Improve tests and MutableClock interface

.gitignore

@@ -10,3 +10,4 @@ buildNumber.properties
 
 # Avoid ignoring Maven wrapper jar file (.jar files are usually ignored)
 !/.mvn/wrapper/maven-wrapper.jar
+/nbproject/

src/main/java/com/trickl/time/MutableClock.java

@@ -4,29 +4,39 @@
 import java.time.Duration;
 import java.time.Instant;
 import java.time.ZoneId;
+import lombok.AccessLevel;
 import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.Setter;
 
-@AllArgsConstructor
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
 final class MutableClock extends Clock {
-  @Getter @Setter private Instant instant;
+  @Setter private Instant time;
   @Getter private final ZoneId zone;
 
+  /**
+   * Create a mutable clock.
+   *
+   * @return A mutable clock
+   */
+  public static MutableClock at(Instant time) {
+    return new MutableClock(time, ZoneId.of("Z"));
+  }
+
   public void advance(Duration duration) {
-    instant = instant.plus(duration);
+    time = time.plus(duration);
   }
 
   @Override
   public Clock withZone(ZoneId zone) {
     if (zone.equals(this.zone)) {
       return this;
     }
-    return new MutableClock(instant, zone);
+    return new MutableClock(time, zone);
   }
 
   @Override
   public Instant instant() {
-    return instant;
+    return time;
   }
 }

src/test/java/com/trickl/time/MutableClockTest.java

@@ -0,0 +1,51 @@
+package com.trickl.time;
+
+import java.time.Clock;
+import java.time.Duration;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.time.LocalDateTime;
+import java.time.LocalTime;
+import java.time.Month;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class MutableClockTest {
+
+    private final Instant clockStart = LocalDateTime.of(
+            LocalDate.of(2018, Month.JANUARY, 1),
+            LocalTime.NOON
+        ).toInstant(ZoneOffset.UTC);
+
+    @Test
+    public void testAdvance() {
+        MutableClock mutableClock = MutableClock.at(clockStart);
+        Duration adjustment = Duration.ofMinutes(5);
+        mutableClock.advance(adjustment);
+        assertEquals(clockStart.plus(adjustment), mutableClock.instant());
+    }
+
+    @Test
+    public void testWithZone() {
+        Clock mutableClock = MutableClock.at(clockStart)
+            .withZone(ZoneId.of("GMT+2"));
+
+        assertEquals(ZoneId.of("GMT+2"), mutableClock.getZone());
+    }
+
+    @Test
+    public void testInstant() {
+        MutableClock mutableClock = MutableClock.at(clockStart);
+        assertEquals(clockStart, mutableClock.instant());
+    }
+
+    @Test
+    public void testSetTime() {
+        MutableClock mutableClock = MutableClock.at(clockStart);
+        Instant adjustedTime = clockStart.plus(Duration.ofMinutes(5));
+        mutableClock.setTime(adjustedTime);
+        assertEquals(adjustedTime, mutableClock.instant());
+    }
+}

src/test/java/com/trickl/time/RelativeClockTest.java

@@ -16,7 +16,7 @@
 public class RelativeClockTest {
 
     private final MutableClock referenceClock =
-        new MutableClock(Instant.MIN, ZoneId.of("Z"));
+        MutableClock.at(Instant.MIN);
 
     private final Instant referenceClockStart = LocalDateTime.of(
             LocalDate.of(2018, Month.JANUARY, 1),
@@ -25,7 +25,7 @@ public class RelativeClockTest {
 
     @Before 
     public void init() {
-      referenceClock.setInstant(referenceClockStart);
+      referenceClock.setTime(referenceClockStart);
     }
 
     @Test