提问关于JAVA中repaint的有关问题

import java.awt.Color;import java.awt.Dimension;import java.awt.Graphics;import javax.swing.JPanel;public class Circle extends JPanel{    public int value;    public Circle(){            }    public Circle(int value){        this.value = value;    }    public int getValue(){        return value;    }        public void setValue(int value){        this.value = value;        repaint();    }            protected void paintComponent(Graphics g){        super.paintComponent(g);        int clockRadius = (int)(Math.min(getWidth(), getHeight())*0.8*0.5);        int xCenter = getWidth()/2;        int yCenter = getHeight()/2;        g.setColor(Color.RED);        g.drawOval(xCenter-clockRadius, yCenter-clockRadius, 2*clockRadius, 2*clockRadius);                int line1 = (int)(clockRadius);        int line1X = (int) (xCenter-clockRadius*Math.cos(value*(2*Math.PI/60)));        int line1Y = yCenter+value;        g.drawLine(xCenter, yCenter, line1X, line1Y);            }    public Dimension getPreferredSize() {        return new Dimension(100,100);    }    }

    public static void main(String[] args) throws Exception {        JFrame app = new JFrame("测试");        app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        app.setSize(400, 500);        Circle panel = new Circle(20);        app.add(panel);        app.setVisible(true);        for(int i = 0;i<100;i++)        {            Thread.sleep(100);            panel.setValue(20 + i*5);        }    }