Adding item to JComboBox from different class

Question

I have been trying to get a JComboBox to update when a new item is added to a backend database.

In the actual code there is a separate class that handles an add dialog, when the new item is added it updates the database and then should add the same item to the dropdown menu by calling a method that accepts a String in the main GUI class. (trying to follow Model-View-Controller).

Below is a minimal example that triggers an error although in the actual application it fails silently.

I have an inkling it has to do with instances of objects. Also to populate the list I'm iterating over a list using addItem() to make sure that is working and the ComboBox is Mutable.

Thanks any help Tom

import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class TestComboBox extends JPanel implements ActionListener{

JComboBox moduleList = new JComboBox(new DefaultComboBoxModel());
TestComboBox testComboBox;
JFrame frame;

public void actionPerformed(ActionEvent e){
    if("additem".equals(e.getActionCommand())){
        addItem("Item");
    }
    if("additemfail".equals(e.getActionCommand())){
        testComboBox.addItemFail("Item Fail");
    }
}

public void addItem(String item){
    moduleList.addItem(item);
}

public void addItemFail(String item){
    testComboBox = new TestComboBox();
    moduleList.addItem(item);
}


protected JPanel createPanel(){
    JPanel panel = new JPanel(false);

    String[] getModuleList = {"MODULE 1", "MODULE 2"};
    moduleList = new JComboBox(new DefaultComboBoxModel(getModuleList));
    panel.add(moduleList);

    JButton additem = new JButton("Add Item");
    additem.setActionCommand("additem");
    additem.addActionListener(this);
    panel.add(additem);

    JButton additemfail = new JButton("Add Item Fail");
    additemfail.setActionCommand("additemfail");
    additemfail.addActionListener(this);
    panel.add(additemfail);

    return panel;
}

public void createAndShowGui(){
    testComboBox = new TestComboBox();
    frame = new JFrame("JComboTest");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(testComboBox.createPanel());
    frame.setSize(450, 150);
    frame.setVisible(true);
}

public static void main(String[] args){
    TestComboBox t = new TestComboBox();
    t.createAndShowGui();
}
}

Exception that is thrown

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at TestComboBox.actionPerformed(TestComboBox.java:16)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2012)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2335)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:404)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:253)
    at java.awt.Component.processMouseEvent(Component.java:6268)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6033)
    at java.awt.Container.processEvent(Container.java:2045)
    at java.awt.Component.dispatchEventImpl(Component.java:4629)
    at java.awt.Container.dispatchEventImpl(Container.java:2103)
    at java.awt.Component.dispatchEvent(Component.java:4455)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4633)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4297)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4227)
    at java.awt.Container.dispatchEventImpl(Container.java:2089)
    at java.awt.Window.dispatchEventImpl(Window.java:2517)
    at java.awt.Component.dispatchEvent(Component.java:4455)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:649)
    at java.awt.EventQueue.access$000(EventQueue.java:96)
    at java.awt.EventQueue$1.run(EventQueue.java:608)
    at java.awt.EventQueue$1.run(EventQueue.java:606)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:116)
    at java.awt.EventQueue$2.run(EventQueue.java:622)
    at java.awt.EventQueue$2.run(EventQueue.java:620)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:105)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:619)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:275)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:200)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:185)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:177)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:138)

row 16 is testComboBox = new TestComboBox();, for why hells on the world ..., you recreate class again, from itself — mKorbel
The problem is a null variable. The error message tells you which line is causing the problem. So look at all the variable used on the line and determine which one is null. Then you need to change the code to make sure the variable isn't null when you try to access it. — camickr

Usman Saleem Usman Saleem · Accepted Answer · 2011-09-19T21:13:21

Instead of testComboBox.addItemFail("Item Fail");, you should simply call addItemFail("Item Fail"); in your actionPerformed method.

Edit: And you should not recreate object of same class again in addItemFail method (whats the point of doing it again?)

Adding item to JComboBox from different class

2 Answers