C-sort

src/main/java/Parser.java

@@ -94,6 +94,9 @@ void parse(String input) throws DukeException {
             }
             tasks.find(input.substring(5));
             break;
+        case "sort":
+            tasks.sort();
+            break;
         default:
             throw new DukeException("I'm sorry, but I don't know what that means :-(");
         }

src/main/java/Task/Task.java

@@ -5,7 +5,7 @@
 /**
  * Represents a task.
  */
-public class Task implements java.io.Serializable {
+public class Task implements java.io.Serializable, Comparable<Task> {
     public String text;
     public boolean isDone;
     LocalDate date;
@@ -25,4 +25,26 @@ public Task(String text, LocalDate date) {
     public String toString() {
         return "[" + (isDone ? "✓" : "✗") + "] " + text;
     }
+
+    /**
+     * Compares this task with the specified task for order based on date. The task with no date is considered greater.
+     * Otherwise, the task with a later date is considered greater.
+     *
+     * @param t the object to be compared.
+     * @return a negative integer, zero, or a positive integer as this object
+     * is less than, equal to, or greater than the specified object.
+     * @throws NullPointerException if the specified object is null
+     * @throws ClassCastException   if the specified object's type prevents it
+     *                              from being compared to this object.
+     */
+    @Override
+    public int compareTo(Task t) {
+        if(this.date == null){
+            return 1;
+        } else if (t.date == null){
+            return -1;
+        } else {
+            return this.date.compareTo(t.date);
+        }
+    }
 }

src/main/java/TaskList.java

@@ -1,6 +1,7 @@
 import Task.Task;
 
 import java.util.ArrayList;
+import java.util.Collections;
 
 /**
  * Stores tasks and supports various operations on them.
@@ -43,15 +44,19 @@ public String numTasks() {
      * Prints the list of tasks through the UI class
      */
     public void print_tasks() {
+        UI.print(this.toString());
+    }
+
+    @Override
+    public String toString(){
         if(tasks.size() == 0){
-            UI.print("There are no tasks!\n");
-            return;
+            return "There are no tasks!\n";
         }
         StringBuilder sb = new StringBuilder();
         for (int i = 0; i < tasks.size(); i++) {
-            sb.append((i + 1) + "." + tasks.get(i).toString());
+            sb.append(i + 1).append(".").append(tasks.get(i).toString());
         }
-        UI.print(sb.toString());
+        return sb.toString();
     }
 
     /**
@@ -111,4 +116,12 @@ public void find(String substring) {
             UI.print("No match found!\n");
         }
     }
+
+    /**
+     * Sorts tasks by date then prints the new list.
+     */
+    public void sort(){
+        Collections.sort(tasks);
+        UI.print("Tasks sorted by date.\n" + this.toString());
+    }
 }