How does one properly handle keypresses and repainting of a JComponent in the context of moving a ball around a screen?

Question

I thought I would try and write a program that would paint a ball, and when the arrow keys are pressed, would move the ball around the screen in the direction pressed. First

Answer 1

Please do have a look at this code example :

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class PaintingPanel
{
    private CustomPanel drawingArea;
    private ActionMap actionMap;
    private int x;
    private int y;

    private enum Direction 
    {
        UP(KeyEvent.VK_UP),
        DOWN(KeyEvent.VK_DOWN),
        LEFT(KeyEvent.VK_LEFT),
        RIGHT(KeyEvent.VK_RIGHT);

        public int key;

        private Direction(int key)
        {
            this.key = key;
        }

        public int getKey()
        {
            return key;
        }
    }

    private class PanelAction extends AbstractAction
    {
        private Direction arrow;

        public PanelAction(Direction a)
        {
            arrow = a;
        }

        @Override
        public void actionPerformed(ActionEvent ae)
        {
            switch(arrow)
            {
                case UP:
                    y -= 1;
                    break;
                case DOWN:
                    y += 1;
                    break;
                case LEFT:
                    x -= 1;
                    break;
                case RIGHT:
                    x += 1;
                    break;
            }
            drawingArea.setValues(x, y);
        }
    }

    public PaintingPanel()
    {
        x = y = 5;
    }

    private void displayGUI()
    {
        JFrame frame = new JFrame("Painting Panel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        drawingArea = new CustomPanel();
        int condition = JComponent.WHEN_IN_FOCUSED_WINDOW;
        InputMap inputMap = drawingArea.getInputMap(condition);
        actionMap = drawingArea.getActionMap();

        for (Direction arrow : Direction.values())
        {
            int key = arrow.getKey();
            String name = arrow.name();
            inputMap.put(KeyStroke.getKeyStroke(
                            key, 0), name);
            actionMap.put(name, new PanelAction(arrow));                    
        }

        frame.setContentPane(drawingArea);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String... args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                new PaintingPanel().displayGUI();
            }
        });
    }
}

class CustomPanel extends JPanel
{
    private final int SIZE = 500;
    private int x;
    private int y;
    private String text = "Hello World!";

    public CustomPanel()
    {
        x = y = 5;
        setOpaque(true);
        setBackground(Color.WHITE);
    }

    public void setValues(int x, int y)
    {
        this.x = x;
        this.y = y;
        repaint();
    }

    @Override
    public Dimension getPreferredSize()
    {
        return (new Dimension(SIZE, SIZE));
    }

    @Override
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.setColor(Color.GREEN.darker());
        g.fillOval(x, y, 50, 50);
    }
}

Answer 2

You can use the KeyboardFocusManager to listen for keystrokes of the arrow keys and update the position of the ball. Here is the reworked code:

class BallFrame extends JFrame {
    private static final int DEFAULT_WIDTH = 500;
    private static final int DEFAULT_HEIGHT = 500;
    final BallComponent ball = new BallComponent();

    public BallFrame() {
        super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        super.setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
        super.setResizable(false);
        super.add(ball);
        super.setVisible(true);

        KeyboardFocusManager.getCurrentKeyboardFocusManager()
                .addKeyEventDispatcher(new KeyEventDispatcher() {
                    @Override
                    public boolean dispatchKeyEvent(KeyEvent e) {
                        if (e.getKeyCode() == KeyEvent.VK_UP)
                            ball.move(0, -10);
                        if (e.getKeyCode() == KeyEvent.VK_RIGHT)
                            ball.move(10, 0);
                        if (e.getKeyCode() == KeyEvent.VK_LEFT)
                            ball.move(-10, 0);
                        if (e.getKeyCode() == KeyEvent.VK_DOWN)
                            ball.move(0, 10);
                        return false;
                    }
                });

    }
}

And for BallComponent class:

static class BallComponent extends JComponent {
    private double x = 225;
    private double y = 225;
    private Ellipse2D.Double ellipse = new Ellipse2D.Double(x, y, 50, 50);

    public void move(int dX, int dY) {
        x += dX;
        y += dY;
        ellipse = new Ellipse2D.Double(x, y, 50, 50);
        repaint();
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        g2d.fill(ellipse);
    }
}

Answer 3

It seems that you never initialized actionPress - try adding this to your BallComponent constructor:

actionPress = new ActionPress();

ie, your constructor would look like this

public BallComponent()
{
    super();
    super.setFocusable(true);

    InputMap imap1 = this.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp1");
    InputMap imap2 = this.getInputMap(JComponent.WHEN_FOCUSED);
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp2");
    InputMap imap3 = this.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW);   
    imap1.put(KeyStroke.getKeyStroke("ctrl Y"), "keyUp3");

    actionPress = new ActionPress();
    ActionMap amap = this.getActionMap();
    amap.put("keyUp1", actionPress);
    amap.put("keyUp2", actionPress);    
    amap.put("keyUp3", actionPress);
}