JTextArea 未附加到 swing GUI 中

Question

我目前正在使用 Swing 开发一个 GUI 计算器程序。我完成了实际的界面 here

但是，每当我点击其中一个数字按钮时，操作数文本区域中什么也没有出现，而 0.0 出现在结果文本区域中，可以看到 here。

这是我的代码：

`
public class Calculator extends JFrame implements ActionListener
{
    private static JTextArea operandArea;
    private static JTextArea resultArea;
    private static JButton[] buttons = new JButton[10];

    public Calculator()
    {
        setTitle("Calculator");
        setSize(800, 800);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);

        BorderLayout calculatorLayout = new BorderLayout();
        setLayout(calculatorLayout);

        JPanel wholeCalculator = new JPanel();
        add(wholeCalculator);
        JPanel textAreas = new JPanel();
        wholeCalculator.add(textAreas, calculatorLayout.NORTH);
        textAreas.setPreferredSize(new Dimension(800, 300));
        JPanel numberPad = new JPanel();
        wholeCalculator.add(numberPad, calculatorLayout.SOUTH);
        numberPad.setPreferredSize(new Dimension(800, 300));

        operandArea = new JTextArea();
        operandArea.setPreferredSize(new Dimension(600,75));
        resultArea = new JTextArea();
        resultArea.setPreferredSize(new Dimension(600,75));

        textAreas.setLayout(new GridLayout(4, 1));
        JLabel operand = new JLabel("Operand");
        JLabel result = new JLabel("Result");
        textAreas.add(operand);
        operand.setHorizontalAlignment(JLabel.CENTER);
        textAreas.add(operandArea);
        textAreas.add(result);
        result.setHorizontalAlignment(JLabel.CENTER);
        textAreas.add(resultArea);

        numberPad.setLayout(new GridLayout(1, 2));

        JPanel numbers = new JPanel();
        numberPad.add(numbers);
        numbers.setLayout(new GridLayout(4, 3));
        numbers.setPreferredSize(new Dimension(400, 400));
        JPanel operations = new JPanel();
        numberPad.add(operations);
        operations.setPreferredSize(new Dimension(200, 400));
        JButton minus = new JButton("-");
        minus.setActionCommand("minus");
        operations.add(minus);
        JButton plus = new JButton("+");
        plus.setActionCommand("plus");
        operations.add(plus);
        JButton multiply = new JButton("X");
        multiply.setActionCommand("multiply");
        operations.add(multiply);
        JButton divide = new JButton("/");
        divide.setActionCommand("divide");
        operations.add(divide);
        JButton decimal = new JButton(".");
        operations.add(decimal);
        JButton reset = new JButton("reset");
        reset.setActionCommand("reset");
        operations.add(reset);
        JButton clear = new JButton("clear");
        operations.add(clear);

        for (int i = 1; i <= 9; i++)
        {
            JButton temp = new JButton(i + " ");
            buttons[i] = temp;
            temp.addActionListener(this);
            numbers.add(temp);
        }
            numbers.add(new JButton("0"));
    }

    public void actionPerformed(ActionEvent e)
    {
        String action = e.getActionCommand();
        double initializer = 0.0;
        String operand = "";

        resultArea.setText(Double.toString(initializer));

        switch (action)
        {
            case "0":
                operandArea.append("0");
                operand = operand + "0";
                break;
            case "1":
                operandArea.append("1");
                operand = operand + "1";
                break;
            case "2":
                operandArea.append("2");
                operand = operand + "2";
                break;
            case "3":
                operandArea.append("3");
                operand = operand + "3";
                break;
            case "4":
                operandArea.append("4");
                operand = operand + "4";
                break;
            case "5":
                operandArea.append("5");
                operand = operand + "5";
                break;
            case "6":
                operandArea.append("6");
                operand = operand + "6";
                break;
            case "7":
                operandArea.append("7");
                operand = operand + "7";
                break;
            case "8":
                operandArea.append("8");
                operand = operand + "8";
                break;
            case "9":
                operandArea.append("9");
                operand = operand + "9";
                break;
            case ".":
                operandArea.append(".");
                operand = operand + ".";
                break;
            case "plus":
                initializer += Double.parseDouble(operand);
                resultArea.setText(Double.toString(initializer));
                break;
            case "minus":
                initializer -= Double.parseDouble(operand);
                resultArea.setText(Double.toString(initializer));
                break;
            case "multiply": initializer *= Double.parseDouble(operand);
                resultArea.setText(Double.toString(initializer));
                break;
            case "divide":
                try
                {
                    if (operand == "0")
                    {
                        throw new DivideByZeroException();
                    }
                    initializer /= Double.parseDouble(operand);
                    resultArea.setText(Double.toString(initializer));
                }
                catch (DivideByZeroException ex)
                {
                    resultArea.setText("Cannot divide by 0");
                }
                break;
            case "clear":
                operandArea.setText("");
                break;
            case "reset":
                operandArea.setText("");
                resultArea.setText("");
                initializer = 0.0;
                break;
        }
    }

    public static void main(String[] args)
    {
        Calculator frame = new Calculator();
        frame.setVisible(true);
    }
}
`

我认为使用追加命令会添加到空白处 space？我将不胜感激。

Answer 1

你的问题是一个复合问题。

首先，您使用...创建按钮...

JButton temp = new JButton(i + " ");

默认情况下，这会将 actionCommmand 设置为 i_ <- 而不是 space！（所以不会让我输入尾随 space，所以我使用 _ 来突出显示）

然后你使用

    switch (action)
    {
        case "0":
        //...

要检查 actionCommand，但命令不是（例如）[0]，它是 [0_]

相反，使用类似...

JButton temp = new JButton(Integer.toString(i));

您还想修复：

if (operand == "0")

因为 == 不是您比较 Strings (and a good IDE should be warning you about it). Instead useequals 的方式，例如...

if ("0".equals(operand))

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