Add increment level 6 and 7.

src/main/java/Deadline.java

@@ -3,11 +3,21 @@ public class Deadline extends Task{
 
     public Deadline(String description, String by) throws DukeException{
         super(description);
-        this.by = by;
+        if(!by.contains("by ")){
+            throw new DukeException("☹ OOPS!!! The description of a deadline must contain \"by\" time.");
+        }else if(by.replace("by ", "").isBlank()){
+            throw new DukeException("☹ OOPS!!! The time of a deadline cannot be empty.");
+        }
+        this.by = by.replace("by ", "");
     }
 
     @Override
     public String toString() {
         return "[D]" + super.toString() + " (by: " + by + ")";
     }
+
+    @Override
+    public String toFile(){
+        return "D | " + super.toFile() + " | " + this.by;
+    }
 }

src/main/java/Duke.java

@@ -1,6 +1,10 @@
+import java.io.FileReader;
 import java.util.Scanner;
 import java.util.List;
 import java.util.ArrayList;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.io.File;
 public class Duke {
     protected final static String LINE = "_______________________________________________";
     protected final static String GREET = "Hello! I'm Elwin\n" + "What can I do for you?";
@@ -14,9 +18,55 @@ private static void print(String message){
         System.out.println(message);
         System.out.println(LINE);
     }
+    private static void writeToFile(List<Task> list){
+        try{
+            FileWriter fileWriter = new FileWriter("./data/duke.txt");
+            for(Task task : list){
+                fileWriter.write(task.toFile()+"\n");
+            }
+            fileWriter.flush();
+            fileWriter.close();
+        }catch(IOException e){
+            System.out.println("Something went wrong: " + e.getMessage());
+        }
+    }
+
     public static void main(String[] args) {
         Scanner scanner = new Scanner(System.in);
-        List<Task> todoList = new ArrayList<>();
+        List<Task> todoList = new ArrayList<Task>();
+        try{
+            FileReader fileReader = new FileReader("./data/duke.txt");
+            Scanner fileScanner = new Scanner(fileReader);
+            while(fileScanner.hasNextLine()){
+                String line = fileScanner.nextLine();
+                if(line.equals("\n")){
+                    break;
+                }
+                String[] task = line.split(" \\| ");
+                if(task[0].equals("T")){
+                    todoList.add(new Todo (task[2]));
+                }else if(task[0].equals("D")){
+                    todoList.add(new Deadline (task[2], "by "+task[3]));
+                }else if(task[0].equals("E")){
+                    todoList.add(new Event (task[2], "from "+task[3], "to "+task[4]));
+                }
+                if(task[1].equals("1")){
+                    todoList.get(todoList.size()-1).markAsDone();
+                }
+            }
+            fileScanner.close();
+            fileReader.close();
+        }catch(IOException e){
+            try{
+                File file = new File("./data/duke.txt");
+                file.getParentFile().mkdirs();
+                file.createNewFile();
+            }catch(IOException ex){
+                System.out.println("☹ OOPS!!! The file has existed.");
+            }
+        }catch(DukeException e){
+            System.out.println("☹ OOPS!!! Wrong format in file.");
+        }
         print(GREET);
         while(true){
             String input = scanner.nextLine();
@@ -32,11 +82,12 @@ public static void main(String[] args) {
             }else if(input.matches("^mark \\d+$")) {
                 int index = Integer.parseInt(input.substring(5));
                 try{
-                    todoList.get(index-1).unmarkAsDone();
+                    todoList.get(index-1).markAsDone();
                 }catch(IndexOutOfBoundsException e){
                     print("☹ OOPS!!! The task number is not in the list.");
                     continue;
                 }
+                writeToFile(todoList);
                 print(MARK+"\n  "+todoList.get(index-1));
             }else if(input.matches("^unmark \\d+$")) {
                 int index = Integer.parseInt(input.substring(7));
@@ -46,37 +97,63 @@ public static void main(String[] args) {
                     print("☹ OOPS!!! The task number is not in the list.");
                     continue;
                 }
+                writeToFile(todoList);
                 print(UNMARK+"\n  "+todoList.get(index-1));
             }else if(input.split(" ")[0].equals("todo")){
                 Todo todo;
                 try{
-                    todo = new Todo(input.substring(4, input.length()));
+                    todo = new Todo(input.substring(5, input.length()));
                 }catch(DukeException e){
                     continue;
+                }catch(StringIndexOutOfBoundsException e){
+                    print("☹ OOPS!!! The description of a todo cannot be empty.");
+                    continue;
                 }
                 todoList.add(todo);
+                writeToFile(todoList);
                 print("Got it. I've added this task:\n  "+todo+"\nNow you have " + todoList.size() + " tasks in the list.");
             }else if(input.split(" ")[0].equals("deadline")){
                 Deadline deadline;
                 try {
-                    deadline = new Deadline(input.split("/")[0].substring(9, input.split("/")[0].length() - 1), input.split("/")[1].replace("by ", ""));
+                    deadline = new Deadline(input.split(" /")[0].substring(9), input.split(" /")[1]);
                 }catch (DukeException e){
                     continue;
+                }catch (ArrayIndexOutOfBoundsException e){
+                    print("☹ OOPS!!! The description of a deadline should use / mark time.");
+                    continue;
+                }catch(StringIndexOutOfBoundsException e){
+                    print("☹ OOPS!!! The description of a deadline cannot be empty.");
+                    continue;
                 }
                 todoList.add(deadline);
+                writeToFile(todoList);
                 print("Got it. I've added this task:\n  "+deadline+"\nNow you have " + todoList.size() + " tasks in the list.");
-            }else if(input.split(" ")[0].equals("event")){
+            }else if(input.split(" ")[0].equals("event")) {
                 Event event;
-                try{
-                    event = new Event(input.split("/")[0].substring(5, input.split("/")[0].length()-1), input.split("/")[1], input.split("/")[2]);
-                }catch(DukeException e){
+                try {
+                    event = new Event(input.split(" /")[0].substring(6), input.split(" /")[1], input.split(" /")[2]);
+                }catch(DukeException e) {
+                    continue;
+                }catch(ArrayIndexOutOfBoundsException e) {
+                    print("☹ OOPS!!! The description of a event should use / mark start time and end time.");
                     continue;
                 }catch(StringIndexOutOfBoundsException e){
-                    print("☹ OOPS!!! The description of a event should use \\ mark start time and end time.");
+                    print("☹ OOPS!!! The description of a event cannot be empty.");
                     continue;
                 }
+                writeToFile(todoList);
                 todoList.add(event);
-                print("Got it. I've added this task:\n  "+event+"\nNow you have " + todoList.size() + " tasks in the list.");
+                print("Got it. I've added this task:\n  " + event + "\nNow you have " + todoList.size() + " tasks in the list.");
+            }else if(input.split(" ")[0].equals("delete")){
+                int index = Integer.parseInt(input.substring(7));
+                try{
+                    Task task = todoList.get(index-1);
+                    todoList.remove(index-1);
+                    writeToFile(todoList);
+                    print("Noted. I've removed this task:\n  "+task+"\nNow you have " + todoList.size() + " tasks in the list.");
+                }catch(IndexOutOfBoundsException e){
+                    print("☹ OOPS!!! The task number is not in the list.");
+                }
             }else{
                 print("☹ OOPS!!! I'm sorry, but I don't know what that means :-(");
             }

src/main/java/DukeException.java

@@ -1,9 +1,9 @@
 public class DukeException extends Exception{
-    protected String line = "_______________________________________________";
+    protected String LINE = "_______________________________________________";
     public DukeException(String message){
         super(message);
-        System.out.println(line);
+        System.out.println(LINE);
         System.out.println(message);
-        System.out.println(line);
+        System.out.println(LINE);
     }
 }

src/main/java/Event.java

@@ -7,13 +7,23 @@ public Event(String description, String from, String by) throws DukeException{
             throw new DukeException("☹ OOPS!!! The description of a event must contain \"from\" time.");
         }else if(!by.contains("to ")){
             throw new DukeException("☹ OOPS!!! The description of a event must contain \"to\" time.");
+        }else if(from.replace("from ", "").isBlank()){
+            throw new DukeException("☹ OOPS!!! The start time of a event cannot be empty.");
+        }else if(by.replace("to ", "").isBlank()){
+            throw new DukeException("☹ OOPS!!! The end time of a event cannot be empty.");
         }
-        this.from = from;
-        this.by = by;
+        this.from = from.replace("from ", "");
+        this.by = by.replace("to ", "");
     }
 
     @Override
     public String toString() {
         return "[E]" + super.toString() + " (from: " + this.from + " to: " + this.by + ")";
     }
+
+
+    @Override
+    public String toFile(){
+        return "E | " + super.toFile() + " | " + this.from + " | " + this.by;
+    }
 }

src/main/java/Task.java

@@ -11,7 +11,7 @@ public Task(String description) throws DukeException {
     }
 
     public String getStatusIcon() {
-        return (isDone ? "X" : " "); // mark done task with X
+        return (isDone ? "X" : " ");
     }
 
     public void markAsDone() {
@@ -24,6 +24,10 @@ public void unmarkAsDone(){
 
     @Override
     public String toString() {
-        return "[" + getStatusIcon() + "]" + this.description;
+        return "[" + getStatusIcon() + "] " + this.description;
+    }
+
+    public String toFile(){
+        return (isDone ? "1" : "0") + " | " + this.description;
     }
 }

src/main/java/Todo.java

@@ -7,4 +7,9 @@ public Todo(String description) throws DukeException{
     public String toString() {
         return "[T]" + super.toString();
     }
+
+    @Override
+    public String toFile(){
+        return "T | " + super.toFile();
+    }
 }