Java画板代码 请问这个flag变量具体是什么作用？

Question

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

public class DrawPanel extends JPanel
{
   private JPanel right,left;//建立2个面板
   private JRadioButton op2;  
   private  int f=2;
   private  int sx, sy, x,y; 
   **private  boolean flag=false;**//【这个flag】
   private Color c;
   private Point point1=null,point2=null;

  public DrawPanel()//DrawPanel无参构造器
  {  c =Color.black;    
   right = new JPanel(); 
   left = new JPanel();
   ButtonGroup g = new ButtonGroup();//new一个按钮组
   op2 = new JRadioButton("直线",false);
   g.add(op2);
   left.setLayout(new BoxLayout(lef__t,BoxLayo_ut_.Y_AXI_S));
   left.add(op2);
   op2.addActionListener(new listen2());     
   right.setBackground(Color.white); //
   right.addMouseListener(new listen1());
   right.addMouseMotionListener(new listen1());   
    setLayout(new BorderLayout());//边界布局
   add(right,"Center");
   add(left,"West");
   setBackground(Color.white);
   setPreferredSize(new Dimension(500,450));
 _
  }

   private class listen1 implements MouseListener,MouseMotionListener
   {


     public void mousePressed(MouseEvent event)//按下鼠标时：
     {
       **flag=false;**
       point1=event.getPoint();//记录按下鼠标时的点坐标
       sx=point1.x;
       sy=point1.y ;
     }

     public void mouseReleased(MouseEvent event)//释放鼠标时：
     {

         Graphics g=right.getGraphics();//

         switch(f){     
              case 2://画直线按钮被选中时：
                   point2=event.getPoint();//获取释放鼠标的坐标点
                   x=point2.x; y=point2.y ;    
                   g.setColor(c);
                   g.drawLine(sx,sy,x,y);//画一条从单击点到释放点的直线
                 break; 


               }
      }

      public void mouseEntered(MouseEvent event){}
      public void mouseExited(MouseEvent event){}
      public void mouseClicked(MouseEvent event){ }  
      public void mouseMoved(MouseEvent event){}
      public void mouseDragged(MouseEvent event)

      //具体化MouseMotionListener中的拖拽方法，使在拖拽鼠标时产生预览效果
      {

      Graphics g=right.getGraphics();
      switch(f){
      case 2:////画直线按钮被选中时：
          **  if(! flag){**
                 point2=event.getPoint();         
                 x=point2.x; y=point2.y ;      
                 g.drawLine(sx,sy,x,y);
                 flag=true;
                      }
            else
            {
              g.setXORMode(Color.red);//设置异或画图模式
              g.drawLine(sx,sy,x,y); //清除上次移动时画出的线
              point2=event.getPoint();        
              x=point2.x; y=point2.y ;        
              g.drawLine(sx,sy,x,y); 
            }
        break;}
     }
  }

   private class listen2 implements ActionListener
    {
       public void actionPerformed(ActionEvent event)
        {
            Object source=event.getSource();//获得事件源

               if(source==op2)f=2;//直线

        }  
    }


 }

Answer 1

   **  if(! flag){**//它的作用就是这里
                 point2=event.getPoint();         
                 x=point2.x; y=point2.y ;      
                 g.drawLine(sx,sy,x,y);
                 flag=true;
                      }
            else
            {
              g.setXORMode(Color.red);//设置异或画图模式
              g.drawLine(sx,sy,x,y); //清除上次移动时画出的线
              point2=event.getPoint();        
              x=point2.x; y=point2.y ;        
              g.drawLine(sx,sy,x,y); 
            }