JPanel 设置为不可见，除默认值之外的组合框选择将其设置为可见，但缺少组件

Question

我正在构建一个欧姆定律应用程序。请记住，这是我的第一个程序（没有教程），所以请友善 =) 这是运行它时的样子：

组合框有计算电压、计算欧姆等的选项...

中心面板设置为不可见，直到您从组合框中做出选择。当您进行选择时，它应该是这样的：

我面临的问题，是我第一次进行选择时，仅显示一个 JLabel/JTextArea 组合（每对位于垂直框内的自己的面板上）：

如果我单击组合框并再次进行相同的选择，它将正确显示。我不明白为什么它在第一次点击时表现不正确。也许我在构建 GUI 时采取了错误的方法。这是到目前为止的代码：

private JFrame frame;
private String[] choiceList = {"", "Calculate Volts", "Calculate Amps", "Calculate Ohms", "Calculate Watts"};
private JTextField textField_2;
private JPanel centerPanel;
private String volts = "Volts";
private String amps = "Amps";
private String ohms = "Ohms";
private String watts = "Watts";
private JLabel var1Label;
private JLabel var2Label;
private JLabel var3Label;
private JFormattedTextField var1TextField;
private JFormattedTextField var2TextField;
private JFormattedTextField var3TextField;



/**
 * Launch the application.
 */
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                GUI window = new GUI();
                window.frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

/**
 * Create the application.
 */
public GUI() {
    initialize();
}

/**
 * Initialize the contents of the frame.
 */
private void initialize() {
    frame = new JFrame();
    frame.setTitle("BotsOne ElectriCalc");
    frame.setBounds(100, 100, 450, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JPanel northPanel = new JPanel();
    northPanel.setIgnoreRepaint(true);
    northPanel.setBorder(new LineBorder(Color.GRAY));
    frame.getContentPane().add(northPanel, BorderLayout.NORTH);

    JLabel choiceLabel = new JLabel("Please make a selection:");
    northPanel.add(choiceLabel);

    JComboBox choiceCombo = new JComboBox(choiceList);
    northPanel.add(choiceCombo);
    choiceCombo.addActionListener(new ChoiceComboListener());

    JPanel southPanel = new JPanel();
    southPanel.setIgnoreRepaint(true);
    southPanel.setBorder(new LineBorder(Color.GRAY));
    frame.getContentPane().add(southPanel, BorderLayout.SOUTH);

    JLabel label = new JLabel("Answer:");
    southPanel.add(label);

    textField_2 = new JTextField();
    textField_2.setColumns(10);
    southPanel.add(textField_2);

    centerPanel = new JPanel();
    centerPanel.setVisible(false);
    centerPanel.setBorder(new LineBorder(Color.GRAY));
    frame.getContentPane().add(centerPanel, BorderLayout.CENTER);

    Box centerPanelVertBox = Box.createVerticalBox();
    centerPanelVertBox.setAlignmentX(Component.CENTER_ALIGNMENT);
    centerPanel.add(centerPanelVertBox);
    centerPanelVertBox.setVisible(true);
    centerPanelVertBox.setBorder(null);

    JLabel pleaseEnterLabel = new JLabel("Please enter 2 of 3 values:");
    pleaseEnterLabel.setBorder(new EmptyBorder(15, 0, 10, 0));
    pleaseEnterLabel.setAlignmentX(Component.CENTER_ALIGNMENT);
    centerPanelVertBox.add(pleaseEnterLabel);

    JPanel var1Panel = new JPanel();
    centerPanelVertBox.add(var1Panel);

    var1Label = new JLabel("xxx");
    var1Label.setAlignmentX(Component.CENTER_ALIGNMENT);
    var1Panel.add(var1Label);

    var1TextField = new JFormattedTextField(NumberFormat.getInstance());
    var1TextField.setColumns(10);
    var1Panel.add(var1TextField);

    Panel var2Panel = new Panel();
    centerPanelVertBox.add(var2Panel);

    var2Label = new JLabel("xxx");
    var2Label.setAlignmentX(Component.CENTER_ALIGNMENT);
    var2Panel.add(var2Label);

    var2TextField = new JFormattedTextField(NumberFormat.getInstance());
    var2TextField.setColumns(10);
    var2Panel.add(var2TextField);

    Panel var3Panel = new Panel();
    centerPanelVertBox.add(var3Panel);

    var3Label = new JLabel("xxx");
    var3Label.setAlignmentX(Component.CENTER_ALIGNMENT);
    var3Panel.add(var3Label);

    var3TextField = new JFormattedTextField(NumberFormat.getInstance());
    var3TextField.setColumns(10);
    var3Panel.add(var3TextField);

    Panel calculatePanel = new Panel();
    centerPanelVertBox.add(calculatePanel);

    JButton calculateButton = new JButton("Calculate");
    calculatePanel.add(calculateButton);
}

public class ChoiceComboListener implements ActionListener {    //combobox actionlistener

    public void actionPerformed(ActionEvent ev) {
        JComboBox cb = (JComboBox)ev.getSource();
        String currentComboSelection = (String)cb.getSelectedItem();
        if (currentComboSelection.equals(choiceList[1])) {                  //choice 1 (Calculate Volts)
            centerPanel.setVisible(true);
            var1Label.setText(amps);
            var2Label.setText(ohms);
            var3Label.setText(watts);               
            //centerPanel.repaint();                
        }
        if (currentComboSelection.equals(choiceList[2])) {                //choice 2 (Calculate Amps)
            centerPanel.setVisible(true);
            var1Label.setText(volts);
            var2Label.setText(ohms);
            var3Label.setText(watts);
            centerPanel.repaint();              
        }
        if (currentComboSelection.equals(choiceList[3])) {
            centerPanel.setVisible(true);
            var1Label.setText(volts);
            var2Label.setText(amps);
            var3Label.setText(watts);
        }
        if (currentComboSelection.equals(choiceList[4])) {
            centerPanel.setVisible(true);
            var1Label.setText(volts);
            var2Label.setText(amps);
            var3Label.setText(ohms);
        }
        if (currentComboSelection.equals(choiceList[0])) {
            centerPanel.setVisible(false);
            //centerPanel.repaint();
        }
    }

}

如果你看一下底部，有一个名为 ChoiceComboListener 的内部类，这是组合框侦听器，我尝试过使用 repaint() 和各种其他东西，但我无法想出任何事物。任何帮助、指导或批评都是值得赞赏的。

Answer 1

感谢任何帮助、指导或批评

另一个选择可能是使用 Card布局。本教程有一个工作示例，可以完全满足您的需求。

Answer 2

“请记住，这是我的第一个程序（没有教程）”——干得好，干得好。一个建议是，您可能希望考虑将电气单元放入枚举中，因为它们似乎是实践这一重要概念的完美场所。另外，您的 JTextField 可能会与 GridBagLayout 更好地对齐，但使用起来可能会非常。

例如，在枚举上：

enum ElectricalUnits {
   AMPS("Amps"), OHMS("Ohms"), WATTS("Watts"), VOLTS("Volts");

   private String text;
   private ElectricalUnits(String text) {
      this.text = text;
   }
   public String getText() {
      return text;
   }
   @Override
   public String toString() {
      return text;
   }
}

那么用户的选择可以是一个枚举，这可以简化 gui 逻辑部分中的一些代码。

Answer 3

我发现，我将第一个面板（正确显示的面板）声明为 JPanel，并将其他 2 个面板声明为常规面板。哎呀！无论如何，如果有人想提供一些建议或批评，我们仍然欢迎！ – 机器人