Null pointer exception error, Please help

[William H Taylor]

This error message occurs when I try to pass a text area into a listener object. I have instantiated all objects so I shouldn't get the error. It is called in the action performed method, and it should be using the class instance variable after it is constructed. I tried instantiating in constructor and as member variable globally. Nothing works. I don't really understand what the problem is other than somehow it is making the inner class object before the outerclass. not possible. I am running windows xp pro, java6. Thank you in advance for the help.

    
C:\Documents and Settings\William Taylor\Desktop\Projects\Learning Life Assistan
t\src>javac *.java

C:\Documents and Settings\William Taylor\Desktop\Projects\Learning Life Assistan
t\src>java LearningTester
The derivative is the slope of line tangent to a curve at point x,y. null
The set of all things T, and the set of all relations on T, R X T, is a set call
ed S for system.
**********************Action Performed Beginning!!!*****************************
******
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at GUIInputter$KnowledgeListener.actionPerformed(GUIInputter.java:122)
        at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
        at javax.swing.AbstractButton$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 Sour
ce)
        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.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)

C:\Documents and Settings\William Taylor\Desktop\Projects\Learning Life Assistan
t\src>javac *.java

C:\Documents and Settings\William Taylor\Desktop\Projects\Learning Life Assistan
t\src>java LearningTester
The derivative is the slope of line tangent to a curve at point x,y. null
The set of all things T, and the set of all relations on T, R X T, is a set call
ed S for system.
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at GUIInputter$KnowledgeListener.actionPerformed(GUIInputter.java:121)
        at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
        at javax.swing.AbstractButton$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 Sour
ce)
        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.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)

C:\Documents and Settings\William Taylor\Desktop\Projects\Learning Life Assistan
t\src>java LearningTester

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class GUIInputter extends JFrame implements Symbol, KnowledgeDisplayer
{
	private JFrame frame;

	private JTextArea declaration;
	private JTextArea question;
	private JTextArea imperative;

	private JButton declare;
	private JButton ask;
	private JButton instructions;

	private Information knowledgeQuestion;
	private Information knowledgeDeclarations;
	private Information KnowledgeInstructions;

	private KnowledgeListener questionListener = new InterrogativeListener("Questions.sav");
	private KnowledgeListener declarationListener = new DeclarationListener("Declarations.sav");
	private KnowledgeListener instructionListener = new InstructionListener("Instructions.sav");

	private FileSaver queSave;
	private FileSaver decSave;
	private FileSaver instructSave;

	public GUIInputter()
	{
		//declarationListener = new DeclarationListener("Questions.sav");
		//instructionListener = new InstructionListener("Declarations.sav");
		//questionListener = new InterrogativeListener("Instructions.sav");
	}
	
	public void symbolize()
	{

	}
	
	public void saveKnowledge(Sentence info, String aFilename, FileSaver saver)
	{
		String filename = aFilename;
		saver = new FileSaver(filename, info);
		saver.save();
	}


	public void knowledgeDisplay()
	{
		frame = new JFrame();

		JPanel decPanel = new JPanel();
		declaration = new JTextArea(10, 30);
		JScrollPane decScroller = new JScrollPane(declaration);
		decScroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
		decScroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
		declare = new JButton("Facts");
		declare.addActionListener(declarationListener);
		decPanel.add(decScroller);
		decPanel.add(declare);
	
		question = new JTextArea(10, 30);
		ask = new JButton("Questions");
		ask.addActionListener(questionListener);
		JScrollPane queScroller = new JScrollPane(question);
		queScroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
		queScroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
		JPanel quePanel = new JPanel();
		quePanel.add(queScroller);
		quePanel.add(ask);
		
		imperative = new JTextArea(10,30);
		instructions = new JButton("Instructions");
		instructions.addActionListener(instructionListener);
		JScrollPane instructScroller = new JScrollPane(imperative);
		instructScroller.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
		instructScroller.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
		JPanel instructPanel = new JPanel();
		instructPanel.add(instructScroller);
		instructPanel.add(instructions);

		MenuBar menubar = new MenuBar();
		Menu fileMenu = new Menu("File");
		Menu edMenu = new Menu("Edit");
		Menu viewMenu = new Menu("View");
		Menu toolMenu = new Menu("Tools");
		Menu helpMenu = new Menu("Help");
		menubar.add(fileMenu);
		menubar.add(edMenu);
		menubar.add(viewMenu);
		menubar.add(toolMenu);
		menubar.add(helpMenu);
		
		frame.add(decPanel,BorderLayout.NORTH);
		frame.add(quePanel, BorderLayout.CENTER);
		frame.add(instructPanel, BorderLayout.SOUTH);
		frame.setMenuBar(menubar);

		frame.setSize(800,600);
		frame.setResizable(false);
		frame.setTitle("William's Learning Assistant!");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setVisible(true);
	}
	
	public abstract class KnowledgeListener implements ActionListener
	{
		String filename;
		FileSaver saver;
		ActionListener listener;
		JTextArea wordText;
		
		public KnowledgeListener(String aFilename, FileSaver aSaver, JTextArea aText)
		{
			filename = aFilename;
			saver = aSaver;
			wordText = aText;
		}
		public void actionPerformed(ActionEvent event)
		{
			String sentence = wordText.getText();
			InterrogativeSentence iSentence = new InterrogativeSentence(sentence);
			saveKnowledge(iSentence, filename, saver);
		}
		public void setFileName(String aFileName)
		{
			filename = aFileName;
		}
	}
	public class DeclarationListener extends KnowledgeListener
	{
		public DeclarationListener(String aFilename)
		{
			super(aFilename, decSave, declaration);
		}
	}
	
	public class InstructionListener extends KnowledgeListener
	{
		public InstructionListener(String aFilename)
		{
			super(aFilename, instructSave, imperative);
		}
	}
	public class InterrogativeListener extends KnowledgeListener
	{
		public InterrogativeListener(String aFilename)
		{
			super(aFilename, queSave, question);
		}
	}
}

[K. Tsang]

Hi William.

You are using the subclass of KnowledgeListener eg InstructionListener. Now KnowledgeListener is abstract. I noticed in the InstructionListener or any concrete KnowledgeListener classes, where are the FileSaver and JTextArea parms coming from in order to call super()? Since all classes are public meaning in separate source files, these 2 parms can't be from the GUIInputter class. So you may need to change those concrete constructors of KnowledgeListener to pass in FileSaver and JTextArea too. Also where is the sentence variable coming from for the InterrogativeSentence class?

So it looks to me that any other 3 variables are null. You need to find out which one.

If you are in doubt in passing the parms correctly, try printing out the variables in the constructor/method of those action listeners.

[Henry Wong]

I don't really understand what the problem is other than somehow it is making the inner class object before the outerclass. not possible.

How is it *not* possible? You are instantiating inner classes, which depends on variables, of the outer class, which you haven't instantiated yet -- so they are assigned to null. Now, you do complete initialization of the outer class later, but that is already too late for the inner class, as they got a copy of the null references.

Henry

[William H Taylor]

Thank you for your help. I put a test for the JTextArea object in actionPerformed, and it works. It did not instantiate in the sequence I thought it would. I now have to work on the design. A thousand and three thank you's

Bill