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计算器的设计和编写方法，主要分为以下几个步骤：

1. 设计计算器的界面

根据计算器的功能，我们可以设计出如下的简单计算器界面：

+--------------------------+
|                          |
|         0                |
|                          |
|    7  8  9  /            |
|                          |
|    4  5  6  *            |
|                          |
|    1  2  3  -            |
|                          |
|    0  .  =  +            |
|                          |
+--------------------------+

其中，0表示显示计算结果的文本框，其他按钮表示不同的计算功能。

2. 实现计算器的界面

使用Java Swing框架可以很容易地实现上述界面，示例代码如下：

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

public class Calculator extends JFrame {
    private JTextField resultField;

    public Calculator() {
        setTitle("Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(250, 200);
        setLocationRelativeTo(null);

        JPanel panel = new JPanel(new GridLayout(4, 4, 5, 5));
        panel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        add(panel, BorderLayout.CENTER);

        resultField = new JTextField("0", 20);
        resultField.setEditable(false);
        add(resultField, BorderLayout.NORTH);

        JButton button7 = new JButton("7");
        panel.add(button7);

        JButton button8 = new JButton("8");
        panel.add(button8);

        JButton button9 = new JButton("9");
        panel.add(button9);

        JButton buttonDivide = new JButton("/");
        panel.add(buttonDivide);

        JButton button4 = new JButton("4");
        panel.add(button4);

        JButton button5 = new JButton("5");
        panel.add(button5);

        JButton button6 = new JButton("6");
        panel.add(button6);

        JButton buttonMultiply = new JButton("*");
        panel.add(buttonMultiply);

        JButton button1 = new JButton("1");
        panel.add(button1);

        JButton button2 = new JButton("2");
        panel.add(button2);

        JButton button3 = new JButton("3");
        panel.add(button3);

        JButton buttonMinus = new JButton("-");
        panel.add(buttonMinus);

        JButton button0 = new JButton("0");
        panel.add(button0);

        JButton buttonDot = new JButton(".");
        panel.add(buttonDot);

        JButton buttonEqual = new JButton("=");
        panel.add(buttonEqual);

        JButton buttonPlus = new JButton("+");
        panel.add(buttonPlus);

        setVisible(true);
    }

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

3. 实现计算按钮不同的计算功能

计算器上的按钮都有不同的计算功能，可以通过为每个按钮添加ActionListener来实现。示例代码如下：

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

public class Calculator extends JFrame implements ActionListener {
    private JTextField resultField;

    private double firstNumber;
    private String operator;

    public Calculator() {
        setTitle("Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(250, 200);
        setLocationRelativeTo(null);

        JPanel panel = new JPanel(new GridLayout(4, 4, 5, 5));
        panel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        add(panel, BorderLayout.CENTER);

        resultField = new JTextField("0", 20);
        resultField.setEditable(false);
        add(resultField, BorderLayout.NORTH);

        JButton button7 = new JButton("7");
        panel.add(button7);
        button7.addActionListener(this);

        JButton button8 = new JButton("8");
        panel.add(button8);
        button8.addActionListener(this);

        JButton button9 = new JButton("9");
        panel.add(button9);
        button9.addActionListener(this);

        JButton buttonDivide = new JButton("/");
        panel.add(buttonDivide);
        buttonDivide.addActionListener(this);

        JButton button4 = new JButton("4");
        panel.add(button4);
        button4.addActionListener(this);

        JButton button5 = new JButton("5");
        panel.add(button5);
        button5.addActionListener(this);

        JButton button6 = new JButton("6");
        panel.add(button6);
        button6.addActionListener(this);

        JButton buttonMultiply = new JButton("*");
        panel.add(buttonMultiply);
        buttonMultiply.addActionListener(this);

        JButton button1 = new JButton("1");
        panel.add(button1);
        button1.addActionListener(this);

        JButton button2 = new JButton("2");
        panel.add(button2);
        button2.addActionListener(this);

        JButton button3 = new JButton("3");
        panel.add(button3);
        button3.addActionListener(this);

        JButton buttonMinus = new JButton("-");
        panel.add(buttonMinus);
        buttonMinus.addActionListener(this);

        JButton button0 = new JButton("0");
        panel.add(button0);
        button0.addActionListener(this);

        JButton buttonDot = new JButton(".");
        panel.add(buttonDot);
        buttonDot.addActionListener(this);

        JButton buttonEqual = new JButton("=");
        panel.add(buttonEqual);
        buttonEqual.addActionListener(this);

        JButton buttonPlus = new JButton("+");
        panel.add(buttonPlus);
        buttonPlus.addActionListener(this);

        setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();

        if (command.equals("0") || command.equals("1") || command.equals("2") ||
                command.equals("3") || command.equals("4") || command.equals("5") ||
                command.equals("6") || command.equals("7") || command.equals("8") ||
                command.equals("9")) {
            if (resultField.getText().equals("0")) {
                resultField.setText(command);
            } else {
                resultField.setText(resultField.getText() + command);
            }
        } else if (command.equals(".")) {
            if (!resultField.getText().contains(".")) {
                resultField.setText(resultField.getText() + ".");
            }
        } else if (command.equals("/")) {
            firstNumber = Double.parseDouble(resultField.getText());
            operator = "/";
            resultField.setText("0");
        } else if (command.equals("*")) {
            firstNumber = Double.parseDouble(resultField.getText());
            operator = "*";
            resultField.setText("0");
        } else if (command.equals("-")) {
            firstNumber = Double.parseDouble(resultField.getText());
            operator = "-";
            resultField.setText("0");
        } else if (command.equals("+")) {
            firstNumber = Double.parseDouble(resultField.getText());
            operator = "+";
            resultField.setText("0");
        } else if (command.equals("=")) {
            double secondNumber = Double.parseDouble(resultField.getText());
            if (operator.equals("/")) {
                resultField.setText(String.valueOf(firstNumber / secondNumber));
            } else if (operator.equals("*")) {
                resultField.setText(String.valueOf(firstNumber * secondNumber));
            } else if (operator.equals("-")) {
                resultField.setText(String.valueOf(firstNumber - secondNumber));
            } else if (operator.equals("+")) {
                resultField.setText(String.valueOf(firstNumber + secondNumber));
            }
        }
    }

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

4. 显示计算器按钮运算后的结果

当用户点击“=”按钮时，我们需要根据之前输入的数字和运算符进行计算，并将计算结果显示在文本框中。代码中已经实现了这一功能。

到此为止，一个简单的Java计算器就完成了。