package myprojects.drawline;

import java.awt.*;
import java.awt.event.*;

class DrawLine extends Frame {
    
    public DrawLine() {
        addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                dispose();
                System.exit(0);
            }
        });

 
      addMouseListener( new MouseAdapter()
          
      { int ox;
        int oy;
          public void mousePressed(MouseEvent e)
              { ox= e.getX();
                oy=e.getY();
                  }

          public void mouseReleased(MouseEvent e)

          {
              Graphics g = getGraphics();
              g.drawLine(ox,oy,e.getX(),e.getY());
              
              }
          
          }
          
          
           );

    }

    public static void main(String args[]) {
        System.out.println("Starting DrawLine...");
        DrawLine mainFrame = new DrawLine();
        mainFrame.setSize(400, 400);
        mainFrame.setTitle("DrawLine");
        mainFrame.setVisible(true);
    }
}

在这个程序中为什么 getGraphics前 加上 mainForm反而不对