java drawOval重复的圆圈

Question

When redrawing circles, the window is not cleared; the new circles get added to the existing content.

The goal is to create three circle, one for each color.

The thread calls move function which draws the circles with different radii.

public void run() {
    try {
          while(true){
              box.removeAll();
              move();
              box.removeAll();
              sleep(500);
          }
    } catch (InterruptedException e) {
    }
}

public synchronized void move() {
    Graphics g = box.getGraphics();
    g.setXORMode(box.getBackground());

    x1= one.x + ofset;
    y1= one.y + ofset;

    System.out.println("My Point ("+ x1 + " , " + y1 +")" );

    g.setColor(Color.green);
    g.drawOval(pointA.x-(int)distance1, pointA.y-(int)distance1, (int)distance1*2, (int)distance1*2);

    g.setColor(Color.blue);
    g.drawOval(pointB.x-(int)distance2, pointB.y-(int)distance2, (int)distance2*2, (int)distance2*2);

    g.setColor(Color.red);
    g.drawOval(pointC.x-(int)distance3, pointC.y-(int)distance3, (int)distance3*2, (int)distance3*2);

    g.dispose();
}

Answer 1

First of all, your approach is not recommended. But, if you only want a quick and dirty fix, you have to clear the panel before drawing the circles.

Graphics g = box.getGraphics();
g.clearRect(0, 0, box.getWidth(), box.getHeight()); // this should do it
g.setXORMode(box.getBackground());

Answer 2

Graphics g = box.getGraphics();

No. Don't use getGraphics(). Any painting done with that Graphics object is only temporary and will be deleted any time Swing determines a component needs to be repainted.

For custom painting override the getPreferredSize() method of a JPanel:

@Override
protected void paintComponent(Graphics g)
{
    super.paintComponent(g); // clears the background

    // add your custom painting here
}

Also, don't forget to override the getPreferredSize() method of your panel. Read the section from the Swing tutorial on Custom Painting for more information.