無法從jcombobox中刪除第一個元素

Question

我無法刪除jcombobox中的第一個元素。我的代碼如下，

JComboBox cBox= cBox= new JComboBox(); 
...  
while (cBox.getItemCount() > 0) 
    cBox.removeItemAt(0); 

對於測試運行，我有3個項目在cBox。當它到達removeItemAt（0）時，調試進入一些文件訪問代碼，這是絕對不相關的。這兩次會得到下面的例外。我試着直接得到相同的異常removeAllItems（）。但是，removeItem（1）按照原樣工作，直到剩下1個元素。該例外不會崩潰的應用程序，我可以看到沒有項目後，所以它的工作一點。我究竟做錯了什麼。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
at util.Gui$4.actionPerformed(Gui.java:111) 
at javax.swing.JComboBox.fireActionEvent(Unknown Source) 
at javax.swing.JComboBox.contentsChanged(Unknown Source) 
at javax.swing.AbstractListModel.fireContentsChanged(Unknown Source) 
at javax.swing.DefaultComboBoxModel.setSelectedItem(Unknown Source) 
at javax.swing.DefaultComboBoxModel.removeElementAt(Unknown Source) 
at javax.swing.JComboBox.removeItemAt(Unknown Source) 
at util.Gui.prepareSubLists(Gui.java:164) 
at util.Gui$3.actionPerformed(Gui.java:97) 
at javax.swing.JComboBox.fireActionEvent(Unknown Source) 
at javax.swing.JComboBox.setSelectedItem(Unknown Source) 
at javax.swing.JComboBox.setSelectedIndex(Unknown Source) 
at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(Unknown Source) 
at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source) 
at java.awt.Component.processMouseEvent(Unknown Source) 
at javax.swing.JComponent.processMouseEvent(Unknown Source) 
at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(Unknown Source) 
at java.awt.Component.processEvent(Unknown Source) 
at java.awt.Container.processEvent(Unknown Source) 
at java.awt.Component.dispatchEventImpl(Unknown Source) 
at java.awt.Container.dispatchEventImpl(Unknown Source) 
at java.awt.Component.dispatchEvent(Unknown Source) 
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) 
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) 
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) 
at java.awt.Container.dispatchEventImpl(Unknown Source) 
at java.awt.Window.dispatchEventImpl(Unknown Source) 
at java.awt.Component.dispatchEvent(Unknown Source) 
at java.awt.EventQueue.dispatchEvent(Unknown Source) 
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source) 
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.pumpEvents(Unknown Source) 
at java.awt.EventDispatchThread.run(Unknown Source) 

Answer 1

是不是你的條件語句錯了？與if更換while，這樣

if(cBox.getItemCount() > 0){ 
    cBox.removeItemAt(0); 
} 

---

下面是一個SSCCE：

public final class JComboBoxDemo { 
    public static void main(String[] args){ 
     SwingUtilities.invokeLater(new Runnable(){ 
      @Override 
      public void run() { 
       createAndShowGUI();    
      } 
     }); 
    } 

    public static void createAndShowGUI(){ 
     final JFrame frame = new JFrame("JComboBox Demo"); 
     frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     frame.getContentPane().setLayout(new FlowLayout()); 
     frame.getContentPane().add(JComboPane.newInstance()); 
     frame.setSize(new Dimension(250, 100)); // for demonstration purposes only 
     //frame.pack(); 
     frame.setLocationRelativeTo(null); 
     frame.setVisible(true); 
    } 

    private static final class JComboPane extends JPanel{ 
     private JComboPane(){ 
      super(); 
      setLayout(new BoxLayout(this, BoxLayout.Y_AXIS)); 
      JCenteredComboBox comboBox = JCenteredComboBox.newInstance(); 
      JCenteredButton button = JCenteredButton.newInstance(comboBox); 
      add(comboBox); 
      add(button); 
     } 

     public static final JComboPane newInstance(){ 
      return new JComboPane(); 
     } 

     private static final class JCenteredComboBox extends JComboBox{ 
      private JCenteredComboBox(){ 
       super(new String[]{"Item 1", "Item 2", "Item 3"}); 
       setAlignmentX(Component.CENTER_ALIGNMENT); 
      } 

      public static final JCenteredComboBox newInstance(){ 
       return new JCenteredComboBox(); 
      } 
     } 

     private static final class JCenteredButton extends JButton{ 
      private JCenteredButton(final JComboBox comboBox){ 
       super("Remove First Item"); 
       setAlignmentX(Component.CENTER_ALIGNMENT); 
       addActionListener(new ActionListener(){ 
        @Override 
        public void actionPerformed(ActionEvent e) { 
         if(comboBox.getItemCount() > 0){ 
          comboBox.removeItemAt(0); // your logic 
         } 
        } 
       }); 
      } 

      public static final JCenteredButton newInstance(final JComboBox comboBox){ 
       return new JCenteredButton(comboBox); 
      } 
     } 
    } 
} 

當你運行它，按JButton將刪除JComboBox的第一個項目。您可以繼續按下直到它爲空。

Answer 2

可能會發生此異常，因爲當組合項目被刪除時觸發事件，並且在此事件處理方法中仍然引用組合框項目。例如，當您在代碼中刪除某處（actionPeformed（）除外）中的最後一個項目時，combo.removeItemAt（0）或removeAllItems（）組合框中的最後一個項目仍然會被觸發/執行。但通常actionPerformed（）方法包含對用戶操作作出反應的代碼（用戶在組合框某處單擊）。因此，當最後一個項目被刪除時，組合框中沒有更多項目，並且對actionPerformed（）中的項目或索引的任何引用都將導致異常。

對此的解決方案是將代碼從actionPerformed移動到例如mouseClicked（）或其他事件處理程序，具體取決於你想要做什麼。