Null pointer exceptions and linked lists

[Tsirist]

I'm trying to determine if a reference in an element of a linked list is null. I'm having trouble doing so.

public void addEngine(Engine engineToBeAdded)
	{
		Engine currentEngine = firstEngine;
		while (currentEngine.nextEngine != null)
			currentEngine = currentEngine.nextEngine;
		currentEngine.nextEngine = engineToBeAdded;
	}

This method is inside the Train class. The Train object has a field that points to the first Engine object, and each Engine object has a field that points to the next Engine.

I need to know if the next Engine doesn't exist, and if that is the case, assign a new Engine to that location.

while (currentEngine.nextEngine != null)

When I try to run the above code, however, I get a NullPointerException. Not sure what to do.

Help is appreciated!

[FlyNn]

It would be useful to see which line of the code throws the exception.

[Tsirist]

Ah, yes. This part gives the problem:

while (currentEngine.nextEngine != null)

[FlyNn]

Is this a separate API? I've never seen this in JAVA? Maybe jMonkeyEngine API?

[Tsirist]

I'm pretty sure this is Java. I refer to the Java API when coding, and I use the Eclipse IDE. When talking in class, we call it Java, not C or this jMonkeyEngine.

[FlyNn]

Where does the firstEngine appear? Is it a global Engine? Maybe if there is nothing in the firstEngine, your currentEngine will be null too.

[Tsirist]

Ah yes, I'd neglected the possibility of firstEngine being null. Thanks a lot! Can't believe I didn't notice that!