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);
}