以前用java写的计算器程序

以前用java写的计算器程序:

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

public class Calculator extends JFrame
{
 private Container container;
 private GridBagLayout layout;
 private GridBagConstraints constraints; 
 private JTextField displayField;//计算结果显示区
 private String lastCommand;//保存+,-,*,/,=命令
 private double result;//保存计算结果
 private boolean start;//判断是否为数字的开始
 
  public Calculator()
  {
  super("Calculator");
  container=getContentPane();
  layout=new GridBagLayout();
  container.setLayout(layout);
  constraints=new GridBagConstraints();
  
  start=true;
  result=0;
  lastCommand = "=";
  
  displayField=new JTextField(20);
  displayField.setHorizontalAlignment(JTextField.RIGHT);
  constraints.gridx=0;
  constraints.gridy=0;
  constraints.gridwidth=4;
  constraints.gridheight=1;
  constraints.fill=GridBagConstraints.BOTH;
  constraints.weightx=100;
  constraints.weighty=100;
  layout.setConstraints(displayField,constraints);
  container.add(displayField);
  
  ActionListener insert = new InsertAction();
    ActionListener command = new CommandAction();
    
  addButton("Backspace",0,1,2,1,insert);
  addButton("CE",2,1,1,1,insert);
  addButton("C",3,1,1,1,insert);
  
  addButton("7",0,2,1,1,insert);
  addButton("8",1,2,1,1,insert);
  addButton("9",2,2,1,1,insert);
  addButton("/",3,2,1,1,command);
  
  addButton("4",0,3,1,1,insert);
  addButton("5",1,3,1,1,insert);
  addButton("6",2,3,1,1,insert);
  addButton("*",3,3,1,1,command);
  
  addButton("1",0,4,1,1,insert);
  addButton("2",1,4,1,1,insert);
  addButton("3",2,4,1,1,insert);
  addButton("-",3,4,1,1,command);
  
  addButton("0",0,5,1,1,insert);
  addButton("+/-",1,5,1,1,insert);//只显示"-"号,"+"没有实用价值
  addButton(".",2,5,1,1,insert);
  addButton("+",3,5,1,1,command);
  
  addButton("=",0,6,4,1,command);
  
  setSize(300,300);
  setVisible(true);
  }
  private void addButton(String label,int row,int column,int with,int height,ActionListener listener)
  {
   JButton button=new JButton(label);
 
   constraints.gridx=row;
   constraints.gridy=column;
   
   constraints.gridwidth=with;
   constraints.gridheight=height;
   
    constraints.fill=GridBagConstraints.BOTH;
   
   button.addActionListener(listener);
   layout.setConstraints(button,constraints);
   container.add(button);
  }
  private class InsertAction implements ActionListener
    {
       public void actionPerformed(ActionEvent event)
       {
          String input=event.getActionCommand();
          if (start)
          {
             displayField.setText("");
             start=false;
             
             if(input.equals("+/-"))
             displayField.setText(displayField.getText()+"-"); 
          }
          if(!input.equals("+/-"))
          {
           if(input.equals("Backspace"))
           {
            String str=displayField.getText();
            if(str.length()>0)
              displayField.setText(str.substring(0,str.length()-1));
            }
           else if(input.equals("CE")||input.equals("C"))
           {
            displayField.setText("0");
            start=true;
           } 
           else
             displayField.setText(displayField.getText()+input);
         }
       }
    }
    private class CommandAction implements ActionListener
    {
       public void actionPerformed(ActionEvent evt)
       {
          String command=evt.getActionCommand();
          if(start)
          {
           lastCommand=command;
          }
          else
          {
          calculate(Double.parseDouble(displayField.getText()));
          lastCommand=command;
          start=true;
          }
         }
      }
   public void calculate(double x)
   {
      if (lastCommand.equals("+")) result+= x;      
      else if (lastCommand.equals("-")) result-=x;
      else if (lastCommand.equals("*")) result*=x;       
      else if (lastCommand.equals("/")) result/=x;
      else if (lastCommand.equals("=")) result=x;
       displayField.setText(""+ result);
    }
   public static void  main(String []args)
   {
     Calculator calculator=new Calculator();
     calculator.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   }
}