What is the flaw in this simple Swing program ?

Question

Hi all ,
I tried one simple animation in Swing .
Here's what the program has to do :
when a button called "play" is clicked a circle should be moved from upper left corner down to the lower right corner.
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class MyAnimation extends JFrame {
   int width;
   int height;
   MyAnimation animation ;
   MyDrawPanel drawpanel ;
   public static void main(String[] args) {
     MyAnimation gui=new MyAnimation();
     gui.go();
   } //close main()
   public void go() {
       animation = new MyAnimation();
       JButton button = new JButton("Play");
       button.addActionListener(new ButtonListener());
       drawpanel = new MyDrawPanel();
       animation.setTitle("Ball Animation");
       animation.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       animation.getContentPane().add(BorderLayout.SOUTH,button);
       animation.getContentPane().add(BorderLayout.CENTER,drawpanel);
       animation.setSize(300,300);
       animation.setVisible(true);
   } //close go()
   class ButtonListener implements ActionListener {
       public void actionPerformed(ActionEvent ae) {
          for(width=0,height=0;width<130&&height<130;width++,height++) {
                   System.out.println("before calling");
                   animation.repaint();
                   System.out.println("W ="+width+"H ="+height);
               try {
                   Thread.sleep(50);
               }catch(Exception ex) { }
         } //close for
    } //close actionPerformed()
} //close ButtonListener
      class MyDrawPanel extends JPanel {
            public void paintComponent(Graphics g) {
               g.setColor(Color.blue);
               g.fillRect(0,0,this.getWidth(),this.getHeight());
               int w= this.getWidth();
               int h = this.getHeight();
               //System.out.print(w,h);
               g.setColor(Color.green);
               g.fillOval(width,height,40,40);
               System.out.println("paintcomponent");
           } //close paintcomponent()
     } //closeMyDrawPanel()
} //close MyAnimation()I noticed when a button is clicked everything goes fine(code reached Listener and loop is executed) expect that repaint() method is not invoked with in a loop(invoked only once at the end of loop).
but if I embed the same loop code with in go() method without a Listener class it's works Fine !
Why it's not happening when the code is within Litener class?
Two more additional questions :
What is the difference between calling the repaint() method with JFrame instance and JPanel instance ?
In that program why System.out.print() method which is in MyDrawPanel class can't be invoked ?
Thanks in advance

Answer 1

Axel_Richter wrote:
May be you could have a chance to animate without threads by using paintImmediately directly. I have never tried this, because to use threads is not so difficult at all.animation without explicit calls to thread:
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class MyAnimation extends JPanel
    private static final int TIME_PERIOD = 10;
    private static final String PLAY = "Play";
    private static final String PAUSE = "Pause";
    private static final String RESET = "Reset";
    int width = 0;
    int height = 0;
    MyDrawPanel drawpanel = new MyDrawPanel();
    private Timer timer = new Timer(TIME_PERIOD, new TimerAL());
    public MyAnimation()
        super(new BorderLayout());
        setPreferredSize(new Dimension(300, 300));
        JButton button = new JButton("Play");
        button.addActionListener(new ButtonListener());
        add(button, BorderLayout.SOUTH);
        add(drawpanel, BorderLayout.CENTER);
    class ButtonListener implements ActionListener
        public void actionPerformed(ActionEvent event)
            String command = event.getActionCommand();
            if (command.equals(PLAY))
                timer.start();
                ((JButton)event.getSource()).setText(PAUSE);
            else if (command.equals(PAUSE))
                timer.stop();
                ((JButton)event.getSource()).setText(RESET);
            else if (command.equals(RESET))
                timer.stop();
                ((JButton)event.getSource()).setText(PLAY);
                width = 0;
                height = 0;
                repaint();
    class TimerAL implements ActionListener
        public void actionPerformed(ActionEvent arg0)
            if (width < (drawpanel.getWidth() - 40) &&
                    height < (drawpanel.getHeight() - 40))
                width++;
                height++;
                drawpanel.repaint();
            else
                timer.stop();
    class MyDrawPanel extends JPanel
        public void paintComponent(Graphics g)
            g.setColor(Color.blue);
            g.fillRect(0, 0, this.getWidth(), this.getHeight());
            int w = this.getWidth();
            int h = this.getHeight();
            // System.out.print(w,h);
            Graphics2D g2 = (Graphics2D)g;
            g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                                RenderingHints.VALUE_ANTIALIAS_ON);
            g2.setColor(Color.green);
            g2.fillOval(width, height, 40, 40);
    private static void createAndShowGUI()
        JFrame frame = new JFrame("Ball Animation");
        frame.getContentPane().add(new MyAnimation());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    public static void main(String[] args)
        SwingUtilities.invokeLater(new Runnable()
            public void run()
                createAndShowGUI();
}