public class Drawboard extends JFrame{       Graphics g;       public static void main(String[] args){              Drawboard d = new Drawboard();              d.showUI();       }         public void showUI(){       Graphics g ;        this.setTitle("画图板");        this.setSize(600,600);        this.setDefaultCloseOperation(3);        this.setResizable(false);                      java.awt.FlowLayout fl = new java.awt.FlowLayout();        this.setLayout(fl);        JPanel p = new JPanel();        this.add(p);     //      创建一个面板        p.setPreferredSize(new Dimension(550,400));     //   设置面板颜色        p.setBackground(Color.green);        JRadioButton jr1 =new JRadioButton("line");        jr1.setActionCommand("直线");        JRadioButton jr2 =new JRadioButton("rectangle");        jr2.setActionCommand("矩形");        JRadioButton jr3 =new JRadioButton("圆角矩形");        jr3.setActionCommand("圆角矩形");        JRadioButton jr4 =new JRadioButton("eclipse");        jr4.setActionCommand("椭圆");        JRadioButton jr5 =new JRadioButton("fill the rectangle");        jr5.setActionCommand("填充矩形");        JRadioButton jr6 =new JRadioButton("填充椭圆");        jr6.setActionCommand("填充椭圆");        JRadioButton jr7 =new JRadioButton("圆弧");        jr7.setActionCommand("圆弧");        //创建一个group（这是个容器）,并把所有单选对象放进去        ButtonGroup group = new ButtonGroup();        group.add(jr1);        group.add(jr2);        group.add(jr3);        group.add(jr4);        group.add(jr5);        group.add(jr6);        group.add(jr7);         this.add(jr1);        this.add(jr2);        this.add(jr3);        this.add(jr4);        this.add(jr5);        this.add(jr6);        this.add(jr7);        this.setVisible(true);        //创建一个画布        g = this.getGraphics();       //创建鼠标监听器对象        mouselistener lis = new mouselistener(g,group);       this.addMouseListener(lis);      }}import java.awt.Graphics;import java.awt.event.MouseEvent;import java.awt.event.MouseListener; import javax.swing.ButtonGroup;public class mouselistener implements MouseListener {       int x1,x2,y1,y2;       private Graphics g;       public String command;       ButtonGroup group;       public mouselistener(Graphics g,ButtonGroup group){              this.g = g;              this.group = group;             }       //鼠标按键在组件上按下时调用       public void mousePressed(MouseEvent e){              x1=e.getX();              y1=e.getY();       }       //鼠标按钮在组件上释放时调用。       public void mouseReleased(MouseEvent e){              x2=e.getX();              y2=e.getY();              String command = group.getSelection().getActionCommand();              if("直线".equals(command))              //画直线              g.drawLine(x1, y1, x2, y2);              else if("矩形".equals(command))              //绘制矩形         g.drawRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1));              else if("椭圆".equals(command))         //绘制椭圆         g.drawOval(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1));              else if("填充矩形".equals(command))         //填充矩形         g.fillRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1));              else if("圆角矩形".equals(command))                 //圆角矩形         g.drawRoundRect(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1),12,12);              else if("填充椭圆".equals(command))         //填充椭圆         g.fillOval(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1));              else if("圆弧".equals(command))         //画圆弧         g.drawArc(Math.min(x1, x2), Math.min(y1, y2), Math.abs(x2-x1), Math.abs(y2-y1),45,90);       }       public void mouseClicked(MouseEvent e){}       public void mouseEntered(MouseEvent e){}       public void mouseExited(MouseEvent e){}}