java设计和编写一个简易的计算器：实现计算器的界面；实现计算按钮不同的计算功能；显示计算器按钮运算后的结果。

这里提供一个简单的Java Swing实现的计算器示例：

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

public class Calculator implements ActionListener {
    JFrame frame;
    JTextField textField;
    JButton button0, button1, button2, button3, button4, button5, button6, button7, button8, button9, buttonAdd, buttonSub, buttonMul, buttonDiv, buttonEqual, buttonClear;
    int num1=0, num2=0, result=0;
    char operator;

    Calculator() {
        frame = new JFrame("计算器");
        frame.setLayout(new GridLayout(5, 4));

        textField = new JTextField();
        textField.setEditable(false);

        button0 = new JButton("0");
        button1 = new JButton("1");
        button2 = new JButton("2");
        button3 = new JButton("3");
        button4 = new JButton("4");
        button5 = new JButton("5");
        button6 = new JButton("6");
        button7 = new JButton("7");
        button8 = new JButton("8");
        button9 = new JButton("9");
        buttonAdd = new JButton("+");
        buttonSub = new JButton("-");
        buttonMul = new JButton("*");
        buttonDiv = new JButton("/");
        buttonEqual = new JButton("=");
        buttonClear = new JButton("C");

        button0.addActionListener(this);
        button1.addActionListener(this);
        button2.addActionListener(this);
        button3.addActionListener(this);
        button4.addActionListener(this);
        button5.addActionListener(this);
        button6.addActionListener(this);
        button7.addActionListener(this);
        button8.addActionListener(this);
        button9.addActionListener(this);
        buttonAdd.addActionListener(this);
        buttonSub.addActionListener(this);
        buttonMul.addActionListener(this);
        buttonDiv.addActionListener(this);
        buttonEqual.addActionListener(this);
        buttonClear.addActionListener(this);

        frame.add(textField);
        frame.add(buttonClear);
        frame.add(buttonDiv);
        frame.add(buttonMul);
        frame.add(button7);
        frame.add(button8);
        frame.add(button9);
        frame.add(buttonSub);
        frame.add(button4);
        frame.add(button5);
        frame.add(button6);
        frame.add(buttonAdd);
        frame.add(button1);
        frame.add(button2);
        frame.add(button3);
        frame.add(buttonEqual);
        frame.add(button0);

        frame.setSize(300, 300);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == button0) {
            textField.setText(textField.getText() + "0");
        }
        if (e.getSource() == button1) {
            textField.setText(textField.getText() + "1");
        }
        if (e.getSource() == button2) {
            textField.setText(textField.getText() + "2");
        }
        if (e.getSource() == button3) {
            textField.setText(textField.getText() + "3");
        }
        if (e.getSource() == button4) {
            textField.setText(textField.getText() + "4");
        }
        if (e.getSource() == button5) {
            textField.setText(textField.getText() + "5");
        }
        if (e.getSource() == button6) {
            textField.setText(textField.getText() + "6");
        }
        if (e.getSource() == button7) {
            textField.setText(textField.getText() + "7");
        }
        if (e.getSource() == button8) {
            textField.setText(textField.getText() + "8");
        }
        if (e.getSource() == button9) {
            textField.setText(textField.getText() + "9");
        }
        if (e.getSource() == buttonAdd) {
            num1 = Integer.parseInt(textField.getText());
            operator = '+';
            textField.setText("");
        }
        if (e.getSource() == buttonSub) {
            num1 = Integer.parseInt(textField.getText());
            operator = '-';
            textField.setText("");
        }
        if (e.getSource() == buttonMul) {
            num1 = Integer.parseInt(textField.getText());
            operator = '*';
            textField.setText("");
        }
        if (e.getSource() == buttonDiv) {
            num1 = Integer.parseInt(textField.getText());
            operator = '/';
            textField.setText("");
        }
        if (e.getSource() == buttonEqual) {
            num2 = Integer.parseInt(textField.getText());
            switch (operator) {
                case '+':
                    result = num1 + num2;
                    break;
                case '-':
                    result = num1 - num2;
                    break;
                case '*':
                    result = num1 * num2;
                    break;
                case '/':
                    result = num1 / num2;
                    break;
            }
            textField.setText(String.valueOf(result));
        }
        if (e.getSource() == buttonClear) {
            textField.setText("");
        }
    }

    public static void main(String[] args) {
        new Calculator();
    }
}

这个计算器界面由一个文本框和16个按钮组成，其中数字按钮用于输入数字，四个运算符按钮用于选择运算符，等号按钮用于计算结果，清除按钮用于清除文本框内容。在按钮的 ActionListener 中，根据不同的按钮执行相应的操作，具体实现方式参见代码注释。