Question

I have a 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);  
}

And I’ve written a method, in a seperate class, to remove a character at a selected index:

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);
    }

}

However when I run it with my button, I get this stack trace:

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)

A lot of the issues I’ve seen googling this, stem from the line: DefaultListModel characterListModel = ....
And the issue they have is that their creating a new instance of the object, now I”m only new to this whole Java thing, but this shouldn’t be an issue with me, as I’ve not made anythign new, I’ve just been specific about which object this is.

Please could someone help me untangle myself?

Answer 1

Those 2 lines can create a lot of problems:

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

If any of the chained methods return null (for whatever reason) the line will throw a NullPointerException.

I suggest you break them down in several lines and check the value of each call to see where you get the null from.

EDIT

Just realised that line 62 is int idx = characterList.getSelectedIndex();. It means that characterList is null.