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 I started off to try and make a program that would just do "UP" arrow key motion.

I've looked around for a solution, and just can't figure out what's wrong with this code. I don't know if it's a problem with my input and action maps (i.e., a problem with the program recognizing key presses) or if it's a problem with how the JComponent and JFrame classes work in swing. I thought maybe the problem might also be focus; I don't really know how to tell when a component has focus. I think the key has been set to CNTRL+Y instead of up for now, just because at some point I thought it might be a problem with my string designating the up arrow in the input map maker.

At this point, I'm so frustrated I'm just trying to get the damn thing to do something, so I'm using more input maps than should be necessary.

the code is as follows, it's pretty short, formatted horribly (sorry):

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

class BallMover
{
    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                BallFrame frame = new BallFrame();
            }
        });

    }
}


class BallFrame extends JFrame
{
    private static final int DEFAULT_WIDTH = 500;
    private static final int DEFAULT_HEIGHT = 500;
    private BallComponent comp;

    public BallFrame()
    {
        super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        super.setSize(this.DEFAULT_WIDTH, this.DEFAULT_HEIGHT);
        super.setResizable(false);
        super.add(new BallComponent());
        super.setVisible(true);
        super.setFocusable(true);
    }
}


class BallComponent extends JComponent
{
    private Ellipse2D.Double ellipse;
    private double x = 225;
    private double y = 225;
    private ActionPress actionPress;

    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");

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

    public void paintComponent(Graphics g)
    {
        super.repaint(); // clear component //
        Graphics2D g2d = (Graphics2D)g;
        this.ellipse = new Ellipse2D.Double(x, y, 50, 50);
        g2d.fill(this.ellipse);
    }

    private class ActionPress extends AbstractAction
    {
        public void actionPerformed(ActionEvent event)
        {
            y = y + 10;
            ellipse = new Ellipse2D.Double(x, y, 50, 50);
            repaint();
        }
    } 
}

Answer 1

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);
}

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

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);
    }
}