Add assertions in Command classes

src/main/java/duke/command/DeleteCommand.java

@@ -21,6 +21,7 @@ public DeleteCommand(String command) {
     @Override
     public String execute(TaskList tasks, Ui ui, Storage storage) throws DukeException {
         String[] inputTokens = this.command.split(" ");
+        assert inputTokens[0] == "delete";
         int deleteIndex;
 
         try {

src/main/java/duke/command/DoneCommand.java

@@ -21,6 +21,7 @@ public DoneCommand(String command) {
     @Override
     public String execute(TaskList tasks, Ui ui, Storage storage) throws DukeException {
         String[] inputTokens = this.command.split(" ");
+        assert inputTokens[0] == "done";
 
         // Mark the task with given index as done
         int doneIndex;

src/main/java/duke/command/ExitCommand.java

@@ -18,6 +18,7 @@ public ExitCommand(String command) {
      */
     @Override
     public String execute(TaskList tasks, Ui ui, Storage storage) {
+        assert command.split(" ")[0] == "bye";
         return ui.showGoodBye();
     }
 }

src/main/java/duke/command/FindCommand.java

@@ -29,6 +29,7 @@ public FindCommand(String command) {
     @Override
     public String execute(TaskList tasks, Ui ui, Storage storage) {
         String[] inputTokens = this.command.split(" ");
+        assert inputTokens[0] == "find";
         String dateString = (inputTokens.length > 1) ? inputTokens[1] : "";
         TaskList filteredTasks = filterByString(tasks, dateString);
         return ui.prettyPrintList(filteredTasks);

src/main/java/duke/command/ListCommand.java

@@ -24,6 +24,7 @@ public ListCommand(String command) {
     @Override
     public String execute(TaskList tasks, Ui ui, Storage storage) {
         String[] inputTokens = this.command.split(" ");
+        assert inputTokens[0] == "list";
         String dateString = (inputTokens.length > 1) ? inputTokens[1] : "";
         TaskList filteredTasks = filterTasksByDate(tasks, dateString);
         return ui.prettyPrintList(filteredTasks);