {
     writer.list("[", ", ", "]", indexExpressions);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelStatement.java
index 38a5d279892c..1f98718f0dd8 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -41,7 +43,7 @@ public <R> R accept(Visitor<R> visitor) {
     return visitor.visit(this);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelTarget.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelTarget.java
index 49cbb4d03b6b..16679ab02343 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelTarget.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/LabelTarget.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -28,7 +30,7 @@ public LabelTarget(String name) {
     this.name = name;
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MemberExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MemberExpression.java
index d746ea19e583..29e6a82f704c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MemberExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MemberExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Field;
 import java.lang.reflect.Modifier;
 import java.util.Objects;
@@ -24,14 +26,14 @@
  * Represents accessing a field or property.
  */
 public class MemberExpression extends Expression {
-  public final Expression expression;
+  public final @Nullable Expression expression;
   public final PseudoField field;
 
   public MemberExpression(Expression expression, Field field) {
     this(expression, Types.field(field));
   }
 
-  public MemberExpression(Expression expression, PseudoField field) {
+  public MemberExpression(@Nullable Expression expression, PseudoField field) {
     super(ExpressionType.MemberAccess, field.getType());
     assert field != null : "field should not be null";
     assert expression != null || Modifier.isStatic(field.getModifiers())
@@ -52,7 +54,7 @@ public <R> R accept(Visitor<R> visitor) {
     return visitor.visit(this);
   }
 
-  public Object evaluate(Evaluator evaluator) {
+  @Override public @Nullable Object evaluate(Evaluator evaluator) {
     final Object o = expression == null
         ? null
         : expression.evaluate(evaluator);
@@ -76,7 +78,7 @@ public Object evaluate(Evaluator evaluator) {
     writer.append('.').append(field.getName());
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodCallExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodCallExpression.java
index f98cfe0be02a..53e0d5bbeae7 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodCallExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodCallExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
@@ -28,13 +30,13 @@
  */
 public class MethodCallExpression extends Expression {
   public final Method method;
-  public final Expression targetExpression; // null for call to static method
+  public final @Nullable Expression targetExpression; // null for call to static method
   public final List<Expression> expressions;
   /** Cached hash code for the expression. */
   private int hash;
 
   MethodCallExpression(Type returnType, Method method,
-      Expression targetExpression, List<Expression> expressions) {
+      @Nullable Expression targetExpression, List<Expression> expressions) {
     super(ExpressionType.Call, returnType);
     assert expressions != null : "expressions should not be null";
     assert method != null : "method should not be null";
@@ -46,7 +48,7 @@ public class MethodCallExpression extends Expression {
     this.expressions = expressions;
   }
 
-  MethodCallExpression(Method method, Expression targetExpression,
+  MethodCallExpression(Method method, @Nullable Expression targetExpression,
       List<Expression> expressions) {
     this(method.getReturnType(), method, targetExpression, expressions);
   }
@@ -64,14 +66,14 @@ public <R> R accept(Visitor<R> visitor) {
     return visitor.visit(this);
   }
 
-  @Override public Object evaluate(Evaluator evaluator) {
+  @Override public @Nullable Object evaluate(Evaluator evaluator) {
     final Object target;
     if (targetExpression == null) {
       target = null;
     } else {
       target = targetExpression.evaluate(evaluator);
     }
-    final Object[] args = new Object[expressions.size()];
+    final @Nullable Object[] args = new Object[expressions.size()];
     for (int i = 0; i < expressions.size(); i++) {
       Expression expression = expressions.get(i);
       args[i] = expression.evaluate(evaluator);
@@ -105,7 +107,7 @@ public <R> R accept(Visitor<R> visitor) {
     writer.append(')');
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodDeclaration.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodDeclaration.java
index e67fa6bb207f..1cd58a48f225 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodDeclaration.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/MethodDeclaration.java
@@ -16,12 +16,14 @@
  */
 package org.apache.calcite.linq4j.tree;
 
-import com.google.common.collect.Lists;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Objects;
+import java.util.stream.Collectors;
 
 /**
  * Declaration of a method.
@@ -30,11 +32,11 @@ public class MethodDeclaration extends MemberDeclaration {
   public final int modifier;
   public final String name;
   public final Type resultType;
-  public final List<ParameterExpression> parameters;
+  public final List<@NonNull ParameterExpression> parameters;
   public final BlockStatement body;
 
   public MethodDeclaration(int modifier, String name, Type resultType,
-      List<ParameterExpression> parameters, BlockStatement body) {
+      List<@NonNull ParameterExpression> parameters, BlockStatement body) {
     assert name != null : "name should not be null";
     assert resultType != null : "resultType should not be null";
     assert parameters != null : "parameters should not be null";
@@ -68,13 +70,13 @@ public void accept(ExpressionWriter writer) {
         .append(' ')
         .append(name)
         .list("(", ", ", ")",
-            Lists.transform(parameters, ParameterExpression::declString))
+            parameters.stream().map(ParameterExpression::declString).collect(Collectors.toList()))
         .append(' ')
         .append(body);
     writer.newlineAndIndent();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewArrayExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewArrayExpression.java
index 54cba746fde3..9380a10ed779 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewArrayExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewArrayExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Objects;
@@ -26,13 +28,13 @@
  */
 public class NewArrayExpression extends Expression {
   public final int dimension;
-  public final Expression bound;
-  public final List<Expression> expressions;
+  public final @Nullable Expression bound;
+  public final @Nullable List<Expression> expressions;
   /** Cached hash code for the expression. */
   private int hash;
 
-  public NewArrayExpression(Type type, int dimension, Expression bound,
-      List<Expression> expressions) {
+  public NewArrayExpression(Type type, int dimension, @Nullable Expression bound,
+      @Nullable List<Expression> expressions) {
     super(ExpressionType.NewArrayInit, Types.arrayType(type, dimension));
     this.dimension = dimension;
     this.bound = bound;
@@ -67,7 +69,7 @@ public <R> R accept(Visitor<R> visitor) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewExpression.java
index d42d67f5cf2e..b65b2c884eee 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/NewExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Objects;
@@ -29,12 +31,12 @@
 public class NewExpression extends Expression {
   public final Type type;
   public final List<Expression> arguments;
-  public final List<MemberDeclaration> memberDeclarations;
+  public final @Nullable List<MemberDeclaration> memberDeclarations;
   /** Cached hash code for the expression. */
   private int hash;
 
   public NewExpression(Type type, List<Expression> arguments,
-      List<MemberDeclaration> memberDeclarations) {
+      @Nullable List<MemberDeclaration> memberDeclarations) {
     super(ExpressionType.New, type);
     this.type = type;
     this.arguments = arguments;
@@ -61,7 +63,7 @@ public <R> R accept(Visitor<R> visitor) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/OptimizeShuttle.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/OptimizeShuttle.java
index c4a508ef803c..0dfb8703644c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/OptimizeShuttle.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/OptimizeShuttle.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.ArrayList;
@@ -215,7 +217,7 @@ && eq(cmp.expression1, expression2)) {
     return super.visit(binary, expression0, expression1);
   }
 
-  private Expression visit0(
+  private @Nullable Expression visit0(
       BinaryExpression binary,
       Expression expression0,
       Expression expression1) {
@@ -361,7 +363,7 @@ && isKnownNotNull(expression0)) {
   }
 
   @Override public Expression visit(MethodCallExpression methodCallExpression,
-      Expression targetExpression,
+      @Nullable Expression targetExpression,
       List<Expression> expressions) {
     if (BOOLEAN_VALUEOF_BOOL.equals(methodCallExpression.method)) {
       Boolean always = always(expressions.get(0));
@@ -381,7 +383,7 @@ private boolean isConstantNull(Expression expression) {
    * Returns whether an expression always evaluates to true or false.
    * Assumes that expression has already been optimized.
    */
-  private static Boolean always(Expression x) {
+  private static @Nullable Boolean always(Expression x) {
     if (x.equals(FALSE_EXPR) || x.equals(BOXED_FALSE_EXPR)) {
       return Boolean.FALSE;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ParameterExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ParameterExpression.java
index fb7eaf2d22d3..03178320d802 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ParameterExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ParameterExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -51,7 +53,7 @@ public <R> R accept(Visitor<R> visitor) {
     return visitor.visit(this);
   }
 
-  public Object evaluate(Evaluator evaluator) {
+  @Override public @Nullable Object evaluate(Evaluator evaluator) {
     return evaluator.peek(this);
   }
 
@@ -69,7 +71,7 @@ String declString(Type type) {
         + " " + name;
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     return this == o;
   }
 
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Primitive.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Primitive.java
index f95105d93917..82dd9a7aff02 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Primitive.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Primitive.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Array;
 import java.lang.reflect.Field;
 import java.lang.reflect.Type;
@@ -28,6 +30,8 @@
 import java.util.List;
 import java.util.Map;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Enumeration of Java's primitive types.
  *
@@ -54,30 +58,30 @@ public enum Primitive {
   VOID(Void.TYPE, Void.class, 3, null, null, null, null, null, -1),
   OTHER(null, null, 4, null, null, null, null, null, -1);
 
-  public final Class primitiveClass;
-  public final Class boxClass;
-  public final String primitiveName; // e.g. "int"
-  public final String boxName;
+  public final @Nullable Class primitiveClass;
+  public final @Nullable Class boxClass;
+  public final @Nullable String primitiveName; // e.g. "int"
+  public final @Nullable String boxName;
   private final int family;
 
   /** The default value of this primitive class. This is the value
    * taken by uninitialized fields, for instance; 0 for {@code int}, false for
    * {@code boolean}, etc. */
-  public final Object defaultValue;
+  public final @Nullable Object defaultValue;
 
   /** The minimum value of this primitive class. */
-  public final Object min;
+  public final @Nullable Object min;
 
   /** The largest value that is less than zero. Null if not applicable for this
    * type. */
-  public final Object maxNegative;
+  public final @Nullable Object maxNegative;
 
   /** The smallest value that is greater than zero. Null if not applicable for
    * this type. */
-  public final Object minPositive;
+  public final @Nullable Object minPositive;
 
   /** The maximum value of this primitive class. */
-  public final Object max;
+  public final @Nullable Object max;
 
   /** The size of a value of this type, in bits. Null if not applicable for this
    * type. */
@@ -98,9 +102,9 @@ public enum Primitive {
     }
   }
 
-  Primitive(Class primitiveClass, Class boxClass, int family,
-      Object defaultValue, Object min, Object maxNegative, Object minPositive,
-      Object max, int size) {
+  Primitive(@Nullable Class primitiveClass, @Nullable Class boxClass, int family,
+      @Nullable Object defaultValue, @Nullable Object min, @Nullable Object maxNegative,
+      @Nullable Object minPositive, @Nullable Object max, int size) {
     this.primitiveClass = primitiveClass;
     this.family = family;
     this.primitiveName =
@@ -123,7 +127,7 @@ public enum Primitive {
    * <code>of(Long.class)</code> and <code>of(String.class)</code> return
    * {@code null}.
    */
-  public static Primitive of(Type type) {
+  public static @Nullable Primitive of(Type type) {
     //noinspection SuspiciousMethodCalls
     return PRIMITIVE_MAP.get(type);
   }
@@ -134,7 +138,7 @@ public static Primitive of(Type type) {
    * <p>For example, <code>ofBox(java.util.Long.class)</code>
    * returns {@link #LONG}.
    */
-  public static Primitive ofBox(Type type) {
+  public static @Nullable Primitive ofBox(Type type) {
     //noinspection SuspiciousMethodCalls
     return BOX_MAP.get(type);
   }
@@ -145,7 +149,7 @@ public static Primitive ofBox(Type type) {
    * <p>For example, <code>ofBoxOr(Long.class)</code> and
    * <code>ofBoxOr(long.class)</code> both return {@link #LONG}.
    */
-  public static Primitive ofBoxOr(Type type) {
+  public static @Nullable Primitive ofBoxOr(Type type) {
     Primitive primitive = of(type);
     if (primitive == null) {
       primitive = ofBox(type);
@@ -216,7 +220,7 @@ public boolean isFixedNumeric() {
    */
   public static Type box(Type type) {
     Primitive primitive = of(type);
-    return primitive == null ? type : primitive.boxClass;
+    return primitive == null ? type : castNonNull(primitive.boxClass);
   }
 
   /**
@@ -225,7 +229,7 @@ public static Type box(Type type) {
    */
   public static Class box(Class type) {
     Primitive primitive = of(type);
-    return primitive == null ? type : primitive.boxClass;
+    return primitive == null ? type : castNonNull(primitive.boxClass);
   }
 
   /**
@@ -234,7 +238,7 @@ public static Class box(Class type) {
    */
   public static Type unbox(Type type) {
     Primitive primitive = ofBox(type);
-    return primitive == null ? type : primitive.primitiveClass;
+    return primitive == null ? type : castNonNull(primitive.primitiveClass);
   }
 
   /**
@@ -243,7 +247,7 @@ public static Type unbox(Type type) {
    */
   public static Class unbox(Class type) {
     Primitive primitive = ofBox(type);
-    return primitive == null ? type : primitive.primitiveClass;
+    return primitive == null ? type : castNonNull(primitive.primitiveClass);
   }
 
   /**
@@ -696,7 +700,7 @@ public void send(Field field, Object o, Sink sink)
   /**
    * Gets an item from an array.
    */
-  public Object arrayItem(Object dataSet, int ordinal) {
+  public @Nullable Object arrayItem(Object dataSet, int ordinal) {
     // Plain old Array.get doesn't cut it when you have an array of
     // Integer values but you want to read Short values. Array.getShort
     // does the right thing.
@@ -727,6 +731,7 @@ public Object arrayItem(Object dataSet, int ordinal) {
   /**
    * Reads value from a source into an array.
    */
+  @SuppressWarnings("argument.type.incompatible")
   public void arrayItem(Source source, Object dataSet, int ordinal) {
     switch (this) {
     case DOUBLE:
@@ -804,7 +809,7 @@ public void arrayItem(Object dataSet, int ordinal, Sink sink) {
    * @param resultSet Result set
    * @param i Ordinal of column (1-based, per JDBC)
    */
-  public Object jdbcGet(ResultSet resultSet, int i) throws SQLException {
+  public @Nullable Object jdbcGet(ResultSet resultSet, int i) throws SQLException {
     switch (this) {
     case BOOLEAN:
       return resultSet.getBoolean(i);
@@ -976,7 +981,7 @@ public interface Sink {
 
     void set(double v);
 
-    void set(Object v);
+    void set(@Nullable Object v);
   }
 
   /**
@@ -999,7 +1004,7 @@ public interface Source {
 
     double getDouble();
 
-    Object getObject();
+    @Nullable Object getObject();
   }
 
   /** Whether a type is primitive (e.g. {@code int}),
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/PseudoField.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/PseudoField.java
index ec1192334106..93f781954a7d 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/PseudoField.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/PseudoField.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 
 /**
@@ -29,7 +31,7 @@ public interface PseudoField {
 
   int getModifiers();
 
-  Object get(Object o) throws IllegalAccessException;
+  @Nullable Object get(@Nullable Object o) throws IllegalAccessException;
 
   Type getDeclaringClass();
 }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ReflectedPseudoField.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ReflectedPseudoField.java
index 4dc11a7b53d3..0372d4359714 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ReflectedPseudoField.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ReflectedPseudoField.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Field;
 import java.lang.reflect.Type;
 
@@ -43,7 +45,7 @@ public int getModifiers() {
     return field.getModifiers();
   }
 
-  public Object get(Object o) throws IllegalAccessException {
+  @Override public @Nullable Object get(@Nullable Object o) throws IllegalAccessException {
     return field.get(o);
   }
 
@@ -51,7 +53,7 @@ public Class<?> getDeclaringClass() {
     return field.getDeclaringClass();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Shuttle.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Shuttle.java
index 52806c0a3bca..ef5aad5595d9 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Shuttle.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Shuttle.java
@@ -16,9 +16,13 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.List;
 import java.util.Objects;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Extension to {@link Visitor} that returns a mutated tree.
  */
@@ -61,7 +65,7 @@ public Shuttle preVisit(GotoStatement gotoStatement) {
     return this;
   }
 
-  public Statement visit(GotoStatement gotoStatement, Expression expression) {
+  public Statement visit(GotoStatement gotoStatement, @Nullable Expression expression) {
     return expression == gotoStatement.expression
         ? gotoStatement
         : Expressions.makeGoto(
@@ -78,8 +82,8 @@ public Shuttle preVisit(ForStatement forStatement) {
   }
 
   public ForStatement visit(ForStatement forStatement,
-      List<DeclarationStatement> declarations, Expression condition,
-      Expression post, Statement body) {
+      List<DeclarationStatement> declarations, @Nullable Expression condition,
+      @Nullable Expression post, Statement body) {
     return declarations.equals(forStatement.declarations)
         && condition == forStatement.condition
         && post == forStatement.post
@@ -116,7 +120,7 @@ public Shuttle preVisit(DeclarationStatement declarationStatement) {
   }
 
   public DeclarationStatement visit(DeclarationStatement declarationStatement,
-      Expression initializer) {
+      @Nullable Expression initializer) {
     return declarationStatement.initializer == initializer
         ? declarationStatement
         : Expressions.declare(
@@ -133,10 +137,10 @@ public Shuttle preVisit(FunctionExpression functionExpression) {
   }
 
   public Expression visit(FunctionExpression functionExpression,
-      BlockStatement body) {
-    return functionExpression.body.equals(body)
+      @Nullable BlockStatement body) {
+    return Objects.equals(body, functionExpression.body)
         ? functionExpression
-        : Expressions.lambda(body, functionExpression.parameterList);
+        : Expressions.lambda(castNonNull(body), functionExpression.parameterList);
   }
 
   public Shuttle preVisit(BinaryExpression binaryExpression) {
@@ -195,7 +199,7 @@ public Shuttle preVisit(MethodCallExpression methodCallExpression) {
   }
 
   public Expression visit(MethodCallExpression methodCallExpression,
-      Expression targetExpression, List<Expression> expressions) {
+      @Nullable Expression targetExpression, List<Expression> expressions) {
     return methodCallExpression.targetExpression == targetExpression
            && methodCallExpression.expressions.equals(expressions)
         ? methodCallExpression
@@ -216,7 +220,7 @@ public Shuttle preVisit(MemberExpression memberExpression) {
   }
 
   public Expression visit(MemberExpression memberExpression,
-      Expression expression) {
+      @Nullable Expression expression) {
     return memberExpression.expression == expression
         ? memberExpression
         : Expressions.field(expression, memberExpression.field);
@@ -231,7 +235,7 @@ public Shuttle preVisit(NewArrayExpression newArrayExpression) {
   }
 
   public Expression visit(NewArrayExpression newArrayExpression, int dimension,
-      Expression bound, List<Expression> expressions) {
+      @Nullable Expression bound, @Nullable List<Expression> expressions) {
     return Objects.equals(expressions, newArrayExpression.expressions)
         && Objects.equals(bound, newArrayExpression.bound)
         ? newArrayExpression
@@ -252,7 +256,7 @@ public Shuttle preVisit(NewExpression newExpression) {
   }
 
   public Expression visit(NewExpression newExpression,
-      List<Expression> arguments, List<MemberDeclaration> memberDeclarations) {
+      List<Expression> arguments, @Nullable List<MemberDeclaration> memberDeclarations) {
     return arguments.equals(newExpression.arguments)
         && Objects.equals(memberDeclarations, newExpression.memberDeclarations)
         ? newExpression
@@ -268,7 +272,7 @@ public Shuttle preVisit(TryStatement tryStatement) {
   }
 
   public Statement visit(TryStatement tryStatement,
-      Statement body, List<CatchBlock> catchBlocks, Statement fynally) {
+      Statement body, List<CatchBlock> catchBlocks, @Nullable Statement fynally) {
     return body.equals(tryStatement.body)
            && Objects.equals(catchBlocks, tryStatement.catchBlocks)
            && Objects.equals(fynally, tryStatement.fynally)
@@ -310,7 +314,7 @@ public Shuttle preVisit(FieldDeclaration fieldDeclaration) {
   }
 
   public MemberDeclaration visit(FieldDeclaration fieldDeclaration,
-      Expression initializer) {
+      @Nullable Expression initializer) {
     return Objects.equals(initializer, fieldDeclaration.initializer)
         ? fieldDeclaration
         : Expressions.fieldDecl(fieldDeclaration.modifier,
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TernaryExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TernaryExpression.java
index 00ef60c370f7..427192cadccd 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TernaryExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TernaryExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Objects;
 
@@ -61,7 +63,7 @@ void accept(ExpressionWriter writer, int lprec, int rprec) {
     expression2.accept(writer, nodeType.rprec, rprec);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ThrowStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ThrowStatement.java
index 0cd427d70d0e..66472d80ad72 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ThrowStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/ThrowStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -43,7 +45,7 @@ public <R> R accept(Visitor<R> visitor) {
     writer.append("throw ").append(expression).append(';').newlineAndIndent();
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TryStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TryStatement.java
index 0ee3d7a96b12..1fd4d6ef4ba1 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TryStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TryStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Objects;
@@ -26,10 +28,10 @@
 public class TryStatement extends Statement {
   public final Statement body;
   public final List<CatchBlock> catchBlocks;
-  public final Statement fynally;
+  public final @Nullable Statement fynally;
 
   public TryStatement(Statement body, List<CatchBlock> catchBlocks,
-      Statement fynally) {
+      @Nullable Statement fynally) {
     super(ExpressionType.Try, body.getType());
     this.body = Objects.requireNonNull(body);
     this.catchBlocks = Objects.requireNonNull(catchBlocks);
@@ -67,7 +69,7 @@ public <R> R accept(Visitor<R> visitor) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TypeBinaryExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TypeBinaryExpression.java
index 967c228849c9..50d03e3dc323 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TypeBinaryExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/TypeBinaryExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Objects;
 
@@ -53,7 +55,7 @@ void accept(ExpressionWriter writer, int lprec, int rprec) {
     writer.append(type);
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Types.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Types.java
index a449a46a925c..ef484432a6ad 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Types.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/Types.java
@@ -18,6 +18,8 @@
 
 import org.apache.calcite.linq4j.Enumerator;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Array;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Field;
@@ -33,6 +35,8 @@
 import java.util.Iterator;
 import java.util.List;
 
+import static org.apache.calcite.linq4j.NullnessUtil.castNonNull;
+
 /**
  * Utilities for converting between {@link Expression}, {@link Type} and
  * {@link Class}.
@@ -60,7 +64,7 @@ public static Type of(Type type, Type... typeArguments) {
    *
    * <p>Returns null if the type is not one of these.</p>
    */
-  public static Type getElementType(Type type) {
+  public static @Nullable Type getElementType(Type type) {
     if (type instanceof ArrayType) {
       return ((ArrayType) type).getComponentType();
     }
@@ -167,7 +171,7 @@ public static Class[] toClassArray(Iterable<? extends Expression> arguments) {
   /**
    * Returns the component type of an array.
    */
-  public static Type getComponentType(Type type) {
+  public static @Nullable Type getComponentType(Type type) {
     if (type instanceof Class) {
       return ((Class) type).getComponentType();
     }
@@ -189,30 +193,20 @@ public static Type getComponentType(Type type) {
 
   static Type getComponentTypeN(Type type) {
     for (;;) {
-      final Type oldType = type;
-      type = getComponentType(type);
-      if (type == null) {
-        return oldType;
+      Type componentType = getComponentType(type);
+      if (componentType == null) {
+        return type;
       }
+      type = componentType;
     }
   }
 
   public static Type box(Type type) {
-    Primitive primitive = Primitive.of(type);
-    if (primitive != null) {
-      return primitive.boxClass;
-    } else {
-      return type;
-    }
+    return Primitive.box(type);
   }
 
   public static Type unbox(Type type) {
-    Primitive primitive = Primitive.ofBox(type);
-    if (primitive != null) {
-      return primitive.primitiveClass;
-    } else {
-      return type;
-    }
+    return Primitive.unbox(type);
   }
 
   static String className(Type type) {
@@ -289,6 +283,7 @@ public static boolean allAssignable(boolean varArgs, Class[] parameterTypes,
    *
    * @return Whether parameter can be assigned from argument
    */
+  @SuppressWarnings("nullness")
   private static boolean assignableFrom(Class parameter, Class argument) {
     return parameter.isAssignableFrom(argument)
            || parameter.isPrimitive()
@@ -404,7 +399,7 @@ static Type gcd(Type... types) {
           return Object.class;
         }
       }
-      return bestPrimitive.primitiveClass;
+      return castNonNull(bestPrimitive.primitiveClass);
     } else {
       for (int i = 1; i < types.length; i++) {
         if (types[i] != types[0]) {
@@ -438,7 +433,7 @@ public static Expression castIfNecessary(Type returnType,
       //   Integer foo(BigDecimal o) {
       //     return o.intValue();
       //   }
-      return Expressions.unbox(expression, Primitive.ofBox(returnType));
+      return Expressions.unbox(expression, castNonNull(Primitive.ofBox(returnType)));
     }
     if (Primitive.is(returnType) && !Primitive.is(type)) {
       // E.g.
@@ -447,7 +442,7 @@ public static Expression castIfNecessary(Type returnType,
       //   }
       return Expressions.unbox(
           Expressions.convert_(expression, Types.box(returnType)),
-          Primitive.of(returnType));
+          castNonNull(Primitive.of(returnType)));
     }
     if (!Primitive.is(returnType) && Primitive.is(type)) {
       // E.g.
@@ -526,10 +521,10 @@ public static Type stripGenerics(Type type) {
   static class ParameterizedTypeImpl implements ParameterizedType {
     private final Type rawType;
     private final List<Type> typeArguments;
-    private final Type ownerType;
+    private final @Nullable Type ownerType;
 
     ParameterizedTypeImpl(Type rawType, List<Type> typeArguments,
-        Type ownerType) {
+        @Nullable Type ownerType) {
       super();
       this.rawType = rawType;
       this.typeArguments = typeArguments;
@@ -563,7 +558,7 @@ public Type getRawType() {
       return rawType;
     }
 
-    public Type getOwnerType() {
+    public @Nullable Type getOwnerType() {
       return ownerType;
     }
   }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/UnaryExpression.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/UnaryExpression.java
index d07e545aa816..5ae22bc60226 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/UnaryExpression.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/UnaryExpression.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.lang.reflect.Type;
 import java.util.Objects;
 
@@ -59,7 +61,7 @@ void accept(ExpressionWriter writer, int lprec, int rprec) {
     }
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/VisitorImpl.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/VisitorImpl.java
index 8d8431f27757..4873f90ad86c 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/VisitorImpl.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/VisitorImpl.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.List;
 
 /**
@@ -24,7 +26,7 @@
  *
  * @param <R> Return type
  */
-public class VisitorImpl<R> implements Visitor<R> {
+public class VisitorImpl<@Nullable R> implements Visitor<R> {
   public VisitorImpl() {
     super();
   }
@@ -84,8 +86,8 @@ public R visit(FieldDeclaration fieldDeclaration) {
 
   public R visit(ForStatement forStatement) {
     R r0 = Expressions.acceptNodes(forStatement.declarations, this);
-    R r1 = forStatement.condition.accept(this);
-    R r2 = forStatement.post.accept(this);
+    R r1 = forStatement.condition == null ? null : forStatement.condition.accept(this);
+    R r2 = forStatement.post == null ? null : forStatement.post.accept(this);
     return forStatement.body.accept(this);
   }
 
@@ -99,7 +101,7 @@ public R visit(FunctionExpression functionExpression) {
     @SuppressWarnings("unchecked") final List<Node> parameterList =
         functionExpression.parameterList;
     R r0 = Expressions.acceptNodes(parameterList, this);
-    return functionExpression.body.accept(this);
+    return functionExpression.body == null ? null : functionExpression.body.accept(this);
   }
 
   public R visit(GotoStatement gotoStatement) {
diff --git a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/WhileStatement.java b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/WhileStatement.java
index 76ab8b0f2860..8e5e0c505c91 100644
--- a/linq4j/src/main/java/org/apache/calcite/linq4j/tree/WhileStatement.java
+++ b/linq4j/src/main/java/org/apache/calcite/linq4j/tree/WhileStatement.java
@@ -16,6 +16,8 @@
  */
 package org.apache.calcite.linq4j.tree;
 
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 
 /**
@@ -49,7 +51,7 @@ public <R> R accept(Visitor<R> visitor) {
         Blocks.toBlock(body));
   }
 
-  @Override public boolean equals(Object o) {
+  @Override public boolean equals(@Nullable Object o) {
     if (this == o) {
       return true;
     }
diff --git a/src/main/config/checkerframework/Constructor.astub b/src/main/config/checkerframework/Constructor.astub
new file mode 100644
index 000000000000..1e7bba8ecb22
--- /dev/null
+++ b/src/main/config/checkerframework/Constructor.astub
@@ -0,0 +1,7 @@
+package java.lang.reflect;
+
+import org.checkerframework.checker.nullness.qual.*;
+
+class Constructor<T> {
+  public @NonNull T newInstance(@Nullable Object... initargs);
+}
diff --git a/src/main/config/checkerframework/Field.astub b/src/main/config/checkerframework/Field.astub
new file mode 100644
index 000000000000..bf9bf25bfd5d
--- /dev/null
+++ b/src/main/config/checkerframework/Field.astub
@@ -0,0 +1,8 @@
+package java.lang.reflect;
+
+import org.checkerframework.checker.nullness.qual.*;
+
+class Field {
+  @Nullable Object get(@Nullable Object obj);
+  int getInt(@Nullable Object obj);
+}
diff --git a/src/main/config/checkerframework/InvocationHandler.astub b/src/main/config/checkerframework/InvocationHandler.astub
new file mode 100644
index 000000000000..27aff377745b
--- /dev/null
+++ b/src/main/config/checkerframework/InvocationHandler.astub
@@ -0,0 +1,7 @@
+package java.lang.reflect;
+
+import org.checkerframework.checker.nullness.qual.*;
+
+interface InvocationHandler {
+    @Nullable Object invoke(Object proxy, Method method, @Nullable Object[] args);
+}
diff --git a/src/main/config/checkerframework/Method.astub b/src/main/config/checkerframework/Method.astub
new file mode 100644
index 000000000000..3a20f211ba1f
--- /dev/null
+++ b/src/main/config/checkerframework/Method.astub
@@ -0,0 +1,7 @@
+package java.lang.reflect;
+
+import org.checkerframework.checker.nullness.qual.*;
+
+class Method {
+  @Nullable Object invoke(@Nullable Object obj, @Nullable Object... args);
+}
diff --git a/src/main/config/checkerframework/Proxy.astub b/src/main/config/checkerframework/Proxy.astub
new file mode 100644
index 000000000000..a77969d949a4
--- /dev/null
+++ b/src/main/config/checkerframework/Proxy.astub
@@ -0,0 +1,7 @@
+package java.lang.reflect;
+
+import org.checkerframework.checker.nullness.qual.*;
+
+class Proxy {
+    Object newProxyInstance(@Nullable ClassLoader loader, Class<?>[] interfaces, InvocationHandler h);
+}