设计GUI界面的计算器程序，用户可以通过鼠标依次输入参加计算的数值，进行加、减、乘、除等综合混合运算。 要求本计算器程序共有3个Java源文件。 ComputerPad.java该文件生成的类负责创建计算器的“主窗口”。该类含有main方法，整个程序的入口。 NumberButton.java生成的类负责为主类创建数字按钮对象，使用鼠标单击数字按钮可依次输入参加运算的数值的各位数字。 OperationButton.java生成的类负责为主类创建运算符号按钮对象。使用鼠标单击运算符号按钮可选择所要进行的运算。

以下是一个简单的GUI计算器程序的设计，共包含3个Java源文件：

1. ComputerPad.java

import javax.swing.*;

public class ComputerPad {
    private JFrame frame;
    private JTextField display;
    private JPanel numberPanel, operationPanel;
    private JButton[] numberButtons, operationButtons;
    private String currentInput = "";
    private double result = 0;
    private String currentOperation = "";
    
    public static void main(String[] args) {
        ComputerPad computerPad = new ComputerPad();
        computerPad.createUI();
    }

    public void createUI() {
        // 创建主窗口
        frame = new JFrame("计算器");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 400);

        // 创建数字按钮面板
        numberPanel = new JPanel();
        numberPanel.setLayout(new GridLayout(4, 3));
        numberButtons = new JButton[10];
        for (int i = 0; i < 10; i++) {
            numberButtons[i] = new NumberButton(i, this);
            numberPanel.add(numberButtons[i]);
        }

        // 创建运算符号按钮面板
        operationPanel = new JPanel();
        operationPanel.setLayout(new GridLayout(4, 1));
        operationButtons = new JButton[4];
        operationButtons[0] = new OperationButton("+", this);
        operationButtons[1] = new OperationButton("-", this);
        operationButtons[2] = new OperationButton("*", this);
        operationButtons[3] = new OperationButton("/", this);
        for (int i = 0; i < 4; i++) {
            operationPanel.add(operationButtons[i]);
        }

        // 创建显示面板
        display = new JTextField();
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setEditable(false);

        // 组装界面
        JPanel mainPanel = new JPanel();
        mainPanel.setLayout(new BorderLayout());
        mainPanel.add(display, BorderLayout.NORTH);
        mainPanel.add(numberPanel, BorderLayout.CENTER);
        mainPanel.add(operationPanel, BorderLayout.EAST);

        frame.add(mainPanel);
        frame.setVisible(true);
    }
    
    public void appendInput(int number) {
        currentInput += String.valueOf(number);
        display.setText(currentInput);
    }
    
    public void setOperation(String operation) {
        if (currentOperation.isEmpty()) {
            result = Double.parseDouble(currentInput);
        } else {
            result = evaluate(result, currentOperation, Double.parseDouble(currentInput));
        }
        currentOperation = operation;
        currentInput = "";
    }
    
    public void calculate() {
        result = evaluate(result, currentOperation, Double.parseDouble(currentInput));
        currentInput = "";
        currentOperation = "";
        display.setText(String.valueOf(result));
    }
    
    private double evaluate(double a, String op, double b) {
        switch (op) {
            case "+":
                return a + b;
            case "-":
                return a - b;
            case "*":
                return a * b;
            case "/":
                return a / b;
            default:
                return 0;
        }
    }
}

2. NumberButton.java

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

public class NumberButton extends JButton implements ActionListener {
    private int number;
    private ComputerPad computerPad;
    
    public NumberButton(int number, ComputerPad computerPad) {
        this.number = number;
        this.computerPad = computerPad;
        setText(String.valueOf(number));
        addActionListener(this);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        computerPad.appendInput(number);
    }
}

3. OperationButton.java

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

public class OperationButton extends JButton implements ActionListener {
    private String operation;
    private ComputerPad computerPad;
    
    public OperationButton(String operation, ComputerPad computerPad) {
        this.operation = operation;
        this.computerPad = computerPad;
        setText(operation);
        addActionListener(this);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        if (operation.equals("=")) {
            computerPad.calculate();
        } else {
            computerPad.setOperation(operation);
        }
    }
}

这个计算器程序的界面是一个三行四列的网格布局，其中第一行是显示面板，第二行是数字按钮面板，第三行是运算符号按钮面板。每个数字按钮和运算符号按钮都是一个独立的JButton对象，通过继承JButton并实现ActionListener接口来实现。在ComputerPad类中，我们定义了一些事件处理方法，例如appendInput()方法用来将数字输入到当前的输入串中，setOperation()方法用来设置当前的运算符号，calculate()方法用来进行计算。