自己写着写着 忘了。。顺便看看书 哈哈~~
Window:

import java.awt.*;
public class Window {
  public static void main(String args[]){
    WindowInit win=new WindowInit();
    Container con=win.getContentPane();
    con.setBackground(Color.green);
  }
}

WindowInit:

import java.awt.*;
import javax.swing.*;
public class WindowInit extends JFrame{
  JTextField input1,input2;
  JComboBox choicefuhao;
  JButton button;
  JTextArea showarea;
  OperatorListener operator;
  ComputerListener computer;
  public WindowInit(){
    init();
    setBounds(100,100,429,329);
    setTitle("计算器");
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
  void init(){
    setLayout(new FlowLayout());
    input1=new JTextField(5);
    input2=new JTextField(5);
    button=new JButton("确定");
    choicefuhao=new JComboBox();
    choicefuhao.addItem("请选择符号");
    choicefuhao.addItem("+");
    choicefuhao.addItem("-");
    choicefuhao.addItem("*");
    choicefuhao.addItem("/");
    showarea=new JTextArea(9,36);
    operator=new OperatorListener();
    computer=new ComputerListener();
    operator.setChoicefuhao(choicefuhao);
    operator.setWorktogether(computer);
    computer.setInput1(input1);
    computer.setInput2(input2);
    computer.setTextArea(showarea);
    button.addActionListener(computer);//感觉这里也应该加上
//input1.addActionListener,input2.addActionListener
    choicefuhao.addItemListener(operator);
    add(input1);
    add(choicefuhao);
    add(input2);
    add(button);
    add(showarea);
  }
}

OperatorListener:

import javax.swing.*;
 import java.awt.event.*;
 public class OperatorListener implements ItemListener{
  JComboBox choicefuhao;
  ComputerListener worktogether;
  public void setChoicefuhao(JComboBox e){
   choicefuhao=e;
  }
  public void setWorktogether(ComputerListener e){
   worktogether=e;
  }
  public void itemStateChanged(ItemEvent e){
   String fuhao=choicefuhao.getSelectedItem().toString();
   worktogether.setfuhao(fuhao);
  }
 }


ComputerListener:

import java.awt.event.*;
import javax.swing.*;
public class ComputerListener implements ActionListener{
  JTextField input1,input2;
  JButton button;
  JTextArea showarea;
  String fuhao;
  public void setInput1(JTextField e){
    input1=e;
  }
  public void setInput2(JTextField e){
    input2=e;
  }
  public void setfuhao(String e){
    fuhao=e;
  }
  public void setTextArea(JTextArea e){
    showarea=e;
  }
  public void actionPerformed(ActionEvent e){
    double num1=Double.parseDouble(input1.getText());
    double num2=Double.parseDouble(input2.getText());
    double result=0;
    if(fuhao.equals("+"))
      result=num1+num2;
    else if(fuhao.equals("-"))
      result=num1-num2;
    else if(fuhao.equals("*"))
      result=num1*num2;
    else 
      result=num1/num2;
    showarea.append(num1+fuhao+num2+"="+result+"\n");
  }

}

这是运行结果：

java简单计算器_java