java实现简易计算器 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。

首先，我们需要设计计算器的界面。可以采用Java Swing库来实现。

以下是一个简单的计算器界面设计：

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

public class CalculatorUI extends JFrame {
    private JTextField displayField;

    public CalculatorUI() {
        initComponents();
    }

    private void initComponents() {
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(300, 300));

        displayField = new JTextField();
        displayField.setPreferredSize(new Dimension(250, 30));

        JPanel buttonPanel = new JPanel(new GridLayout(4, 4, 5, 5));
        String[] buttonLabels = {
                "7", "8", "9", "/",
                "4", "5", "6", "*",
                "1", "2", "3", "-",
                "0", ".", "=", "+"
        };
        for (String label : buttonLabels) {
            JButton button = new JButton(label);
            buttonPanel.add(button);
        }

        JPanel contentPanel = new JPanel(new BorderLayout());
        contentPanel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        contentPanel.add(displayField, BorderLayout.NORTH);
        contentPanel.add(buttonPanel, BorderLayout.CENTER);

        getContentPane().add(contentPanel);
        pack();
    }

    public static void main(String[] args) {
        new CalculatorUI().setVisible(true);
    }
}

接下来，我们需要实现计算器按钮的不同计算功能。我们可以使用Java的ScriptEngine类来解析计算表达式并计算结果。

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class Calculator {
    private ScriptEngine engine;

    public Calculator() {
        ScriptEngineManager mgr = new ScriptEngineManager();
        engine = mgr.getEngineByName("JavaScript");
    }

    public double evaluate(String expression) throws ScriptException {
        return (double) engine.eval(expression);
    }
}

最后，我们需要将计算结果显示在计算器界面上。我们可以为每个计算按钮添加ActionListener，当用户点击按钮时，我们将所点击的按钮的文本内容添加到计算器显示文本框中，并在用户点击“=”按钮时计算表达式并显示结果。

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

public class CalculatorUI extends JFrame {
    private JTextField displayField;
    private Calculator calculator;

    public CalculatorUI() {
        initComponents();
        calculator = new Calculator();
    }

    private void initComponents() {
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(300, 300));

        displayField = new JTextField();
        displayField.setPreferredSize(new Dimension(250, 30));

        JPanel buttonPanel = new JPanel(new GridLayout(4, 4, 5, 5));
        String[] buttonLabels = {
                "7", "8", "9", "/",
                "4", "5", "6", "*",
                "1", "2", "3", "-",
                "0", ".", "=", "+"
        };
        for (String label : buttonLabels) {
            JButton button = new JButton(label);
            button.addActionListener(new CalculatorButtonActionListener());
            buttonPanel.add(button);
        }

        JPanel contentPanel = new JPanel(new BorderLayout());
        contentPanel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));
        contentPanel.add(displayField, BorderLayout.NORTH);
        contentPanel.add(buttonPanel, BorderLayout.CENTER);

        getContentPane().add(contentPanel);
        pack();
    }

    private class CalculatorButtonActionListener implements ActionListener {
        @Override
        public void actionPerformed(ActionEvent e) {
            String buttonLabel = ((JButton) e.getSource()).getText();
            if (buttonLabel.equals("=")) {
                try {
                    String expression = displayField.getText();
                    double result = calculator.evaluate(expression);
                    displayField.setText(Double.toString(result));
                } catch (ScriptException ex) {
                    JOptionPane.showMessageDialog(CalculatorUI.this,
                            "Invalid expression", "Error", JOptionPane.ERROR_MESSAGE);
                }
            } else {
                displayField.setText(displayField.getText() + buttonLabel);
            }
        }
    }

    public static void main(String[] args) {
        new CalculatorUI().setVisible(true);
    }
}