这样用Graphics画矩形为什么不行呢？

[Yanbin_Q]

//Draw.java

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

public class Draw extends JFrame{

public static void main(String[] args){

JFrame draw=new Draw();
draw.setSize(320,240);
Graphics g=draw.getContentPane().getGraphics();//这里
g.drawRect(2,2,100,100);//还有这里
draw.show();
}
}

错误出在上面两处，要怎么改造才行呢？我希望在此处直接画出来。

[ppqq2099]

有没试过
Graphics g=draw.getGraphics();
g.drawRect(2,2,100,100);

[Yanbin_Q]

也是一样的，没用，出错为
--------------------Configuration: j2sdk1.4.2 <Default>--------------------
java.lang.NullPointerException
at Draw.main(Draw.java:33)
Exception in thread "main"
Process completed.

[realzealy]

不能这么直接画，你要写paint方法，调用repaint()做
import javax.swing.*;
import java.awt.*;

public class Draw extends JFrame
{
Draw()
{
repaint();
}
public void paint(Graphics g)
{
g.drawRect(2,2,100,100);
}


public static void main(String[] args){

JFrame draw=new Draw();
draw.setSize(320,240);
draw.setVisible(true);
}
}

[Yanbin_Q]

那getGraphics()还有用吗？

[Yanbin_Q]

我发现问题啊，我一直没注意paint(Graphics g)方法中的g的生命周期，它不能离开paint方法而存在。