Add compareTo for Task

src/main/java/seedu/task/model/task/Task.java

@@ -12,7 +12,7 @@
  * Represents a Task in the task list.
  * Guarantees: field values are validated.
  */
-public class Task implements ReadOnlyTask {
+public class Task implements ReadOnlyTask, Comparable<Task> {
 
     private Name name;
     private DateTime openTime;
@@ -149,4 +149,26 @@ public int hashCode() {
     public String toString() {
         return getAsText();
     }
+
+    //@@author A0141052Y
+    /**
+     * Uses Comparator.compareTo.
+     * The comparison is based on:<br/>
+     * - Starred tasks have higher priority<br/>
+     * - Completed tasks have lower priority<br/>
+     * - For tasks with same completion/starred status,
+     *   base comparison on endDate only
+     */
+    @Override
+    public int compareTo(Task o) {
+        if ((this.isCompleted || o.getComplete()) && !(this.isCompleted && o.getComplete())) {
+            return (this.isCompleted) ? 1 : -1;
+        } else if ((this.isImportant || o.getImportance()) && !(this.isImportant && o.getImportance())) {
+            return (this.isImportant) ? -1 : 1;
+        } else if (!this.closeTime.isEmpty() && !o.getCloseTime().isEmpty()) {
+            return this.closeTime.compareTo(o.getCloseTime());
+        } else {
+            return 0;
+        }
+    }
 }