关于画直线的问题（进来看下，没你想的那么＃＃＃＃）

wuyan19831013 2006-08-15 10:49:55

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

public class drawonpanel {
Graphics2D g2d;
public drawonpanel(JPanel p) {
g2d =(Graphics2D) p.getGraphics();
System.out.println("inside of drawonpanel");
g2d.drawString("i am a boy!",50,50);
}
}

import javax.swing.*;
public class Main {
JPanel p;
JFrame f;
drawonpanel pp;
public Main() {
p = new JPanel();
f = new JFrame();
pp = new drawonpanel(p);
f.getContentPane().add(p);
f.pack();
f.show();
}

public static void main(String args[]){
new Main();
}
}

两个类，drawonpanle负责画图，Main负责显示，我把一个JPanle传到drawonpanle里了，但是程序执行到:

g2d.drawString("i am a boy!",50,50);
会有异常显示：
java.lang.NullPointerException

这个问题如何处理？

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sxnucseven 2006-08-15

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f.getContentPane().add(p);之前加上
f.setSize(300,300);
f.setLayout(new BorderLayout());
应该就行了

千里冰封820 2006-08-15

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/*
* Tests.java
*
* Created on 2006年8月15日, 上午11:09
*
* To change this template, choose Tools | Template Manager
* and open the template in the editor.
*/

package ss;

/**
*
* @author lbf
*/
import java.awt.*;
import javax.swing.*;
public class Tests extends JFrame implements Runnable{
private JPanel jp;
private int arc;
private Color c;
/** Creates a new instance of Tests */
public Tests(){
super("test");
jp=new JPanel();
this.getContentPane().add(jp,BorderLayout.CENTER);
this.setBounds(300,300,400,400);
this.setVisible(true);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
new Thread(this).start();
}
public void run(){
while(true){
try{
arc++;
Thread.sleep(10);
draw(jp);
if(arc==360){
arc=0;
c=new Color((int)(Math.random()*0xffffff));
}

}
catch(Exception exe){
exe.printStackTrace();
}
}
}
private void draw(JPanel p){
Graphics g=p.getGraphics();
g.setColor(c);
g.fillArc(50,50,100,100,0,arc);
}
public static void main(String args[]){
new Tests();
}
}
你试试这个,和你实现是一样的,我这里用了一个线程一直在画,这样可以看得更清楚些

wuyan19831013 2006-08-15

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多谢楼上的２个兄弟

我改了下，没有异常了，但是还是画不出图形

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

public class drawonpanel {
Graphics2D g2d;
public drawonpanel(JPanel p) {
g2d =(Graphics2D) p.getGraphics();
g2d.setBackground(Color.BLUE);
g2d.drawString("i am a boy!",50,50);
g2d.drawLine(20,30,100,20);
}
}

import javax.swing.*;
public class Main {
JPanel p;
JFrame f;
drawonpanel pp;
public Main() {
p = new JPanel();
f = new JFrame();
f.getContentPane().add(p);
f.pack();
f.show();
pp = new drawonpanel(p);
}

public static void main(String args[]){
new Main();
}
}

Areslp 2006-08-15

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g2d =(Graphics2D) p.getGraphics();
System.out.println("inside of drawonpanel");
这里g2d是null

UnAgain 2006-08-15

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

class drawonpanel extends JPanel{
Graphics2D g2d;
public drawonpanel() {
}
public void paint(Graphics g){
g2d =(Graphics2D) g;
System.out.println("inside of drawonpanel");
g2d.drawString("i am a boy!",50,50);
}
}

public class Main {
JPanel p;
JFrame f;
drawonpanel pp;
public Main() {
//p = new JPanel();
f = new JFrame();
pp = new drawonpanel();
f.getContentPane().add(pp);
f.pack();
f.show();
}

public static void main(String args[]){
new Main();
}
}

千里冰封820 2006-08-15