①使用鼠标随意画线、②画直线（显示直线轨迹）、③画直线（不显示直线轨迹）

package drawLine;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class PaintJFrame extends JFrame
{
    public PaintJFrame()
    {
        super("使用鼠标随意画线");
        this.setBounds(400,300,400,300);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.getContentPane().add(new PaintCanvas());      //创建自定义画布组件
        this.setVisible(true);
    }
    
    class PaintCanvas extends Canvas implements MouseListener, MouseMotionListener //画布组件
    {
        Point start, end;
        public PaintCanvas()
        {
            start=end=null;
            this.addMouseMotionListener(this);
            this.addMouseListener(this);
        }
        
        public void mousePressed(MouseEvent ev)            //按下鼠标事件处理方法，以下实现MouseListener接口
        {
            end = new Point(ev.getX(), ev.getY());
        }
        public void mouseReleased(MouseEvent ev) {}        //释放鼠标
        public void mouseClicked(MouseEvent ev) {}         //单击鼠标
        public void mouseEntered(MouseEvent ev) {}         //鼠标进入
        public void mouseExited(MouseEvent ev) {}          //鼠标离开

        public void mouseMoved(MouseEvent ev){}              //移动鼠标，实现MouseMotionListener接口
        public void mouseDragged(MouseEvent ev)            //拖动鼠标，实现MouseMotionListener接口
        {
            start = end;
            end = new Point(ev.getX(), ev.getY());
            repaint();
        }
        
        public void paint(Graphics g)                      //在Canvas上作图
        {
            g.setColor(Color.blue);
            g.fillOval(10, 10, 20,30);
            g.drawOval(10, 40, 20, 30);
            if (start!=null && end!=null)
            {
                g.setColor(Color.blue);                    //设置画线颜色
                g.drawLine(start.x, start.y, end.x, end.y);//画直线
            }
        }
        public void update(Graphics g)                     //更新组件
        {
            paint(g);
        }
    }//PaintCanvas类结束
    
    public static void main(String arg[])
    {
        new PaintJFrame();
    }
}

package drawLine;
// 使用鼠标画直线
//显示直线轨迹

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class PaintLineJFrame extends JFrame
{
    public PaintLineJFrame()
    {
        super("使用鼠标画直线");
        this.setBounds(400,300,400,300);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.getContentPane().add(new PaintLineCanvas());      //创建自定义画布组件
        this.setVisible(true);
    }
    
    //画布内部类，响应鼠标事件和鼠标移动事件
    class PaintLineCanvas extends Canvas implements MouseListener, MouseMotionListener
    {
        Point start, end, lastend;
        public PaintLineCanvas()
        {
            start=end=lastend=null;
            this.addMouseMotionListener(this);
            this.addMouseListener(this);
        }

        public void mousePressed(MouseEvent ev)                //按下鼠标事件处理方法，以下实现MouseListener接口
        {
            start = new Point(ev.getX(), ev.getY());
        }
        public void mouseReleased(MouseEvent ev)               //释放鼠标
        {
            end = new Point(ev.getX(), ev.getY());
            this.repaint();
        } 
        public void mouseClicked(MouseEvent ev) {}             //单击鼠标
        public void mouseEntered(MouseEvent ev) {}             //鼠标进入
        public void mouseExited(MouseEvent ev) {}              //鼠标离开

        public void mouseMoved(MouseEvent ev){}                //移动鼠标，实现MouseMotionListener接口
        public void mouseDragged(MouseEvent ev)                //拖动鼠标，实现MouseMotionListener接口
        {
            lastend = end;
            end = new Point(ev.getX(), ev.getY());
            this.repaint();                                    //调用paint(Graphics)方法，重画
        }
        
        public void paint(Graphics g)                          //在Canvas上作图
        {
            if (start!=null && lastend!=null)
            {
//??                g.setXORMode(getBackground());
                g.setColor(this.getBackground()); 
                g.drawLine(start.x,start.y,lastend.x,lastend.y);//用背景色画线即擦除原线
//                g.drawOval(start.x, start.y, lastend.x, lastend.y);      //画椭圆
                g.setColor(Color.blue);                        //设置画线颜色
                g.drawLine(start.x, start.y, end.x, end.y);    //画直线
//            g.drawOval(start.x, start.y, end.x, end.y);      //画椭圆
//            g.drawRect(start.x, start.y, end.x, end.y);      //画圆角矩形
            }
        }
        public void update(Graphics g)                         //更新组件
        {
            this.paint(g);
        }
    }//PaintLineCanvas类结束
    
    public static void main(String arg[])
    {
        new PaintLineJFrame();
    }
}

package drawLine;

// 使用鼠标画直线。
//只响应鼠标事件,没有响应鼠标拖动事件,松开鼠标时才画,没有显示直线轨迹。

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class PaintLineJFrame1 extends JFrame
{
  public PaintLineJFrame1()
  {
      super("使用鼠标画直线");
      this.setBounds(400,300,400,300);
      this.setDefaultCloseOperation(EXIT_ON_CLOSE);
      this.getContentPane().add(new PaintLineCanvas());      //创建自定义画布组件
      this.setVisible(true);
  }
  
  class PaintLineCanvas extends Canvas implements MouseListener //画布
  {
      Point start, end;
      public PaintLineCanvas()
      {
          start=end=null;
          this.addMouseListener(this);
      }

      public void mousePressed(MouseEvent ev)                //按下鼠标事件处理方法，以下实现MouseListener接口
      {
          start = new Point(ev.getX(), ev.getY());
      }
      public void mouseReleased(MouseEvent ev)               //释放鼠标
      {
          end = new Point(ev.getX(), ev.getY());
          repaint();
      } 
      public void mouseClicked(MouseEvent ev) {}             //单击鼠标
      public void mouseEntered(MouseEvent ev) {}             //鼠标进入
      public void mouseExited(MouseEvent ev) {}              //鼠标离开
      
      public void paint(Graphics g)                          //在Canvas上作图
      {
          System.out.println("paint(Graphics g)");
          if (start!=null && end!=null)
          {
              g.setColor(Color.blue);                        //设置画线颜色
              g.drawLine(start.x, start.y, end.x, end.y);    //画直线
          }
      }
      public void update(Graphics g)                         //更新组件时，自动执行
      {
          System.out.println("update(Graphics g)");
          paint(g);
      }
  }//PaintLineCanvas类结束
  
  public static void main(String arg[])
  {
      new PaintLineJFrame1();
  }
}