我想要实现一个动画功能，一个矩形从上落到下；但下面的代码不知道怎么回事，结果把整个运动过程中的所以矩形都画出来了，谁能帮忙解决下。

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Gamef extends JPanel{
    static Zuhe z;
    int x = 70;
    static int y = 40;
    int j = 0;
    public static void main(String[] args) {
        final Gamef gf = new Gamef();
        JFrame jf = new JFrame();
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.setVisible(true);
        jf.setBackground(Color.DARK_GRAY);
        jf.setBounds(400,130,300,400);
        jf.setResizable(false);
        gf.setBackground(Color.DARK_GRAY);
        jf.add(gf);
        gf.stat();
    }
    public void stat() {
        new Thread(new abcd()).start();
    }
   
    public void paint(Graphics g) {
        Color c = g.getColor();
        g.fillRect(x, y, 30, 40);
        if(j == 1){
            g.setColor(Color.RED);
            g.fillRect(x, y, 30, 40);
        }else if(j == 2){
            g.setColor(Color.BLUE);
            g.fillRect(x, y, 30, 40);
        }
        g.setColor(c);
    }
   
    private class abcd implements Runnable {
        public void run() {
            while(true) {
                y += 10;
                j ++;
                if(j >3) j = 1;
                repaint();
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

为什么需要g.clearRect(0, 0, getWidth(), getHeight());这句？
  我使用Frame都没有使用这句的。