下面是一个“计算器”的程序，不过因为遇到困难，所以还没有编写完毕！这个程序在编译的时候能够通过，但当我点击“＝”的时候，就会出现下列报错：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at Counter$1.actionPerformed(ceshi.java:85)
        at java.awt.Button.processActionEvent(Unknown Source)
        at java.awt.Button.processEvent(Unknown Source)
        at java.awt.Component.dispatchEventImpl(Unknown Source)
        at java.awt.Component.dispatchEvent(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(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)
请问一下，我的程序哪里出错了？请大家赐教！！   
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class Counter extends JFrame
{  
      Double x,y;
      boolean z;
      //实例化2个显示屏
      JTextField Jtextfield1=new JTextField(20);
      TextField textfield2=new TextField(8);
      
      //实例化5个面板
      JPanel Jpanel1=new JPanel();
      JPanel Jpanel2=new JPanel();
      JPanel Jpanel3=new JPanel();
      JPanel Jpanel4=new JPanel();
      JPanel Jpanel5=new JPanel(); 
       
      //实例化一个空字符缓冲区
      StringBuffer str=new StringBuffer();
       
      //实例化计算器上的按钮
      Button[][] b=new Button[][]{{new Button("MC"),
                                   new Button("MR"),
                                   new Button("MS"),
                                   new Button("M+")},
                            
                                  {new Button("Backspace"),
                                   new Button("CE"),
                                   new Button("C")},
                           
                                  {new Button("7"),
                                   new Button("8"),
                                   new Button("9"),
                                   new Button("/"),
                                   new Button("sqrt"),
                                   new Button("4"),
                                   new Button("5"),
                                   new Button("6"),
                                   new Button("*"),
                                   new Button("%"),
                                   new Button("1"),
                                   new Button("2"),
                                   new Button("3"),
                                   new Button("-"),
                                   new Button("1/x"),
                                   new Button("0"),
                                   new Button("+/-"),
                                   new Button("."),                                                                                           
                                   new Button("+"),
                                   new Button("=")}                                                                                                       
                                 };                    
      ActionListener action=new ActionListener()
      {
            public void actionPerformed(ActionEvent e)
            {
                   if(e.getSource()==b[1][0])
                   {
                         if(str.length()>1)  //"Backspace"
                         {
                               str.setLength((str.length())-1);
                               Jtextfield1.setText(str.toString());
                         }
                         else
                             Jtextfield1.setText("0.");
                   }
                   else if(e.getSource()==b[1][1])  //"CE"
                   {
                           Jtextfield1.setText("0.");
                           str.setLength(0);
                   }
                   else if(e.getSource()==b[1][2])  //"C"
                   {
                            Jtextfield1.setText("0.");
                           str.setLength(0);
                   }
                   else if(e.getSource()==b[2][18])  //"+"
                   {
                            str.setLength(0);
                            z=false;
                   }
                   else if(e.getSource()==b[2][19])  //"="
                   {  
                            Jtextfield1.setText(Double.toString(x+y));
                            z=true;
                   }
                   else if(str.length()<=10)  //"数字键"，“10”表示有限位数为10位
                   {
                           Jtextfield1.setText(str.append(e.getActionCommand()).toString());
                           if(z=true)  
                             x=Double.parseDouble(str.toString());
                           else
                             y=Double.parseDouble(str.toString());    
                 }
            }
      }; 
      
      public Counter(String str)
      {
              super(str);
              Jtextfield1.setBackground(Color.white);
              Jtextfield1.setEditable(false);
              Jtextfield1.setText("0.");
              setLayout(null);
        setJPanel1();
        setJPanel2();
        setJPanel3();
        setJPanel4();
        setJPanel5();
        add(Jpanel1);
        add(Jpanel2);
        add(Jpanel3);
        add(Jpanel4);
        add(Jpanel5);
        for(int i=0;i<b.length;i++)
        { 
          for(int j=0;j<b[i].length;j++)
          {
                  b[i][j].addActionListener(action);
          }
        } 
      }
      public void setJPanel1()
      {
              GridLayout GLayout1=new GridLayout(1,1);
              Jpanel1.setLayout(GLayout1);
              Jpanel1.add(Jtextfield1);
              Jtextfield1.setHorizontalAlignment(JTextField.RIGHT );
              Jpanel1.setBounds(10,10,252,22);
      }
      public void setJPanel2()
      {
              GridLayout GLayout2=new GridLayout(5,1,10,6);
              Jpanel2.setLayout(GLayout2);
              for(int i=0;i<4;i++)
                b[0][i].setForeground(Color.red);
              Jpanel2.add(b[0][0]);
              Jpanel2.add(b[0][1]);
              Jpanel2.add(b[0][2]);
              Jpanel2.add(b[0][3]);
              Jpanel2.setBounds(10,74,40,165);
      }
      public void setJPanel3()
      {
              GridLayout GLayout3=new GridLayout(1,3,5,0);
              Jpanel3.setLayout(GLayout3);
              for(int i=0;i<3;i++)
                b[1][i].setForeground(Color.red);
              Jpanel3.add(b[1][0]);
              Jpanel3.add(b[1][1]);
              Jpanel3.add(b[1][2]);
              Jpanel3.setBounds(57,39,205,25);
      }
      public void setJPanel4()
      {
            GridLayout GLayout4=new GridLayout(4,5,5,6);
            Jpanel4.setLayout(GLayout4);
            for(int i=0;i<b[2].length;i++)
            {
                  Jpanel4.add(b[2][i]);
              b[2][i].setForeground(Color.blue);
          }
          b[2][4].setForeground(Color.red);
          b[2][9].setForeground(Color.red);
          b[2][14].setForeground(Color.red);
          Jpanel4.setBounds(57,74,205,132);
      }
      public void setJPanel5()
      {
              GridLayout GLayout5=new GridLayout(1,1);
              Jpanel5.setLayout(GLayout5);
              textfield2.setEditable(false);
              Jpanel5.add(textfield2);
              Jpanel5.setBounds(15,40,30,25);
      }
        public static void main(String[] args)
        {
                Counter counter=new Counter("计算器");
                counter.setSize(280,245);
                counter.setLocation(300,250);
                counter.setVisible(true);
                counter.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
}