java.lang.NullPointerException

[hudu1]

import java.awt.*;
//import java.awt.event.MouseListener;
import javax.swing.*;
public class tm18{
private JTextField t1,t2;
private JButton b1,b2,b3;
private JLabel x1,x2;
private JPanel j1,j2,j3;
public static void main(String args[]){
tm18 x=new tm18();
x.go();
}
public void go(){
JFrame xin=new JFrame("Jframe");
t1=new JTextField(30);
t2=new JTextField(30);
Container xx=xin.getContentPane();
xin.setLayout(new FlowLayout());
b1=new JButton("相加");
b2=new JButton("清空");
b3=new JButton("退出");
x1=new JLabel("frist number:");
x2=new JLabel("second number");
j1=new JPanel();
j2=new JPanel();
j1.add(x1);
j1.add(t1);
j2.add(x2);
j2.add(t2);
j3.add(b1);
j3.add(b2);
j3.add(b3);
xx.add(j1);
xx.add(j2);
xx.add(j3,BorderLayout.SOUTH);
xin.pack();
xin.setVisible(true);
}
}
我不知道什么地方错了,请大家帮助

[unika_ly12]

把异常信息发出来看看

[hudu1]

Exception in thread "main" java.lang.NullPointerException
at tm18.go(tm18.java:30)
at tm18.main(tm18.java:11)

[bloodstars]

j3 没有 new

[interpb]

j3 没有构造对象

[hudu1]

OK 了