有谁可以告诉我这个程序的问题,帮帮忙
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;
public class 图形界面入门 {
public static void main(String argv[])
{draw p=new draw();
}
}
class draw extends JFrame
{
public draw()
{setLayout(new GridLayout(2,1));
getContentPane().add(new in1());
getContentPane().add(new in());
Dimension p=Toolkit.getDefaultToolkit().getScreenSize();
int x=p.width;
int y=p.height;
setLocation((x-500)/2,(y-500)/2);
setSize(500,500);
setVisible(true);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
class in extends JPanel
{
private JButton p;
private JButton q;
private in1 s=new in1();
public in()
{p=new JButton( "OK ");
q=new JButton( "Cancel ");
add(p);
add(q);
p.addActionListener(s);
q.addActionListener(s);
}
}
class in1 extends JPanel implements ActionListener
{private Graphics g;
public void actionPerformed(ActionEvent e)
{g=getGraphics();
super.paintComponent(g);
if(e.getActionCommand().equals( "OK "))
//总是下面这里出错
g.drawString( "OK is clicked ",getWidth()/2,getHeight()/5);
if(e.getActionCommand().equals( "Cancel "))
//总是下面这里出错
g.drawString( "Cancel is clicked ",getWidth()/2,getHeight()/5);
g.dispose();
}
}
这里是问题
Exception in thread "AWT-EventQueue-0 " java.lang.NullPointerException
at in1.paintComponent(图形界面入门.java:61)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JLayeredPane.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paintWithOffscreenBuffer(Unknown Source)
at javax.swing.JComponent.paintDoubleBuffered(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)
at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)
at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
at java.awt.Container.paint(Unknown Source)
at sun.awt.RepaintArea.paintComponent(Unknown Source)
at sun.awt.RepaintArea.paint(Unknown Source)
at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
[解决办法]
在new in1的时候,传一个this过来,然后g= in.getGraphics()试试
用一个匿名内部类多方便
