java - 访问 DefaultListModel 的索引时出现空指针异常

Question

我有一个 list :

JList characterList = new JList(characterListModel);
characterListModel = new DefaultListModel();        
String myCharacters[]={"Dean Winchester","Sam Winchester",
            "Bobby Singer","Castiel"};
for (String myCharacter : myCharacters) {
        ((DefaultListModel) characterList.getModel()).addElement(myCharacter);  
}

我已经在一个单独的类中编写了一个方法，用于删除选定索引处的字符:

public void removeCharacter() {
    DefaultListModel characterListModel = ((PlayerContentPane) IViewManager.Util.getInstance()
        .getMyContainerPane().getMyPlayerManagerPane().getContentPane())
        .getCharacterListModel();
    JList characterList = ((PlayerContentPane) IViewManager.Util.getInstance()
        .getMyContainerPane().getMyPlayerManagerPane().getContentPane())
        .getCharacterList();
    int idx = characterList.getSelectedIndex(); //<---line 62
    int size = characterListModel.getSize();
    characterListModel.remove(idx);
    if (size == 0) { 
        //do nothing
    } else { 
        if (idx == characterListModel.getSize()) {
            idx--;
        }

        characterList.setSelectedIndex(idx);
        characterList.ensureIndexIsVisible(idx);
    }

}

但是，当我使用按钮运行它时，我得到以下堆栈跟踪:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.detica.LarpDB.Controller.Controller.removeCharacter(Controller.java:62)
at com.detica.LarpDB.view.PlayerContentPane$3.actionPerformed(PlayerContentPane.java:94)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$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$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$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)

我在谷歌上看到的很多问题都源于这一行:DefaultListModel characterListModel = ....他们遇到的问题是他们创建了一个新的对象实例，现在我对整个 Java 事物来说只是个新手，但这对我来说不应该是一个问题，因为我'我没有做任何新的事情，我只是具体说明了这是哪个对象。

请问有人可以帮我解开吗？

Answer 1

这两行会产生很多问题:

DefaultListModel characterListModel = ((PlayerContentPane) IViewManager.Util.getInstance()
    .getMyContainerPane().getMyPlayerManagerPane().getContentPane())
    .getCharacterListModel();
JList characterList = ((PlayerContentPane) IViewManager.Util.getInstance()
    .getMyContainerPane().getMyPlayerManagerPane().getContentPane())
    .getCharacterList();

如果任何链接方法返回 null(无论出于何种原因)，该行将抛出 NullPointerException。

我建议您将它们分成几行，然后检查每个调用的值，看看从哪里获得 null。

编辑

刚刚意识到第 62 行是 int idx = characterList.getSelectedIndex();。这意味着 characterList 为空。