如何减少矩形的高度？ （Java，AWT）

Question

I'm beginner in Java. So, please help me with my problem.

I can do animation when a rectangle's height increases. But I have problem with decreasing rectangle's height. Please look at this code:

public class Animation extends JPanel implements ActionListener {

    Timer timer;
    int i = 100;

public Animation() {
    timer = new Timer(10, this);
    timer.start();
}

 public void paint(Graphics g) {

    Graphics2D g2d1 = (Graphics2D) g;

    g2d1.fillRect(0, 100, 30, i);

}

public static void main(String[] args) {

    JFrame frame = new JFrame("animation");
    frame.add(new Animation());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(800, 800);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}

public void actionPerformed(ActionEvent e)
{     
    --i;
    repaint();
}

} 

Please help me.

Best regards Pawel

Answer 1

It isn't clearing the screen between draws, so it draws over the old larger rectangle.

Try this:

public void paint(Graphics g) {
    Graphics2D g2d1 = (Graphics2D) g;
    g.setColor(getBackground());
    g.fillRect(0,0,getWidth(),getHeight()); // draw a rectangle over the display area in the bg color
    g.setColor(Color.BLACK);
    g2d1.fillRect(0, 100, 30, i);
}

Or:

public void paint(Graphics g) {
    super.paint(g); // call superclass method, which does clear the screen
    Graphics2D g2d1 = (Graphics2D) g;
    g2d1.fillRect(0, 100, 30, i);
}

And as camickr pointed out below, custom painting should be done in paintComponent not paint, so you should change the name of the method to paintComponent.