Using keypad to move a circle at angles in java

Question

So I have an application that successfully moves a ball (ellipse) left, right, up, or down depending on which button of the keypad is being pressed. However, I\'m having tro

Answer 1

Because the key events will only report the last key that triggered the event, your last if statement will never work, as the keyCode will only ever equal the last key that triggered the event.

Instead, you should maintain a "state" of each key and process against that state instead.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestKeyListener01 {

    public static void main(String[] args) {
        new TestKeyListener01();
    }

    public TestKeyListener01() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private boolean arrowUp = false;
        private boolean arrowDown = false;
        private boolean arrowLeft = false;
        private boolean arrowRight = false;

        public TestPane() {
            setFocusable(true);
            addKeyListener(new KeyAdapter() {

                @Override
                public void keyReleased(KeyEvent e) {
                    switch (e.getKeyCode()) {
                        case KeyEvent.VK_DOWN:
                            arrowDown = false;
                            break;
                        case KeyEvent.VK_UP:
                            arrowUp = false;
                            break;
                        case KeyEvent.VK_LEFT:
                            arrowLeft = false;
                            break;
                        case KeyEvent.VK_RIGHT:
                            arrowRight = false;
                            break;
                    }
                    repaint();
                }

                @Override
                public void keyPressed(KeyEvent e) {
                    switch (e.getKeyCode()) {
                        case KeyEvent.VK_DOWN:
                            arrowDown = true;
                            break;
                        case KeyEvent.VK_UP:
                            arrowUp = true;
                            break;
                        case KeyEvent.VK_LEFT:
                            arrowLeft = true;
                            break;
                        case KeyEvent.VK_RIGHT:
                            arrowRight = true;
                            break;
                    }
                    repaint();
                }
            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            FontMetrics fm = g2d.getFontMetrics();
            String text = "Up";
            int x = (getWidth() - fm.stringWidth(text)) / 2;
            int y = ((getHeight() / 2) - fm.getHeight());
            draw(g2d, text, x, y, arrowUp);

            text = "Down";
            x = (getWidth() - fm.stringWidth(text)) / 2;
            y = ((getHeight() / 2) + fm.getHeight()) + fm.getAscent();
            draw(g2d, text, x, y, arrowDown);

            text = "Left";
            x = (getWidth() / 2) - fm.stringWidth(text) - 4;
            y = ((getHeight() - fm.getHeight()) / 2) + fm.getAscent();
            draw(g2d, text, x, y, arrowLeft);

            text = "Right";
            x = (getWidth() / 2) + 4;
            y = ((getHeight() - fm.getHeight()) / 2) + fm.getAscent();
            draw(g2d, text, x, y, arrowRight);

            g2d.dispose();
        }

        protected void draw(Graphics2D g2d, String text, int x, int y, boolean pressed) {
            g2d.setColor(pressed ? Color.BLACK : Color.GRAY);
            g2d.drawString(text, x, y);
        }
    }
}

I would also recommend that instead of KeyListener, you should use Key Bindings instead. They solve annoying focus issues that plague KeyListener