一个计算器的计算方法的问题
代码如下:
import   java.awt.*;
import   java.awt.event.*;
import   javax.swing.*;

public   class   JFrameCalculator{
public   static   void   main(String   args[]){
JFrameCalculatorClass   JFC=new   JFrameCalculatorClass();
JFC.CalculatorDisplay();
}
}

class   JFrameCalculatorClass   extends   JFrame{
double   result;
boolean   start;
String   lastStrMethod;
Container   container=getContentPane();
JPanel   p1=new   JPanel();
JPanel   p2=new   JPanel();
JPanel   p3=new   JPanel();
JTextField   txtField=new   JTextField( "0 ",24);
JButton   []btn=new   JButton[11];
ActionListener   btnNumAction=new   NumberCountAction();
ActionListener   CountMethod=new   CountMethodClass();
//构造函数
JFrameCalculatorClass(){
result=0;
start=true;
lastStrMethod= "= ";

p1.add(txtField);

btn[0]=new   JButton( "1 ");
btn[1]=new   JButton( "2 ");
btn[2]=new   JButton( "3 ");
btn[3]=new   JButton( "4 ");
btn[4]=new   JButton( "5 ");
btn[5]=new   JButton( "6 ");
btn[6]=new   JButton( "7 ");
btn[7]=new   JButton( "8 ");
btn[8]=new   JButton( "9 ");
btn[9]=new   JButton( "0 ");  
btn[10]=new   JButton( ". ");

p2.setLayout(new   GridLayout(4,3,7,7));
for(int   i=0;i <11;i++){
btn[i].addActionListener(btnNumAction);
p2.add(btn[i]);
}

addButton( "+ ",CountMethod);
addButton( "- ",CountMethod);
addButton( "* ",CountMethod);
addButton( "/ ",CountMethod);
addButton( "= ",CountMethod);

}
//实例化数字按键
void   addButton(String   method,ActionListener   listener){
JButton   btn=new   JButton(method);
btn.addActionListener(listener);
p3.add(btn);
}

//构建窗体
public   void   CalculatorDisplay(){
container.add(new   JLabel( "       "),BorderLayout.WEST);
container.add(new   JLabel( "       "),BorderLayout.EAST);
container.add(p1,BorderLayout.NORTH);
container.add(p2,BorderLayout.CENTER);
container.add(p3,BorderLayout.SOUTH);
setSize(300,250);
setLocation(250,250);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);  
}
//获得文本框内的数据
  class   NumberCountAction   implements   ActionListener{
public   void   actionPerformed(ActionEvent   e){
String   input=e.getActionCommand();
if(start){
txtField.setText( " ");
start=false;
}
txtField.setText(txtField.getText()+input);
}
}
//设置文本框输入符号
class   CountMethodClass   implements   ActionListener{
public   void   actionPerformed(ActionEvent   e){
String   strMethod=e.getActionCommand();
if(start){
if(strMethod.equals( "- ")){
txtField.setText(strMethod);
start=false;
}else{
lastStrMethod=strMethod;
}
}else{
Calculate(Double.parseDouble(txtField.getText()));
lastStrMethod=strMethod;
start=true;