JComboBox项目侦听器选定项目

Question

I am creating a combo box and it keeps giving me an error with the item listener that I don't understand. If you could also explain your answer that would be nice. Thanks in advance:

Here's the item listeners for the combo boxes:

combo1.addItemListener(new ItemListener(){
            @Override
            public void itemStateChanged(ItemEvent e){
               if(e.getStateChange() == ItemEvent.SELECTED){
                  JComboBox localCombo = (JComboBox)e.getSource();
                  ic1[0] = localCombo.getSelectedItem().toString();     
               }  
            }
        });
        combo2.addItemListener(new ItemListener(){
            @Override
            public void itemStateChanged(ItemEvent e){

                if(e.getStateChange() == ItemEvent.SELECTED){
                    JComboBox localCombo = (JComboBox)e.getSource();
                    ic1[1] = localCombo.getSelectedItem().toString();
                }
            }
        });

The error says it is on the .getSelectedItem()... lines. It only gives me the error when I run the program and select a word in the box. Thanks!

Here's the error in the run (line 61 is the .getSelectedItem()... line):

    run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at thisprogramisforfun.guiClasses.guiClassConversions$1.itemStateChanged(guiClassConversions.java:61)
    at javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1225)
    at javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1282)
    at javax.swing.JComboBox.contentsChanged(JComboBox.java:1329)
    at javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118)
    at javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93)
    at javax.swing.JComboBox.setSelectedItem(JComboBox.java:578)
    at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:624)
    at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:835)
    at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
    at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(BasicComboPopup.java:499)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

Your problem is here:

combo1 = (JComboBox)e.getSource();
ic1[0] = combo2.getSelectedItem().toString();

You're getting combo1 but calling a method on combo2

Better would be this:

combo1 = (JComboBox)e.getSource();
ic1[0] = combo1.getSelectedItem().toString();

But even better, use a local variable for this sort of work, not a field.

JComboBox localCombo = (JComboBox)e.getSource();
ic1[0] = localCombo.getSelectedItem().toString();

Answer 2

The error says it is on the .getSelectedItem()... lines

So you only have two varaibles on that line:

1. The ic1 array
2. the localCombo variable

Do you know how to use System.out.println(...) to display the value of those variables?

My guess in the ici array is not initialized, since you get the localCombo variable from the event source.