java计算器退格功能

以下是一个简单的Java计算器，具有退格功能：

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

public class Calculator extends JFrame implements ActionListener {
    private JTextField display;
    private JButton[] buttons;
    private String[] buttonLabels = { "7", "8", "9", "/", "4", "5", "6", "*", "1", "2", "3", "-", "0", ".", "=", "+" };
    private JPanel buttonPanel;

    public Calculator() {
        display = new JTextField("0", 20);
        display.setEditable(false);
        buttons = new JButton[buttonLabels.length];
        buttonPanel = new JPanel(new GridLayout(4, 4));

        for (int i = 0; i < buttonLabels.length; i++) {
            buttons[i] = new JButton(buttonLabels[i]);
            buttons[i].addActionListener(this);
            buttonPanel.add(buttons[i]);
        }

        getContentPane().add(display, BorderLayout.NORTH);
        getContentPane().add(buttonPanel, BorderLayout.CENTER);
        pack();
    }

    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        if (command.equals("C")) {
            display.setText("0");
        } else if (command.equals("=")) {
            String expression = display.getText();
            try {
                double result = evaluate(expression);
                display.setText(Double.toString(result));
            } catch (Exception ex) {
                display.setText("Error");
            }
        } else if (command.equals("Backspace")) {
            String text = display.getText();
            if (text.length() > 0) {
                text = text.substring(0, text.length() - 1);
                if (text.length() == 0) {
                    text = "0";
                }
                display.setText(text);
            }
        } else {
            String text = display.getText();
            if (text.equals("0")) {
                text = "";
            }
            display.setText(text + command);
        }
    }

    private double evaluate(String expression) throws Exception {
        return new Object() {
            int pos = -1, ch;

            void nextChar() {
                ch = (++pos < expression.length()) ? expression.charAt(pos) : -1;
            }

            boolean eat(int charToEat) {
                while (ch == ' ')
                    nextChar();
                if (ch == charToEat) {
                    nextChar();
                    return true;
                }
                return false;
            }

            double parse() throws Exception {
                nextChar();
                double x = parseExpression();
                if (pos < expression.length())
                    throw new Exception("Unexpected: " + (char) ch);
                return x;
            }

            double parseExpression() throws Exception {
                double x = parseTerm();
                for (;;) {
                    if (eat('+'))
                        x += parseTerm();
                    else if (eat('-'))
                        x -= parseTerm();
                    else
                        return x;
                }
            }

            double parseTerm() throws Exception {
                double x = parseFactor();
                for (;;) {
                    if (eat('*'))
                        x *= parseFactor();
                    else if (eat('/'))
                        x /= parseFactor();
                    else
                        return x;
                }
            }

            double parseFactor() throws Exception {
                if (eat('+'))
                    return parseFactor();
                if (eat('-'))
                    return -parseFactor();

                double x;
                int startPos = this.pos;
                if (eat('(')) {
                    x = parseExpression();
                    eat(')');
                } else if ((ch >= '0' && ch <= '9') || ch == '.') {
                    while ((ch >= '0' && ch <= '9') || ch == '.')
                        nextChar();
                    x = Double.parseDouble(expression.substring(startPos, this.pos));
                } else if (ch >= 'a' && ch <= 'z') {
                    while (ch >= 'a' && ch <= 'z')
                        nextChar();
                    String func = expression.substring(startPos, this.pos);
                    x = parseFactor();
                    if (func.equals("sqrt"))
                        x = Math.sqrt(x);
                    else if (func.equals("sin"))
                        x = Math.sin(Math.toRadians(x));
                    else if (func.equals("cos"))
                        x = Math.cos(Math.toRadians(x));
                    else if (func.equals("tan"))
                        x = Math.tan(Math.toRadians(x));
                    else
                        throw new Exception("Unknown function: " + func);
                } else {
                    throw new Exception("Unexpected: " + (char) ch);
                }

                if (eat('^'))
                    x = Math.pow(x, parseFactor());

                return x;
            }
        }.parse();
    }

    public static void main(String[] args) {
        Calculator calculator = new Calculator();
        calculator.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        calculator.setVisible(true);
    }
}