java - itemStateChanged 和 actionPerformed 问题

Question

我有一个实现 ActionListner、FocusListner 和 ItemListner 的处理程序类。我已经从 ConfrenceGUI 类实例化了一个 gui 对象:

      public ConferenceGUI()
   {
      //Create a new JPanel and set its latyout mgr   
      conference = new JPanel(); 
      setLayout(new BorderLayout());  
      //create a RegPanel panel           
      regPanel = new RegPanel();
      //create new WorkshopPanel workshopPanel
      workshopPanel = new WorkshopPanel();      
      //build a buttonpanel
      buildButtonPanel();
      //Create a title panel      
      titlePanel = new JPanel(new FlowLayout());
      //create and set a font object
      titlePanel.setFont(new Font ("sansserif", Font.BOLD, 18));
      //add a label
      titlePanel.add(new Label("Select Registration Options"));      
      //Add the instantiated subpanels to the main conference gui panel
      add(titlePanel,        BorderLayout.NORTH);
      add(regPanel,          BorderLayout.WEST);
      add(workshopPanel,     BorderLayout.EAST);
      add(buttonPanel,       BorderLayout.SOUTH);
      //add an item listener to the combo box
      ConferenceHandler handler = new ConferenceHandler(this);
      regPanel.regComboBox.addItemListener(handler);
      //add a focus listener to the name field
      ConferenceHandler fhandler = new ConferenceHandler(this);
      regPanel.regTextField.addFocusListener(fhandler);
   }

我正在尝试从监听器获取信息(包括来 self 的 ConferenceGUI 类中的单独方法的两个按钮监听器，但我没有包含该信息)。

这是我的处理程序的代码片段:

  public void itemStateChanged(ItemEvent e)
  {
     String name = gui.regPanel.regTextField.getText(); 
     if (e.getSource() == gui.regPanel.regComboBox)
       {
          if (gui.regPanel.getRegType() == "Please select a type")
          JOptionPane.showMessageDialog(null, "Please select a registraion type",
                                        "Type Error", JOptionPane.ERROR_MESSAGE);     
          else gui.textArea.setText(name+" is a " +
                                        gui.regPanel.getRegType()+ " registration");
       }

按钮:

      public void actionPerformed (ActionEvent e)
  {
    String name = gui.regPanel.regTextField.getText();
    DecimalFormat $ = new DecimalFormat("$#,##0.00");
    if (e.getSource() == gui.calcButton)//if the calculate buttton is pressed
    {
       //dislplay error box if user selects index 0
       if (gui.regPanel.getRegType() == "Please select a type")
         JOptionPane.showMessageDialog(null, "Please select a registraion type",
                                                "Type Error",JOptionPane.ERROR_MESSAGE);
       //prints to textarea if registrant will be attending keynote or not
       if (gui.regPanel.regCheckBox.isSelected())
          gui.textArea.setText("Keynote address will be attended");
            else
              gui.textArea.setText("Keynote address will not be attended");
       //prints to textarea which workshops registrant will be attending
       gui.textArea.setText(name+" is registered in these workshops:" +
                            gui.workshopPanel.getWorkshopList());
       //prints total registration fees to textarea
       gui.textArea.setText("Total charges for" + name + " are " + $.format(calcTotalCharges()));
    }
    else if (e.getSource() == gui.clearButton)//if the clear button is pressed
    {
       //clear the textarea 
       gui.textArea.setText("");  
       //reset the list
       gui.workshopPanel.workshopList.setSelectedIndex(0);
       //reset the combobox to index 0
       gui.regPanel.regComboBox.setSelectedIndex(0);
    }
  }

问题对你们所有人来说都是显而易见的，但由于我刚刚开始，我不明白为什么我不能在 GUI 的文本区域中写入任何文本。对于代码量，我深表歉意，但我想尽量做到彻底。

这是文本区域的来源(这是我的 ConferenceGUI 类中包含的一个单独的方法:

   private void buildButtonPanel()
   {
      //create the buttonpanel
      buttonPanel = new JPanel(new FlowLayout());
      //create the buttons
      calcButton = new JButton("Calculate Charges");
      clearButton = new JButton    ("Clear");
      //add listeners to the buttons
      ConferenceHandler ahandler = new ConferenceHandler(this);
      calcButton.addActionListener(ahandler);  
      clearButton.addActionListener(ahandler);
      //create a text area
      JTextArea textArea = new JTextArea(5,30); 
      textArea.setLineWrap(true); textArea.setWrapStyleWord(true);
      //add everything to the buttonpanel
      buttonPanel.add(calcButton); buttonPanel.add(clearButton); buttonPanel.add(new JScrollPane(textArea));
   }

还有其他三个类 RegPanel 和 WorkshopPanel，它们都为 ConferenceGUI 创建几个面板，而 ConferenceGUI 又由小程序 (gui) 实例化。

Answer 1

我不确定我是否理解你的问题(什么不起作用？你期望什么以及会发生什么？)，但可以肯定的是在 JTextArea 上多次调用 setText()不是一个好主意:setText 替换文本区域的全部内容。您应该使用 append() 将多行文本添加到文本区域。

已编辑:

既然您向我们展示了如何构建文本区域，问题就更清楚了:您在 buildButtonPanel 方法中实例化了一个局部变量 textArea，但 GUI 中的实例变量指向另一个文本区域(或空)。

再次编辑:

您的代码太复杂，不够完整，无法纠正，但情况如下所示:

public class Bug extends JPanel {
    private JTextArea textArea = new JTextArea(); // first text area

    private void build()  {
        JTextArea textArea = new JTextArea(); // second text area. Inaccessible outside of this method
        this.add(new JScrollPane(textArea));
    }

    public void actionPerformed(ActionEvent e) {
        this.textArea.setText("foo"); // here, we modify the first text area, but it hasn't been added to the GUI, so it's invisible
    }
}

要修复它，您必须将其更改为:

public class NoBug extends JPanel {
    private JTextArea textArea = new JTextArea(); // unique text area

    private void build()  {
        this.add(new JScrollPane(this.textArea));
    }

    public void actionPerformed(ActionEvent e) {
        this.textArea.setText("foo"); // here, we modify the unique text area, which has been added to the GUI in the build() method        }
}