Refactor the codebase to remove unnecessary runtime exceptions

src/main/java/duke/Duke.java

@@ -6,7 +6,8 @@
 import duke.parser.Parser;
 import duke.storage.Storage;
 import duke.task.TaskList;
-import duke.ui.Ui;
+
+import java.io.IOException;
 
 /**
  * A task management application.
@@ -16,20 +17,14 @@ public class Duke {
     private boolean isActive;
     private Storage storage;
     private TaskList tasks;
-    private Ui ui;
 
     /**
      * Returns a {@link Duke}.
      */
-    public Duke() {
+    public Duke() throws DukeException, IOException {
         isActive = true;
-        ui = new Ui();
         storage = new Storage(STORAGE_PATHNAME);
-        try {
-            tasks = new TaskList(storage.load());
-        } catch (DukeException e) {
-            tasks = new TaskList();
-        }
+        tasks = new TaskList(storage.load());
     }
 
     /**
@@ -38,10 +33,10 @@ public Duke() {
      * @param input The input to parse and execute
      * @return The response from Duke.
      */
-    public String getResponse(String input) {
+    public String getResponse(String input) throws IOException {
         try {
             Command c = Parser.parse(input);
-            c.setData(tasks, ui, storage);
+            c.setData(tasks, storage);
             CommandResult result = c.execute();
             isActive = !c.isExit();
             return result.feedback;

src/main/java/duke/Main.java

@@ -1,5 +1,6 @@
 package duke;
 
+import duke.exception.DukeException;
 import duke.ui.MainWindow;
 import javafx.application.Application;
 import javafx.fxml.FXMLLoader;
@@ -13,6 +14,9 @@ public class Main extends Application {
 
     private Duke duke = new Duke();
 
+    public Main() throws IOException, DukeException {
+    }
+
     @Override
     public void start(Stage primaryStage) {
         try {

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

@@ -2,8 +2,12 @@
 
 import duke.exception.DukeException;
 import duke.task.Task;
+import duke.ui.Ui;
+
+import java.io.IOException;
 
 import static java.lang.System.lineSeparator;
+import static java.util.Objects.requireNonNull;
 
 /**
  * Represents a {@link Command} to add a {@link Task}.
@@ -12,20 +16,26 @@ public class AddCommand extends Command {
 
     private final Task toAdd;
 
+    /**
+     * Creates a {@link Command} that will create a {@link Task} when executed.
+     *
+     * @param task the Task to be created
+     */
     public AddCommand(Task task) {
-        this.toAdd = task;
+        this.toAdd = requireNonNull(task);
     }
 
     @Override
-    public CommandResult execute() throws DukeException {
+    public CommandResult execute() throws IOException {
         tasks.add(toAdd);
         String data = toAdd.stringify();
         if (tasks.size() > 1) {
             data = lineSeparator() + data;
         }
         storage.append(data);
         return new CommandResult("Got it. I've added this task:"
-                + lineSeparator() + ui.INDENT + toAdd
-                + lineSeparator() + "Now you have " + tasks.size() + " tasks in the list.");
+                + lineSeparator() + Ui.INDENT + toAdd
+                + lineSeparator() + "Now you have " + tasks.size() + " tasks in the list."
+        );
     }
 }

src/main/java/duke/command/Command.java

@@ -4,15 +4,17 @@
 import duke.exception.DukeException;
 import duke.storage.Storage;
 import duke.task.TaskList;
-import duke.ui.Ui;
+
+import java.io.IOException;
+
+import static java.util.Objects.requireNonNull;
 
 /**
  * Represents a user command.
  */
 public abstract class Command {
 
     protected TaskList tasks;
-    protected Ui ui;
     protected Storage storage;
     protected boolean isExit;
 
@@ -21,19 +23,17 @@ public abstract class Command {
      *
      * @throws DukeException If the command failed to execute.
      */
-    public abstract CommandResult execute() throws DukeException;
+    public abstract CommandResult execute() throws DukeException, IOException;
 
     /**
      * Sets the data to be used by the {@link Command}.
      *
-     * @param tasks List of tasks.
-     * @param ui UI interface.
+     * @param tasks   List of tasks.
      * @param storage Storage.
      */
-    public void setData(TaskList tasks, Ui ui, Storage storage) {
-        this.tasks = tasks;
-        this.ui = ui;
-        this.storage = storage;
+    public void setData(TaskList tasks, Storage storage) {
+        this.tasks = requireNonNull(tasks);
+        this.storage = requireNonNull(storage);
     }
 
     /**

src/main/java/duke/command/DeadlineCommand.java

@@ -10,8 +10,11 @@
 public class DeadlineCommand extends AddCommand {
 
     public static final String COMMAND_WORD = "deadline";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Adds a task with a deadline" + System.lineSeparator()
+            + "Usage: " + COMMAND_WORD + " <description> /by <dueDate>";
 
-    public DeadlineCommand(String description, Date by) {
-        super(new Deadline(description, by));
+    public DeadlineCommand(String description, Date dueDate) {
+        super(new Deadline(description, dueDate));
     }
 }

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

@@ -1,7 +1,9 @@
 package duke.command;
 
-import duke.exception.DukeException;
 import duke.task.Task;
+import duke.ui.Ui;
+
+import java.io.IOException;
 
 import static java.lang.System.lineSeparator;
 
@@ -11,18 +13,30 @@
 public class DeleteCommand extends Command {
 
     public static final String COMMAND_WORD = "delete";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Deletes the task with the specified ID" + System.lineSeparator()
+            + "Usage: " + COMMAND_WORD + " <id>";
     private final int toDelete;
 
+    /**
+     * Creates a {@link Command} that deletes a {@link Task} when executed.
+     *
+     * @param id the id of the {@link Task} to be deleted.
+     */
     public DeleteCommand(int id) {
         this.toDelete = id;
     }
 
     @Override
-    public CommandResult execute() throws DukeException {
+    public CommandResult execute() throws IOException {
+        if (toDelete < 0 || toDelete >= tasks.size()) {
+            return new IncorrectCommand(MESSAGE_USAGE).execute();
+        }
         final Task task = tasks.delete(toDelete);
         storage.save(tasks.stringify());
         return new CommandResult("Noted. I've removed this task:"
-                + lineSeparator() + ui.INDENT + task
-                + lineSeparator() + "Now you have " + tasks.size() + " tasks in the list.");
+                + lineSeparator() + Ui.INDENT + task
+                + lineSeparator() + "Now you have " + tasks.size() + " tasks in the list."
+        );
     }
 }

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

@@ -1,7 +1,9 @@
 package duke.command;
 
-import duke.exception.DukeException;
 import duke.task.Task;
+import duke.ui.Ui;
+
+import java.io.IOException;
 
 import static java.lang.System.lineSeparator;
 
@@ -11,23 +13,29 @@
 public class DoneCommand extends Command {
 
     public static final String COMMAND_WORD = "done";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Marks the task with the specified ID as complete"
+            + lineSeparator() + "Usage: " + COMMAND_WORD + " <id>";
 
     private int toMarkAsDone;
 
+    /**
+     * Creates a {@link Command} that marks a {@link Task} as complete when executed.
+     *
+     * @param id the id of the task to be marked as complete
+     */
     public DoneCommand(int id) {
         this.toMarkAsDone = id;
     }
 
     @Override
-    public CommandResult execute() throws DukeException {
-        try {
-            Task task = tasks.get(toMarkAsDone);
-            task.markAsDone();
-            storage.save(tasks.stringify());
-            return new CommandResult("Nice! I've marked this task as done:"
-                    + lineSeparator() + ui.INDENT + task);
-        } catch (IndexOutOfBoundsException e) {
-            throw new DukeException("Marking task with ID " + toMarkAsDone + " failed: Invalid ID");
+    public CommandResult execute() throws IOException {
+        if (toMarkAsDone < 0 || toMarkAsDone >= tasks.size()) {
+            return new IncorrectCommand(MESSAGE_USAGE).execute();
         }
+        Task task = tasks.get(toMarkAsDone);
+        task.markAsDone();
+        storage.save(tasks.stringify());
+        return new CommandResult("Nice! I've marked this task as done:" + lineSeparator() + Ui.INDENT + task);
     }
 }

src/main/java/duke/command/EventCommand.java

@@ -10,8 +10,11 @@
 public class EventCommand extends AddCommand {
 
     public static final String COMMAND_WORD = "event";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Creates a task with a date" + System.lineSeparator()
+            + "Usage: " + COMMAND_WORD + " <description> /at <date>";
 
-    public EventCommand(String description, Date at) {
-        super(new Event(description, at));
+    public EventCommand(String description, Date date) {
+        super(new Event(description, date));
     }
 }

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

@@ -4,14 +4,18 @@
 import duke.task.TaskList;
 
 import static java.lang.System.lineSeparator;
+import static java.util.Objects.requireNonNull;
 
 public class FindCommand extends Command {
 
     public static final String COMMAND_WORD = "find";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Finds and displays a list of Tasks that match the keyword"
+            + lineSeparator() + "Usage: " + COMMAND_WORD + " <keyword>";
     private final String keyword;
 
     public FindCommand(String keyword) {
-        this.keyword = keyword;
+        this.keyword = requireNonNull(keyword);
     }
 
     @Override

src/main/java/duke/command/IncorrectCommand.java

@@ -1,12 +1,20 @@
 package duke.command;
 
+import static java.util.Objects.requireNonNull;
+
 /**
  * Represents a {@link Command} to inform the user that he has entered an incorrect command.
  */
 public class IncorrectCommand extends Command {
 
+    private final String message;
+
+    public IncorrectCommand(String message) {
+        this.message = requireNonNull(message);
+    }
+
     @Override
     public CommandResult execute() {
-        return new CommandResult("☹ OOPS!!! I'm sorry, but I don't know what that means :-(");
+        return new CommandResult(message);
     }
 }

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

@@ -1,6 +1,7 @@
 package duke.command;
 
 import duke.task.Task;
+import duke.ui.Ui;
 
 import static java.lang.System.lineSeparator;
 
@@ -20,7 +21,7 @@ public CommandResult execute() {
         StringBuilder feedback = new StringBuilder("Here are the tasks in your list:");
         for (int i = 1; i <= tasks.size(); i++) {
             Task task = tasks.get(i - 1);
-            feedback.append(lineSeparator() + i + ". " + task);
+            feedback.append(lineSeparator() + Ui.INDENT + i + ". " + task);
         }
         return new CommandResult(feedback.toString());
     }

src/main/java/duke/command/ToDoCommand.java

@@ -2,12 +2,17 @@
 
 import duke.task.ToDo;
 
+import static java.lang.System.lineSeparator;
+
 /**
  * Represents a {@link Command} to add a {@link ToDo}.
  */
 public class ToDoCommand extends AddCommand {
 
     public static final String COMMAND_WORD = "todo";
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Create a new task" + lineSeparator()
+            + "Usage: " + COMMAND_WORD + " <description>";
 
     public ToDoCommand(String description) {
         super(new ToDo(description));

src/main/java/duke/command/UnknownCommand.java

@@ -0,0 +1,12 @@
+package duke.command;
+
+/**
+ * Represents a {@link Command} to inform the user that he has entered an unknown command.
+ */
+public class UnknownCommand extends Command {
+
+    @Override
+    public CommandResult execute() {
+        return new CommandResult("☹ OOPS!!! I'm sorry, but I don't know what that means :-(");
+    }
+}