swing的一段代码，貌似是焦点的有关问题，求指教

import java.awt.*;import javax.swing.*;import java.awt.event.*;class DrawPic extends JFrame implements MouseListener,MouseMotionListener{    int x1,x2,y1,y2;    boolean line,circle,rect;    static CheckboxGroup cbg ;    DrawPic(){        addMouseListener(this);        x1 = 0;        x2 = 0;        y1 = 0;        y2 = 0;        line = false;        circle = false;        rect = false;    }    public void go(){      setLayout(new GridLayout(3,1));      cbg = new CheckboxGroup();      add(new Checkbox("line",cbg,true));      add(new Checkbox("rect",cbg,false));      add(new Checkbox("circle",cbg,false));   }    public static void main(String args[]){        DrawPic draw = new DrawPic();        draw.setSize(400,300);        //draw.go();        draw.setVisible(true);    }    public void mouseClicked(MouseEvent e){        System.out.println("Mouse Clicked");    }    public void mouseReleased(MouseEvent e){        x2 = e.getX();        y2 = e.getY();        /*if(cbg.getSelectedCheckbox().getLabel().equals("line"))            line = true;        else if(cbg.getSelectedCheckbox().getLabel().equals("rect"))            rect = true;        else if(cbg.getSelectedCheckbox().getLabel().equals("circle"))            circle = true;*/        repaint();        line = false;        circle = false;        rect = false;    }    public void mouseEntered(MouseEvent e){}    public void mouseExited(MouseEvent e){}    public void mousePressed(MouseEvent e){        x1 = e.getX();        y1 = e.getY();    }    public void mouseDragged(MouseEvent e){            }    public void mouseMoved(MouseEvent e){}    public void paint(Graphics g){        System.out.println("painting..");        line = true;        g = this.getContentPane().getGraphics();        if(line)            g.drawLine(x1,y1,x2,y2);        else if(circle)            g.drawOval(x1,y1,x2-x1,y2-y1);        else if(rect)            g.drawRect(x1,y1,x2-x1,y2-y1);    }}

/* * To change this template, choose Tools | Templates * and open the template in the editor. */package test;import java.awt.*;import javax.swing.*;import java.awt.event.*;class DrawPic extends JFrame implements MouseListener,MouseMotionListener{    int x1,x2,y1,y2;    boolean line,circle,rect;    static CheckboxGroup cbg ;    DrawPic(){        addMouseListener(this);        x1 = 0;        x2 = 0;        y1 = 0;        y2 = 0;        line = false;        circle = false;        rect = false;        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);    }    public void go(){      setLayout(new GridLayout(3,1));      JPanel pnl = new JPanel();      add(pnl);           cbg = new CheckboxGroup();      pnl.add(new Checkbox("line",cbg,true));      pnl.add(new Checkbox("rect",cbg,false));      pnl.add(new Checkbox("circle",cbg,false));   }    public static void main(String args[]){        DrawPic draw = new DrawPic();        draw.setSize(400,300);        draw.go();        draw.setVisible(true);    }    public void mouseClicked(MouseEvent e){        System.out.println("Mouse Clicked");    }    public void mouseReleased(MouseEvent e){        x2 = e.getX();        y2 = e.getY();        line = false;        circle = false;        rect = false;        if(cbg.getSelectedCheckbox().getLabel().equals("line"))            line = true;        else if(cbg.getSelectedCheckbox().getLabel().equals("rect"))            rect = true;        else if(cbg.getSelectedCheckbox().getLabel().equals("circle"))            circle = true;        repaint();            }    public void mouseEntered(MouseEvent e){}    public void mouseExited(MouseEvent e){}    public void mousePressed(MouseEvent e){        x1 = e.getX();        y1 = e.getY();    }    public void mouseDragged(MouseEvent e){            }    public void mouseMoved(MouseEvent e){}    public void paint(Graphics g){        System.out.println("painting..");        //line = true;        g = this.getContentPane().getGraphics();        if(line)            g.drawLine(x1,y1,x2,y2);        else if(circle)            g.drawOval(x1,y1,x2-x1,y2-y1);        else if(rect)            g.drawRect(x1,y1,x2-x1,y2-y1);        else{            g.drawLine(x1,y1,x2,y2);        }    }}