求助，急...！

本人初学者，遇到一个问题，麻烦哪位朋友帮忙解决哈。。。。。万分感谢！！！

 

问题：  编译一个简单的计算机图形窗口，要求（在点击相应数字时在文本域中显示点击数字）

             可是编译出.class文件，并也显示出图形窗口，但点击上面的数字时，编译器报错。

           求解决方法.  谢谢.........（代码是自己编写，如有不规范的地方请多多见谅）

错误提示如下：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
 at awt.layout.Calculator$TestListener.actionPerformed(Calculator.java:53)
 at java.awt.Button.processActionEvent(Button.java:392)
 at java.awt.Button.processEvent(Button.java:360)
 at java.awt.Component.dispatchEventImpl(Component.java:4501)
 at java.awt.Component.dispatchEvent(Component.java:4331)
 at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
 at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
 at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
 at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
 at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
------------------------------------------------------------------------------------------------

原代码如下：

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

public class Calculator
{
 Frame f;
 Button b;
 TextField t;
 Panel p;
 
 public static void main(String[] args)
 {
  Calculator c = new Calculator();
  c.init();
  
 }
 
 public void init()
 {   
  Frame f = new Frame("计算器");
  TextField t = new TextField(50);
  Panel p = new Panel();
  TestListener ts = new TestListener();
  p.setLayout(new GridLayout(6,5));
  Button [] bt =new Button[30];
  String [] buttonname ={"MC","MR","MS","M+","M-","←","CE","C","±","√","7","8","9","/","%","4","5","6","*","1/x","1","2","3","-","=","0",".","+","a","b","c","d"};
  for(int i=0;i<=29;i++)
  {
   p.add(bt[i] = new Button(buttonname[i]));
   bt[i].addActionListener(ts);
  }
  
  
  f.addWindowListener(new WindowAdapter(){
   public void windowClosing(WindowEvent e)
   {
    System.exit(1);
   }
  });
  f.add(t,BorderLayout.NORTH);
  f.add(p,BorderLayout.CENTER);
  f.pack();
  f.setVisible(true);
 }
 
 public class TestListener implements ActionListener
 {
  public void actionPerformed(ActionEvent e)
  {
   String s = e.getActionCommand();
   t.setText(s);
  }
 }
 
}