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);
    }
}