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);

   }

}