Repaint（）不会清除帧

Question

public class Graphics2DTest extends JPanel implements ActionListener{
private Timer time = new Timer(5,(ActionListener) this);
int x = 0,y = 0;
public void paintComponent(Graphics g){

    Graphics2D gui = (Graphics2D) g;
    Rectangle2D rectangle = new Rectangle2D.Double(x,y,100,150);
    gui.setPaint(Color.GREEN);
    gui.fill(rectangle);
    time.start();
}

public void actionPerformed(ActionEvent arg0) {
    x++;
    y++;
    repaint();
}
}

The problem is repaint() is supposed to clear the frame and draw the rectangle in the position, but the previously painted rectangle remains. So, how to do it? Please explain your answers.

Answer 1

Have you tried calling super.paintComponent(g) in your paintComponent method? This will clear prior images drawn in your JPanel:

public void paintComponent(Graphics g){
  super.paintComponent(g);
  Graphics2D gui = (Graphics2D) g;
  Rectangle2D rectangle = new Rectangle2D.Double(x,y,100,150);
  gui.setPaint(Color.GREEN);
  gui.fill(rectangle);
  //time.start();
}

Also, don't start a timer or do any program logic within the paintComponent method. First of all you cannot absolutely control when or if the method will be called, and secondly, this method must be concerned only with painting and nothing else, and needs to be as fast as possible.

For instance:

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;

import javax.swing.*;

public class Graphics2DTest extends JPanel implements ActionListener {
    private Timer time = new Timer(5, (ActionListener) this);
    int x = 0, y = 0;

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D gui = (Graphics2D) g;
        Rectangle2D rectangle = new Rectangle2D.Double(x, y, 100, 150);
        gui.setPaint(Color.GREEN);
        gui.fill(rectangle);
        //time.start();
    }

    public void actionPerformed(ActionEvent arg0) {
        x++;
        y++;
        repaint();
    }

    public Graphics2DTest() {
        setPreferredSize(new Dimension(700, 500));
        time.start();
    }

    private static void createAndShowUI() {
        JFrame frame = new JFrame("Graphics2DTest");
        frame.getContentPane().add(new Graphics2DTest());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                createAndShowUI();
            }
        });
    }
}

Answer 2

You need to repaint the background each time as well. Add code to paint the background before you paint the rectangle.

Answer 3

You need to clear the background first.

A resource is this:

http://java.sun.com/products/jfc/tsc/articles/painting/