• Change the ball's X or Y velocity depending on the key pressed.
• This situation is different from single key presses like pressing Escape to exit the game. For single strokes, we use a KeyListener.
• This situation involves processing multiple key presses during a game. For example, the user could press the left and up arrow simultaneously to move the ball. This requires the game engine's MultiKeyListener which keeps track of all the strokes. We use the onKeysPolled method for this:

  @Override
  protected void onKeysPolled(int[] keyCodes) {
            
    for (int code : keyCodes) {
      switch(code) {
        case KeyEvent.VK_W:
        case KeyEvent.VK_UP:
          // change the ball's X velocity here
          break;
                  
        case ?
          // add other cases here
                  
        default: // this is needed to make the ball stop
                 // when no keys are pressed
          ball.setXVelocity(0);
          ball.setYVelocity(0);
         }
      }
    }

Read about setXVelocity, setYVelocity, KeyEvent, and MultiKeyListener.

• Set the coefficient of restitution for your ball in setAppearance().
• Set the ball's X and Y velocities and Y acceleration in the controller when you add the ball entity.
• Use setRotationAngle to add spin to the ball in your sprite model's updateParameters method.
• Use the Physics.rebound method in the onOutOfBounds method to make your ball rebound when it collides with a wall:

 @Override
  protected void onOutOfBounds(OutOfBoundsEvent[] events) {
		for (OutOfBoundsEvent e : events) {
			ScreenBoundary sb = e.getBoundary();
			EntityModel em = e.getEntity();
			Physics.rebound(em, sb);
		}
  }

• Experiment until you get the behavior you want.

Read about setCoefficientOfRestitution, setRotationAngle, ScreenBoundary, and Physics.

What is happening when the ball reaches the bottom of the screen? Read about it in the Book. We'll handle this situation in the next exercise.

• Add a mouse listener.
• Add the code to generate a new ball:

@Override
public void mouseReleased(MouseEvent e) {
  addBall(e.getX(), e.getY());
}

• In your updateParameters method, handle the "stopped" balls, those that reach the bottom of the screen with their velocity approaching 0 and still having acceleration downward. Use notifyController to send a message about the situation.

System.out.println(speed);
if (speed <= 10.0f)
  notifyController("stopped");

• In your controller, check for the message, print a message to the console, and dispose of the ball.

@Override
protected void onMessagesPolled(MessageEvent[] events) {
	for (MessageEvent me : events) {
		String msg = me.getDescription();
		EntityModel em = me.getSender();
		if (msg.equalsIgnoreCase("stopped")) {
			System.out.println("The ball has stopped");
			em.dispose();
		}
	}
}

If you want, you can show the Collision Bounds width and height (hit box width and height) for each ball using showCollisionBounds. Later, it may be useful to adjust this.

Read about MouseListener, Physics, .dispose, EntityModel, notifyController, and onMessagesPolled.

• Here's a class diagram containing the classes you'll need for the exercise:

• Create a scoreboard by extending TextModel.
• The scoreboard needs some attributes:

private int score = 0;
private String message;

• Set the scoreboard's appearance (be creative with this if you want):

@Override
protected void setAppearance() {
  Font font = new Font("Courier New", Font.BOLD|Font.PLAIN, 40);
  message = String.format("Score: %03d", score);
  setView(message, font, Color.YELLOW);
} 

• Increment the score in onOutOfBounds:

@Override
	protected void onOutOfBounds(OutOfBoundsEvent[] events) {
		for (OutOfBoundsEvent e : events) {
			ScreenBoundary sb = e.getBoundary();
			// more here
			if (sb.getEdge() == Direction.South)
				sboard.increment(); 
		}
	}

• You can use the TextModel's modifyMessage method to change the message contained in the scoreboard text entity.
• Code updateGameState() to check for the end of the game:

@Override
protected void updateGameState() {
	if (sboard.getScore() >= 10) {
		sboard.modifyMessage("Game Over!");
		gameOver = true;
	}
}

• Use setGhost(true) for the scoreboard since we don't want it involved in collision processing.

Read about TextModel, ScreenBoundary, and updateGameState.

Exercise 3E_Barrier

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Put a sloping barrier in the window and call the .rebound() method in onBlocked() when a soccer ball hits the barrier.

If you would like, go back and use a BarrierModel to fix the "stopped" balls in 3C.

Read about BarrierModel.

Read about setFlipped.

Challenge 3G_JumpingSprite

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Create a game where a green man (greenman-4.png) can move left and right with the arrow keys. Make the man able to jump using the spacebar giving him the acceleration of gravity. Create a bee that appears at the right edge of the screen, moves to the left, and then wraps around to appear at the right again. Use a SceneryModel (ufoscene-1.png) to create a static background for the game. Each time the bee crosses the screen and the man successfully avoids getting stung by jumping over it, a point is scored. Make a scoreboard that shows the total points. When three bee stings have been avoided, display “Game Over”.

Tip #1. With most games, you'll have to tweak the velocities, acceleration, and sprite hit boxes. Use showCollisionBounds and setCollisionBounds to experiment.

Tip #2. For this challenge, you may run into a situation where a BarrierModel will come in handy to solve your problem.

Challenge 3H_BeachPailGame

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Make a pail move left and right across the bottom of the screen using keystrokes. The game should begin by automatically releasing a single beach ball from the top of the screen. Move the pail left and right to collect balls. When the ball comes in contact with the pail, the ball should disappear, the score should be incremented, and another ball should be released. When 10 balls have been “collected,” display “Game Over”.

Tip: If you have objects created during the game loop, don’t do it in enlistEntities. enlistEntities is done once at the beginning for objects that are present at the beginning of the game.

For all exercises and challenges, try to incorporate a few features of your own. Get creative and make it unique!

Draw it online