Question

I made the applet Bouncing Ball and in the class Ball.java I made inner class TimerListener with method repaint(), and when I run the applet, instead of repaint the ball, java paint the ball again and again(not delete, and then paint).

here is my code for the class Ball.java

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

public class Ball extends JPanel {
private int delay = 10;

Timer timer=new Timer(delay, new TimerListener());

private int x=0;
private int y=0;
private int dx=20;
private int dy=20;
private int radius=5;

private class TimerListener implements ActionListener{
    public void actionPerformed(ActionEvent e) {
        repaint();
    }
}

public void paintComponent(Graphics g){

    g.setColor(Color.red);
    if(x<radius) dx=Math.abs(dx);
    if(x>(getWidth()-radius)) dx=-Math.abs(dx);
    if(y>(getHeight()-radius)) dy=-Math.abs(dy);
    if(y<radius) dy=Math.abs(dy);
    x+=dx;
    y+=dy;
    g.fillOval(x-radius, y-radius, radius*2, radius*2);
        }
public void suspend(){
    timer.stop();
}
public void resume(){
    timer.start();
}
public void setDelay(int delay){
this.delay=delay;
timer.setDelay(delay);
}   
}

here is my code for class BallControl.java

import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
public class BallControl extends JPanel{

private Ball ball = new Ball();
private JButton jbtSuspend = new JButton("Suspend");
private JButton jbtResume = new JButton("Resume");
private JScrollBar jsbDelay = new JScrollBar();

public BallControl(){
    JPanel panel = new JPanel();
    panel.add(jbtSuspend);
    panel.add(jbtResume);
    //ball.setBorder(new javax.swing.border.LineBorder(Color.red));
    jsbDelay.setOrientation(JScrollBar.HORIZONTAL);
    ball.setDelay(jsbDelay.getMaximum());
    setLayout(new BorderLayout());
    add(jsbDelay, BorderLayout.NORTH);
    add(ball, BorderLayout.CENTER);
    add(panel, BorderLayout.SOUTH);

    jbtSuspend.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            ball.suspend();
        }
    });

    jbtResume.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            ball.resume();
        }
    });

    jsbDelay.addAdjustmentListener(new AdjustmentListener() {
        public void adjustmentValueChanged(AdjustmentEvent e) {
            ball.setDelay(jsbDelay.getMaximum() - e.getValue());
        }
    });

}
    }

Answer 1

Isn’t the Timer supposed to also change the Ball object’s position? In other words, isn’t it supposed to change its x and y values? i.e.,

private class TimerListener implements ActionListener{
    public void actionPerformed(ActionEvent e) {

        // first change x and y here *****

        repaint();
    }
}

Else, how is the ball supposed to move much less bounce?

You seem to have this change position code in your paintComponent(...) method, and that is not good since you don’t have full control over when or even if this method gets called. For this reason, program logic and code that changes this object’s state does not belong inside of that method.

Also, your paintComponent(...) method override needs a call to to the super’s paintComponent(...) method on its first line so that the old ball can be erased before a new ball is drawn.