how do I manipulate JList with JList?

Question

I have two JLists, the other has categories of weapons (I'm doing roleplaying character creation software) and the other has items in that specific category. I want the items to change according to category

At the moment jList with items swaps its DefaultListModel every time user changes category (every DefaultListModel variable has different sets of items) this somehow produces NullPointerException whenever user clicks an Item after changing category.

I've tried to search this online and I have read the examples with JList's, but I haven't been able to solve the problem.

Here is the related code

private void jList1ValueChanged(javax.swing.event.ListSelectionEvent evt) { //Kategory
    String selection = jList1.getSelectedValue().toString();

    if(selection.equals(settings.weapon_classes[0])){
        jList2.clearSelection();
        jList2.setModel(model_l_pistols);
        jList2.validate();

    }
    if(selection.equals(settings.weapon_classes[1])){
        jList2.clearSelection();
        jList2.setModel(model_m_pistols);
        jList2.validate();
    }
}

private void wpn_available_valuechanged_listener(javax.swing.event.ListSelectionEvent evt) {                                                     

     Weapon wpn = new Weapon(); //The details of selected weapon are stored here for desplaying
        try{
            String wpn_name = jList2.getSelectedValue().toString();
            if(jList1.getSelectedValue().equals("Light pistols")){
                wpn = controller.get_Weapon(wpn_name, settings.light_pistols); //Stores right item details from array in settings class
            }
            if(jList1.getSelectedValue().equals("Medium pistols")){
                wpn = controller.get_Weapon(wpn_name, settings.medium_pistols);
            }

        }catch(Exception e){
            System.out.println(e + "String wpn_name = jList2.getSelectedValue().toString();");
        }
}

And here is the errormessage I get when running my program with NetBeans IDE (first line includes my try/catch block printing of exception:

java.lang.NullPointerExceptionString wpn_name = jList2.getSelectedValue().toString();
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at charcreation.GUI.wpn_available_valuechanged_listener(Unknown Source)
    at charcreation.GUI.access$6900(Unknown Source)
    at charcreation.GUI$67.valueChanged(Unknown Source)
    at javax.swing.JList.fireSelectionValueChanged(JList.java:1765)
    at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1779)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:388)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:398)
    at javax.swing.DefaultListSelectionModel.removeSelectionIntervalImpl(DefaultListSelectionModel.java:559)
    at javax.swing.DefaultListSelectionModel.clearSelection(DefaultListSelectionModel.java:403)
    at javax.swing.JList.clearSelection(JList.java:2013)
    at charcreation.GUI.jList1ValueChanged(Unknown Source)
    at charcreation.GUI.access$6800(Unknown Source)
    at charcreation.GUI$66.valueChanged(Unknown Source)
    at javax.swing.JList.fireSelectionValueChanged(JList.java:1765)
    at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1779)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
    at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:388)
    at javax.swing.DefaultListSelectionModel.changeSelection(DefaultListSelectionModel.java:398)
    at javax.swing.DefaultListSelectionModel.setSelectionInterval(DefaultListSelectionModel.java:442)
    at javax.swing.JList.setSelectionInterval(JList.java:2035)
    at javax.swing.plaf.basic.BasicListUI$Handler.adjustSelection(BasicListUI.java:2731)
    at javax.swing.plaf.basic.BasicListUI$Handler.mousePressed(BasicListUI.java:2687)
    at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:263)
    at java.awt.Component.processMouseEvent(Component.java:6260)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6028)
    at java.awt.Container.processEvent(Container.java:2041)
    at java.awt.Component.dispatchEventImpl(Component.java:4630)
    at java.awt.Container.dispatchEventImpl(Container.java:2099)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4574)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4235)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
    at java.awt.Container.dispatchEventImpl(Container.java:2085)
    at java.awt.Window.dispatchEventImpl(Window.java:2475)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)

Answer 1

Possible error could be in this line

String selection = jList1.getSelectedValue().toString();

jList1.getSelectedValue() can be a null value so when the toString() function calls against a null it throws an exception

Use this instead to avoid the exception

String selection = String.valueOf(jList1.getSelectedValue());

This way if it is a null selection you will just get back the String value of null;