Add assertion statements

data/duke.txt

@@ -0,0 +1 @@
+T | 0 | ask

src/main/java/duke/command/AddCommand.java

@@ -19,6 +19,7 @@ public class AddCommand extends Command {
      * @param task task to be added
      */
     public AddCommand(Task task) {
+        assert task != null : "task cannot be null";
         this.task = task;
     }
 

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

@@ -45,12 +45,15 @@ public boolean isExit() {
     public String execute(TaskList tasks, Ui ui, Storage storage) throws DukeException {
         try {
             Task deletedTask = tasks.deleteTaskAtPos(this.index);
+            assert this.index <= tasks.getCount() + 1 && this.index > 0 :
+                    "Position argument should be more than 0 and less than or equal to the task list size";
             storage.save(tasks);
             return ui.showDeleted(deletedTask);
         } catch (IndexOutOfBoundsException e) {
             if (tasks.getCount() == 0){
                 throw new DukeException(Message.INVALID_ACCESS_EMPTY_TASKLIST);
             } else {
+                assert tasks.getCount() > 0 : "task list should have 1 or more tasks";
                 throw new DukeException(Message.returnTaskNotFound(tasks));
             }
         }

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

@@ -10,6 +10,7 @@ public class FindCommand extends Command {
     private String keyword;
 
     public FindCommand(String keyword) {
+        assert keyword != null : "keyword cannot be null";
         this.keyword = keyword;
     }
 

src/main/java/duke/command/MarkCommand.java

@@ -46,12 +46,15 @@ public String execute(TaskList tasks, Ui ui, Storage storage) throws DukeExcepti
         try {
             tasks.markTaskAtPos(this.index);
             Task currentTask = tasks.getTask(this.index);
+            assert this.index <= tasks.getCount() && this.index > 0 :
+                    "Index should be more than 0 and less than or equal to the task list size";
             storage.save(tasks);
             return ui.showMarked(currentTask);
         } catch (IndexOutOfBoundsException e) {
             if (tasks.getCount() == 0) {
                 throw new DukeException(Message.INVALID_ACCESS_EMPTY_TASKLIST);
             } else {
+                assert tasks.getCount() > 0 : "task list should have 1 or more tasks";
                 throw new DukeException(Message.returnTaskNotFound(tasks));
             }
         }

src/main/java/duke/command/UnmarkCommand.java

@@ -46,12 +46,15 @@ public String execute(TaskList tasks, Ui ui, Storage storage) throws DukeExcepti
         try {
             tasks.unmarkTaskAtPos(this.index);
             Task currentTask = tasks.getTask(this.index);
+            assert this.index <= tasks.getCount() && this.index > 0 :
+                    "Position argument should be more than 0 and less than or equal to the task list size";
             storage.save(tasks);
             return ui.showUnmarked(currentTask);
         } catch (IndexOutOfBoundsException e) {
             if (tasks.getCount() == 0) {
                 throw new DukeException(Message.INVALID_ACCESS_EMPTY_TASKLIST);
             } else {
+                assert tasks.getCount() > 0 : "task list should have 1 or more tasks";
                 throw new DukeException(Message.returnTaskNotFound(tasks));
             }
         }

src/main/java/duke/parser/Parser.java

@@ -8,6 +8,7 @@
 import duke.task.ToDo;
 
 import java.time.LocalDate;
+import java.util.Arrays;
 
 public class Parser {
 
@@ -114,10 +115,16 @@ private static DeleteCommand deleteTask(String command) throws DukeException {
     }
 
     private static FindCommand findTask(String command) throws DukeException {
-        String[] commandList = command.strip().split(" ");
+        String[] commandList = command.strip().split(" ", 2);
         try {
-            return new FindCommand(commandList[1]);
+            String keyword = commandList[1].strip();
+            if (keyword.equals("")) {
+                throw new DukeException(Message.INVALID_FIND_TASK_FORMAT);
+            }
+            assert !keyword.equals("") : "keyword cannot be empty";
+            return new FindCommand(keyword);
         } catch (IndexOutOfBoundsException e) {
+            assert commandList.length < 2 : "Command list should contain at most 1 element";
             throw new DukeException(Message.INVALID_FIND_TASK_FORMAT);
         }
     }

src/main/java/duke/task/TaskList.java

@@ -66,6 +66,8 @@ public int getCount(){
      */
     public void markTaskAtPos(int position) throws IndexOutOfBoundsException{
         Task currTask = getTask(position);
+        assert position <= this.count && position > 0 :
+                "Position argument should be more than 0 and less than or equal to the task list size";
         currTask.markAsDone();
     }
 
@@ -77,6 +79,8 @@ public void markTaskAtPos(int position) throws IndexOutOfBoundsException{
      */
     public void unmarkTaskAtPos(int position) throws IndexOutOfBoundsException{
         Task currTask = getTask(position);
+        assert position <= this.count && position > 0 :
+                "Position argument should be more than 0 and less than or equal to the task list size";
         currTask.unmark();
     }
 
@@ -90,6 +94,8 @@ public void unmarkTaskAtPos(int position) throws IndexOutOfBoundsException{
      */
     public Task deleteTaskAtPos(int position) throws IndexOutOfBoundsException {
         Task deletedTask = getTask(position);
+        assert position <= this.count && position > 0 :
+                "Position argument should be more than 0 and less than or equal to the task list size";
         this.taskArray.remove(position - 1);
         this.count -= 1;
         return deletedTask;