Using KeyListener with Java

Question

For a homework assignment I need to create a program that essentially displays a ball and the user should be able to move it using the left and right keys. However, the program isn't responding to the keys. I don't know where the bug is, and I'd appreciate it very much if someone could help! This is the code:

public class GraphicsComponent extends JComponent
{
Ellipse2D.Double ball = new Ellipse2D.Double(200, 400, 80, 80);

     public void paintComponent(Graphics g)
     {
         Graphics2D g2 = (Graphics2D) g;
         g2.setColor(Color.RED); 
         g2.fill(ball); 
         g2.draw(ball); 
     }

}


public class BallViewer
{
    public static void main(String[] args)
    {
        JFrame frame = new JFrame(); //creates a new JFrame called frame

        frame.setSize(600,600); //invokes the method setSize on the implicit parameter frame
        frame.setTitle("Move this Ball"); //sets the title of the fram
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        final GraphicsComponent g = new GraphicsComponent(); //creates a new GraphicsComponent called g, is final so that the inner class can access it
        frame.add(g);//adds component g to the frame

        frame.setVisible(true); //sets the visibility of the frame

        class PressListener implements KeyListener //creates an inner class that implements MouseListener interface
        {
            public void keyPressed(KeyEvent e)
            {

                if (e.getKeyCode() == KeyEvent.VK_LEFT)
                {
                    System.out.println("Left key pressed");
                }

                if (e.getKeyCode() == KeyEvent.VK_RIGHT)
                {
                    System.out.println("Right key pressed");
                }
            }

            public void keyReleased(KeyEvent e)
            {
            }

            public void keyTyped(KeyEvent e)
            {
            }
        }

        PressListener listener = new PressListener(); 
        g.addKeyListener(listener);
    }
}

Answer 1

KeyListener will only respond when the component it's registered is focusable AND has focusable, JComponent is not focusable by default.

Instead, use key bindings, they save you all the hassel of messing with focus issues related to KeyListener

You're also going to have problems with making the Ellipse2D move, start by setting it's position as 0x0 and translating the Graphics context to the position you want to paint the ball

As an example...

InputMap im = getInputMap(WHEN_IN_FOCUSED_WINDOW);
ActionMap am = getActionMap();

im.put(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), "up");
im.put(KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0), "down");
im.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0), "left");
im.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0), "right");

am.put("up", new DeltaAction(0, -10));
am.put("down", new DeltaAction(0, 10));
am.put("left", new DeltaAction(-10, 0));
am.put("right", new DeltaAction(10, 0));

And the DeltaAction...point is the location at which the Ellipse is painted...

public class DeltaAction extends AbstractAction {

    private int deltaX;
    private int deltaY;

    public DeltaAction(int deltaX, int deltaY) {
        this.deltaX = deltaX;
        this.deltaY = deltaY;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        point.x += deltaX;
        point.y += deltaY;

        if (point.x < 0) {
            point.x = 0;
        } else if (point.x + DIAMETER >= getWidth()) {
            point.x = getWidth() - DIAMETER;
        }
        if (point.y < 0) {
            point.y = 0;
        } else if (point.y + DIAMETER >= getHeight()) {
            point.y = getHeight() - DIAMETER;
        }
        repaint();
    }

}