Null pointer when accessing a JComboBox

Hi,
I have this code here which asks the for the selectedindex of a combo box.

Code:

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action.ChangeColor(manager.getWords()[SelectWord.getSelectedIndex()],
                                                action.getColors()[SelectColor.getSelectedIndex()]);

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However, when getSelectedIndex runs it does not return anything but a null pointer that looks like this :( :

Code:

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Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at AlgorithmAnalyzer.GUI$SelectListener.actionPerformed(GUI.java:60)
        at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
        at javax.swing.AbstractButton$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)

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How do I fix this? Those are some of the scariest errors Ive ever seen :s:


this is the rest of the code:

Code:

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import java.awt.BorderLayout;
import java.awt.FlowLayout;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class GUI extends JFrame {
       
        private JComboBox SelectColor;
        private JComboBox SelectWord;
        private JButton Button;
        private WordManager manager;
       
        Actions action = new Actions();
       
        public GUI (WordManager manager){
               
                super ("Algorithm Analyzer 0.1");
                this.setLayout(new FlowLayout());
               
                this.manager = manager;
               
                for (int i = 0; i < manager.getWords().length; i++){
                       
                        this.add(manager.getWords()[i].getLabel());
                }
               
                SelectListener handler = new SelectListener ();
               
                final JComboBox SelectWord = new JComboBox(manager.getWordNames());
                SelectWord.setSelectedIndex(0);
                SelectWord.addActionListener(handler);
                this.add(SelectWord);
               
               
                final JComboBox SelectColor = new JComboBox(action.getColorNames());
                SelectColor.setSelectedIndex(0);
                SelectColor.addActionListener(handler);
                this.add(SelectColor);
               
                JButton Button = new JButton ("Submit");
                Button.addActionListener(handler);
                this.add(Button);
                       
                }
       
       
        private class SelectListener implements  ActionListener  {
               
                @Override
                public void actionPerformed(ActionEvent e) {
                        // TODO Auto-generated method stub
                        if (e.getActionCommand() == "Submit"){
                               
                                action.ChangeColor(manager.getWords()[SelectWord.getSelectedIndex()],
                                                action.getColors()[SelectColor.getSelectedIndex()]);

                               
                        }
                       
                }
        }

}

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Thanks in advance to anyone who can help out :D

Code:

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action.ChangeColor(manager.getWords()[SelectWord.getSelectedIndex()],
        action.getColors()[SelectColor.getSelectedIndex()]);

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The NullPointerException means that something is null in that line but you are treating it as if it referenced an actual thing.

So, use System.out.println() to display the possible culprits: action, manager, manager.getWords(), SelectWord, action.getColors(), and SelectColor. (Also make sure you see why you can't use . or [] with something that is null). Once you have found which variable is causing the problem, go back through your code to the place you thought you were assigning it a nonnull value and figure out why that didn't happen.

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Follow Java coding standards: variables start with a lowercase letter, like selectColor. Personally I would use colorCB because that says what it is (a colour combo box), but the lowercase starting letter is important. Also action (not actions?) should be private unless you have a very good reason for it not to be.

I followed your instructions and pinpointed the null pointer to my two ComboBoxes. However I can't see anything wrong with their declarations here:

Code:

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private JComboBox SelectColor;
        private JComboBox SelectWord;

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or their assignments:

Code:

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JComboBox SelectWord = new JComboBox(manager.getWordNames());
                SelectWord.setSelectedIndex(0);
                SelectWord.addActionListener(handler);
                this.add(SelectWord);
               
               
                JComboBox SelectColor = new JComboBox(action.getColorNames());
                SelectColor.setSelectedIndex(0);
                SelectColor.addActionListener(handler);
                this.add(SelectColor);

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I cant say what I'm missing

You're shadowing the class fields by re-declaring the variables in the constructor.

Yes, it's the shadowing that means the (intended) assignment didn't happen: you assigned to something else. If you haven't seen this before there is a brief discussion in Wikipedia, or google for more.