java用鼠标点击画圆和方块出现白线(画出来的矩形一边消失，圆中出现白线），有时点击不出现图形

Question

package homework;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;

class MousePolice implements MouseListener{
int x1, y1, x2, y2; //定义四个坐标记录光标位置
private Graphics g;//定义私有属性画笔g
//将表示画笔的参数赋给成员属性的方法
public void setGraphics(Graphics g) {
this.g = g;
}
@Override
public void mouseClicked(MouseEvent e) {
// TODO Auto-generated method stub
if(e.getButton()==MouseEvent.BUTTON1&&e.getClickCount()==1) {

        x1 = e.getX();
        y1 = e.getY();//获取光标位置
        g.setColor(Color.green);//填充颜色
        g.fillOval(x1,y1,50,50);//绘制圆形的位置和大小
    }else if(e.getButton()==MouseEvent.BUTTON3&&e.getClickCount()==1) {
        x2 = e.getX();
        y2 = e.getY();
        g.setColor(Color.red);
        g.drawRect(x2,y2,50,50);
    }else if(e.getClickCount()==2&&e.getButton()==MouseEvent.BUTTON1) {
        g.clearRect(0, 0, 300, 300);//调用clearRect方法清空窗体画板//清除
    }
}
@Override
public void mousePressed(MouseEvent e) {
    // TODO Auto-generated method stub
    
}
@Override
public void mouseReleased(MouseEvent e) {
    // TODO Auto-generated method stub
    
}
@Override
public void mouseEntered(MouseEvent e) {
    // TODO Auto-generated method stub
    
}
@Override
public void mouseExited(MouseEvent e) {
    // TODO Auto-generated method stub
    
}

}

public class Demo{
public static void main(String []args){
JFrame windows = new JFrame();
windows.setBounds(100,100,300,300);
MousePolice listener = new MousePolice();
windows.addMouseListener(listener);

    windows.setVisible(true);
    Graphics g = (Graphics) windows.getGraphics();//定义Graphics的对象g并获取        
     listener.setGraphics(g);//调用监听器类中的setGraphics方法
    windows.setVisible(true);
    windows.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
}

}

Answer 1

最好是在paint方法里面实现画图的功能，其他事件改变变量的值，再调用repaint方法。