yiding_he's opinions

public class MyCalculator implements ActionListener {

    private JFrame frame = new JFrame();

    private JTextField text = new JTextField("0");

    private Processor processor = new Processor();

    private void show() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            // nothing happens
        }
        initControls();
        frame.setVisible(true);
    }

    private void initControls() {
        initFrame();
        initTextField();
        initTopPanel();
        initButtonsPanel();
    }

    private void initFrame() {
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setTitle("计算器");
        frame.setLayout(new BorderLayout());
        frame.setSize(300, 250);
        frame.setResizable(false);
    }

    private void initButtonsPanel() {
        JPanel buttons_panel = new JPanel(new GridLayout(4, 4, 3, 3));
        buttons_panel.setBorder(new EmptyBorder(0, 10, 10, 10));
        String[] button_texts = {"7", "8", "9", "+", "4", "5", "6", 
                "-", "1", "2", "3", "*", "0", ".", "=", "/"};
        for (String button_text : button_texts) {
            buttons_panel.add(createButton(button_text));
        }
        frame.add(buttons_panel);
    }

    private void initTopPanel() {
        JPanel top_panel = new JPanel(new BorderLayout());
        top_panel.setBorder(new EmptyBorder(10, 10, 5, 10));
        top_panel.add(text, BorderLayout.CENTER);
        frame.add(top_panel, BorderLayout.NORTH);
    }

    private void initTextField() {
        text.setEditable(false);
        text.setHorizontalAlignment(JTextField.RIGHT);
        text.setBackground(Color.white);
    }

    private JButton createButton(String button_text) {
        JButton button = new JButton(button_text);
        button.addActionListener(this);
        return button;
    }

    /**
     * 处理按钮事件
     *
     * @param e 按钮事件
     */
    public void actionPerformed(ActionEvent e) {
        if (!(e.getSource() instanceof JButton)) {
            return;
        }

        JButton btn = (JButton) e.getSource();
        String btn_text = btn.getText();
        if ("0123456789".indexOf(btn_text) != -1) {
            processor.numButtonClicked(btn_text);
        } else if (".".indexOf(btn_text) != -1) {
            processor.dotButtonClicked();
        } else if ("+-*/".indexOf(btn_text) != -1) {
            processor.operateButtonClicked(btn_text);
        } else if ("=".indexOf(btn_text) != -1) {
            processor.equalsButtonClicked();
        }

        text.setText(processor.getResult());
    }

    /**
     * 程序入口
     *
     * @param args 命令行参数
     */
    public static void main(String[] args) {
        new MyCalculator().show();
    }
}

/**
 * 运算部分
 */
class Processor {

    private String result = "0";

    private boolean start_new = false;

    private double last_value = 0;

    private String operation = "";

    public String getResult() {
        return result;
    }

    public void numButtonClicked(String text) {
        if (!start_new) {
            result += text;
        } else {
            result = text;
        }

        // 去掉开头的 0
        while (result.startsWith("0") && !result.startsWith("0.") && result.length() > 1) {
            result = result.substring(1);
        }

        start_new = false;
    }

    public void dotButtonClicked() {
        if (result.indexOf(".") == -1) {
            result += ".";
        }
    }

    public void operateButtonClicked(String text) {
        operation = text;
        if (result.length() > 0) {
            last_value = Double.parseDouble(result);
        }
        start_new = true;
    }

    public void equalsButtonClicked() {
        double value = Double.parseDouble(result);

        // 执行计算
        if (operation.equals("+")) {
            result = String.valueOf(value + last_value);
        } else if (operation.equals("-")) {
            result = String.valueOf(last_value - value);
        } else if (operation.equals("*")) {
            result = String.valueOf(last_value * value);
        } else if (operation.equals("/")) {
            if (value == 0) {
                result = "ERROR";
            } else {
                result = String.valueOf(last_value / value);
            }
        }

        // 去掉结尾的“.0000...”
        if (result.matches(".+\\.0+")) {
            result = result.substring(0, result.lastIndexOf("."));
        }

        // 设置状态
        start_new = true;
        operation = "";
    }
}