New to Java

Hi,

I'm getting the following error and i'm really struggling to figure out what the problem is.
the error reads

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1 < 0
        at java.util.Vector.elementAt(Vector.java:437)
        at MemPool.calcStimulation(MemPool.java:106)
        at Test.main(Test.java:50)

i have a MemPool class which begins as follows

public class MemPool
{
private Vector MemoryPool;

public Vector calcStimulation(Vector MemPool, Vector Training)
{

and a test class with has the main method to call this calss which is

public class Test
{
	public static void main(String[] args)
  	{
	MemPool m = new MemPool();
	Vector stim = m.calcStimulation(pool,trainingData);
	}
}

The above is just snippets of the code.
Can anyone point me in the right direction on how to solve this?

Cheers