當在可編輯的JComboBox上調用getSelectedItem（）時發生NullPointerException

我有一個可編輯的組合框和一個按鍵監聽器。我想寫一些文本和每一個字母，它必須與我插入（或刪除）的文本的字符串。但是，當我想要得到的字符串，我得到一個NullPointerException。當在可編輯的JComboBox上調用getSelectedItem（）時發生NullPointerException

combobox.getEditor().getEditorComponent() 
    .addKeyListener(new KeyListener() 
    { 

     String value=""; 
     @Override 
     public void keyTyped(KeyEvent e) 
     { 
      value = combobox.getSelectedItem().toString(); 
      System.out.println(value); 
     } 

     @Override 
     public void keyReleased(KeyEvent e) 
     { 
     } 

     @Override 
     public void keyPressed(KeyEvent e) 
     { 

     } 
    });

堆棧跟蹤：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException 
    at auftrag_paket.Neuer_Auftrag_ohneMwSt$25.keyTyped(Neuer_Auftrag_ohneMwSt.java:2037) 
    at java.awt.Component.processKeyEvent(Unknown Source) 
    at javax.swing.JComponent.processKeyEvent(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.KeyboardFocusManager.redispatchEvent(Unknown Source) 
    at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source) 
    at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source) 
    at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source) 
    at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source) 
    at java.awt.Component.dispatchEventImpl(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.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$400(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) 
    at java.awt.WaitDispatchSupport$2.run(Unknown Source) 
    at java.awt.WaitDispatchSupport$4.run(Unknown Source) 
    at java.awt.WaitDispatchSupport$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.awt.WaitDispatchSupport.enter(Unknown Source) 
    at java.awt.Dialog.show(Unknown Source) 
    at java.awt.Component.show(Unknown Source) 
    at java.awt.Component.setVisible(Unknown Source) 
    at java.awt.Window.setVisible(Unknown Source) 
    at java.awt.Dialog.setVisible(Unknown Source) 
    at auftrag_paket.Auftragsverwaltung$14.actionPerformed(Auftragsverwaltung.java:1865) 
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) 
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) 
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source) 
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) 
    at java.awt.Component.processMouseEvent(Unknown Source) 
    at javax.swing.JComponent.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.dispatchEventImpl(Unknown Source) 
    at java.awt.EventQueue.access$400(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.awt.EventQueue$3.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.awt.EventQueue$4.run(Unknown Source) 
    at java.security.AccessController.doPrivileged(Native Method) 
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) 
    at java.awt.EventQueue.dispatchEvent(Unknown Source) 
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) 
    at java.awt.EventDispatchThread.pumpEventsForFilter(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)

有任何人任何想法？

來源

2014-12-24 user2942606

正如我所看到的，當'toString'在返回的'null'上被調用時發生異常。我草繪了一小段代碼，所有工作都很好。可能是其他地方的錯誤？ – cybersoft

必須根據（摘要）Document來搜索代碼，注意DocumentListener被指定通知關於編輯器（到另一個JComponent或變量）的更改，否則使用DocumentFilter – mKorbel

要捕獲使用KeyListener在組合框中輸入任何字符，你應該這樣做內keyReleased方法，試試下面的代碼：

combobox.getEditor().getEditorComponent() 
     .addKeyListener(new KeyListener() 
     { 

      String value=""; 
      @Override 
      public void keyTyped(KeyEvent e) 
      { 

      } 

      @Override 
      public void keyReleased(KeyEvent e) 
      { 
       value = combo.getSelectedItem().toString(); 
       value = ((JTextField)combo.getEditor().getEditorComponent()).getText(); 
       System.out.println(value); 
      } 

      @Override 
      public void keyPressed(KeyEvent e) 
      { 

      } 
     });

來源

2014-12-24 16:40:07

可以使用'combobox.getEditor（）。getItem （）' – cybersoft

不要使用KeyListener的使用AWT時是舊代碼。 Swing有更新更好的API。您應該使用DocumentListener。閱讀Swing教程中有關如何編寫文檔監聽器的部分以獲取更多信息。

來源

2014-12-24 17:07:43 camickr

當在可編輯的JComboBox上調用getSelectedItem（）時發生NullPointerException

回答

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