I'm stuck with a problem considering my Display class, which extends Canvas. A single thread is running within the very same class. In this thread, the repaint method is called. However, while the thread works fine, the paint method is never called!

Here's my code (I left out everything unrelated):

package display;

public final class Display extends Canvas implements Runnable {
private static 开发者_运维知识库final long serialVersionUID = 1L;

private Frame frame;

private GraphicsEnvironment ge;
private GraphicsDevice gd;

public BDisplay() {     
    ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
    gd = ge.getDefaultScreenDevice();

    frame = new Frame();

    //frame.setUndecorated(true);
    frame.setLocationRelativeTo(null);
    frame.setResizable(false);
    frame.setVisible(true);

    frame.add(this);

    frame.setLayout(null);

    //gd.setFullScreenWindow(frame);
}

@Override
public final void paint(Graphics g) {
    //THIS METHOD IS NEVER CALLED

    System.out.println("Paint method was called!");
    super.paint(g);

    //...
}

@Override
public synchronized void run() {
    while (isRunning()) {
        //THIS LOOP WORKS FINE.
        this.repaint();
    }
}
}

It has nothing to do with some missing functions like isRunning(). They exist, i just left those out.

I never stumbled across a problem like this before, although I haven't done anything with SWING or AWT for some time now.

Can anyone help?

The thread loop works fine, but the repaint just doesn't seem to get scheduled...

You're realizing the JFrame before you add the component (i.e. Canvas, and previously JPanel).

Try this

public BDisplay() {     
    ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
    gd = ge.getDefaultScreenDevice();

    frame = new Frame();
    frame.setResizable(false);
    frame.add(this);
    //frame.setLayout(null); // Why are you using a null layout?
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

EDIT

By the way, I'd recommend using a JPanel instead of a Canvas since it's not recommended that you mix heavy and light components. For more detail, see Mixing heavy and light components.

and now ??? by using javax.swing.Timer

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

public class AnimationJPanel extends JPanel {

    private static final long serialVersionUID = 1L;
    private int cx = 0;
    private int cy = 150;
    private int cw = 20;
    private int ch = 20;
    private int xinc = 1;
    private int yinc = 1;

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                AnimationJPanel panel = new AnimationJPanel();
                panel.setPreferredSize(new Dimension(400, 300));
                panel.animate();
                JFrame frame = new JFrame("Test");
                frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
                frame.getContentPane().add(panel);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public AnimationJPanel() {
        setLayout(new BorderLayout());
        JLabel label = new JLabel("This is an AnimationJPanel");
        label.setForeground(Color.RED);
        label.setHorizontalAlignment(SwingConstants.CENTER);
        add(label);
        setBackground(Color.BLACK);
        setForeground(Color.RED);
        setOpaque(true);
    }

    public void animate() {
        new Timer(15, new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                Rectangle oldCircle = new Rectangle(cx - 1, cy - 1, cw + 2, ch + 2);
                cx += xinc;
                cy += yinc;
                if (cx >= getWidth() - cw || cx <= 0) {
                    xinc *= -1;
                }
                if (cy >= getHeight() - ch || cy <= 0) {
                    yinc *= -1;
                }
                repaint(oldCircle);
                repaint(cx - 1, cy - 1, cw + 2, ch + 2);
            }
        }).start();
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawOval(cx, cy, cw, ch);
    }
}

or

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.Point;
import java.awt.Polygon;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;
import java.util.Random;
import javax.swing.AbstractAction;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class AnimationBackground {

    public AnimationBackground() {
        Random random = new Random();
        JFrame frame = new JFrame("Animation Background");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);
        final MyJPanel panel = new MyJPanel();
        panel.setBackground(Color.BLACK);
        for (int i = 0; i < 50; i++) {
            Star star = new Star(new Point(random.nextInt(490), random.nextInt(490)));
            star.setColor(new Color(100 + random.nextInt(155), 100 + random.nextInt(155), 100 + random.nextInt(155)));
            star.setxIncr(-3 + random.nextInt(7));
            star.setyIncr(-3 + random.nextInt(7));
            panel.add(star);
        }
        panel.setLayout(new GridLayout(10, 1));
        JLabel label = new JLabel("This is a Starry background.", JLabel.CENTER);
        label.setForeground(Color.WHITE);
        panel.add(label);
        JPanel stopPanel = new JPanel();
        stopPanel.setOpaque(false);
        stopPanel.add(new JButton(new AbstractAction("Stop this madness!!") {

            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e) {
                panel.stopAnimation();
            }
        }));
        panel.add(stopPanel);
        JPanel startPanel = new JPanel();
        startPanel.setOpaque(false);
        startPanel.add(new JButton(new AbstractAction("Start moving...") {

            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e) {
                panel.startAnimation();
            }
        }));
        panel.add(startPanel);
        frame.add(panel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                AnimationBackground animationBackground = new AnimationBackground();
            }
        });
    }

    class Star extends Polygon {

        private static final long serialVersionUID = 1L;
        private Point location = null;
        private Color color = Color.YELLOW;
        private int xIncr, yIncr;
        static final int WIDTH = 500, HEIGHT = 500;

        Star(Point location) {
            int x = location.x;
            int y = location.y;
            this.location = location;
            this.addPoint(x, y + 8);
            this.addPoint(x + 8, y + 8);
            this.addPoint(x + 11, y);
            this.addPoint(x + 14, y + 8);
            this.addPoint(x + 22, y + 8);
            this.addPoint(x + 17, y + 12);
            this.addPoint(x + 21, y + 20);
            this.addPoint(x + 11, y + 14);
            this.addPoint(x + 3, y + 20);
            this.addPoint(x + 6, y + 12);
        }

        public void setColor(Color color) {
            this.color = color;
        }

        public void move() {
            if (location.x < 0 || location.x > WIDTH) {
                xIncr = -xIncr;
            }
            if (location.y < 0 || location.y > WIDTH) {
                yIncr = -yIncr;
            }
            translate(xIncr, yIncr);
            location.setLocation(location.x + xIncr, location.y + yIncr);
        }

        public void setxIncr(int xIncr) {
            this.xIncr = xIncr;
        }

        public void setyIncr(int yIncr) {
            this.yIncr = yIncr;
        }

        public Color getColor() {
            return color;
        }
    }

    class MyJPanel extends JPanel {

        private static final long serialVersionUID = 1L;
        private ArrayList<Star> stars = new ArrayList<Star>();
        private Timer timer = new Timer(20, new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                for (Star star : stars) {
                    star.move();
                }
                repaint();
            }
        });

        public void stopAnimation() {
            if (timer.isRunning()) {
                timer.stop();
            }
        }

        public void startAnimation() {
            if (!timer.isRunning()) {
                timer.start();
            }
        }

        @Override
        public void addNotify() {
            super.addNotify();
            timer.start();
        }

        @Override
        public void removeNotify() {
            super.removeNotify();
            timer.stop();
        }

        MyJPanel() {
            this.setPreferredSize(new Dimension(512, 512));
        }

        public void add(Star star) {
            stars.add(star);
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            ((Graphics2D) g).setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            for (Star star : stars) {
                g.setColor(star.getColor());
                g.fillPolygon(star);
            }
        }
    }
}