程序代码：

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"); //调用父类的方法。构造函数的名称与类名必须一致。
container=getContentPane(); //返回些对话框的contentPane对象。（是JDialog类的一个方法。）
start=true;
result=0;
lastCommand = "="; 

layout=new GridBagLayout(); //设置布局窗口为GridBagLayout.
container.setLayout(layout);
constraints=new GridBagConstraints();
displayField=new JTextField(20);
displayField.setHorizontalAlignment(JTextField.RIGHT); //设置对齐方式为右对起。
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);
} 

}