java.lang.NullPointerException

[Travis]

Hey,
I've been having a problem with one of my programming assignments.
I keep getting the message:

Exception in thread "main" java.lang.NullPointerException
at queueTask.TpQueue.front(TpQueue.java:29)
at queueTask.TpQueue.main(TpQueue.java:53)

on this code:

package queueTask;

import simpleLists.*;

public class TpQueue<E> implements Queue<E> {
	
	private SimpleList<E> _contents;
	

	@Override
	public E deQueue() throws EmptyQueueException {
		// TODO Auto-generated method stub
		if (_contents.isEmpty() == true)
			throw new EmptyQueueException("Queue is empty.");
		E tmp =_contents.removeFirst();
		return tmp;
	}

	@Override
	public void enQueue(E item) {
		// TODO Auto-generated method stub
		Node<E> node = new Node<E>();
		node.setElement(item);
		node.setNext(null);
		}
	@Override
	public E front() throws EmptyQueueException {
		// TODO Auto-generated method stub
		if (_contents.isEmpty()== true)
			throw new EmptyQueueException("Queue is empty");
		else
			return _contents.firstElement();
	}

	@Override
	public boolean isEmpty() {
		// TODO Auto-generated method stub
		if( _contents.isEmpty()== true)
			return true;
		else
			return false;
	}

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// some limited testing
		TpQueue<String> testCase = new TpQueue<String>();
		for (int i=0;i<10;i++)
			testCase.enQueue("Robert");
		
		System.out.println("First = " + testCase.front());
		
		for (int i = 0; i<10; i++) 
			testCase.deQueue();
		
		if (testCase.isEmpty())
			System.out.println("It WORKS! YIPPIE!");
		else 
			System.out.println("It does not work.  Darn.");
	

	}

}

you will probably need this code too, to help me figure out the problem:

/**
 * 
 */
package simpleLists;

/**
 * @author robert
 *
 */
public class TpList<E> implements SimpleList<E> {
	
	// instance variables: the first and last Node in the list
	private Node<E> _front;
	private Node<E> _rear;
	
	/* (non-Javadoc)
	 * @see simpleLists.SimpleList#addAtFront(java.lang.Object)
	 */
	@Override
	public void addAtFront(E item) {
		// TODO Auto-generated method stub
		_front = new Node<E>(item, _front);
	}

	/* (non-Javadoc)
	 * @see simpleLists.SimpleList#addAtRear(java.lang.Object)
	 */
	@Override
	public void addAtRear(E item) {
		// TODO Auto-generated method stub
		if (_front != null){
			_rear = _front;
			while (_rear.getNext() != null)
				_rear = _rear.getNext();
			_rear.setNext(new Node<E>(item,null));}
		else _front = _rear = new Node<E>(item,null);
		}

	/* (non-Javadoc)
	 * @see simpleLists.SimpleList#firstElement()
	 */
	@Override
	public E firstElement() throws EmptyListException {
		// TODO Auto-generated method stub
		if (_front == null){
			throw new EmptyListException ("This list is empty.");
		}
		else {
			E v = _front.getElement();
			return v; 
		}
	}

	/* (non-Javadoc)
	 * @see simpleLists.SimpleList#isEmpty()
	 */
	@Override
	public boolean isEmpty() {
		// TODO Auto-generated method stub
		if (_front == null)
			return true;
		else
			return false;
	}

	/* (non-Javadoc)
	 * @see simpleLists.SimpleList#removeFirst()
	 */
	@Override
	public E removeFirst() throws EmptyListException {
		// TODO Auto-generated method stub
		if (_front == null)
			throw new EmptyListException("The list is empty.");
		E t = _front.getElement();
		_front = _front.getNext();
		return t;

	}

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		// This may be useful for simple testing
		TpList<String> lister = new TpList<String>();
		lister.addAtRear("my");
		lister.addAtRear("hero");
		lister.addAtFront("is");
		lister.addAtFront("Robert");
		while (!lister.isEmpty()) 
			System.out.print(lister.removeFirst() + " " );
		System.out.println("!");			
	}
}

Please if you can help at all, i'd really appreciate it. I've been up all night trying to figure it out, and I'm super tired now xD.

[RamyaSivakanth]

Hi.
_contents is just a reference know.U have not created an object.Might be it is throwing nullpointer exception.

[r035198x]

Read this article.

[Travis]

Thanks for the replies, but i still wasn't able to fix it, mainly because there was so much the link, that i didn't know what to actually look for. But is there anything wrong with those specific lines of code that i should fix?

[r035198x]

If you can't read that article and apply it to your problem then you can't solve your problem at all.

[RamyaSivakanth]

Hi,
ro35198x has given explanation in the below link.Take some time and gothru.

But,as per the code posted and line numbers,the below reference variable is not pointing to any object.Please check ur code fully and put proper debug stattements as S.O.Ps.

private SimpleList<E> _contents;

[Travis]

thanks ramya, i instantiated it and i got rid of the error. However, now its giving me

Exception in thread "main" queueTask.EmptyQueueException: Queue is empty.
at queueTask.TpQueue.front(TpQueue.java:29)
at queueTask.TpQueue.main(TpQueue.java:53)

for those same pieces of code. This one i can't even find on the internet. Any ideas of whats causing it?

[StormyWaters]

Look at your TpQueue class at the line specified(29) and you'll notice you're throwing it.

With that information you should be able to backtrack why it is being thrown.