Java APIs

Hi,

I am trying to write an ItIterator class which would iterate over the elements of multiple iterator.

import java.util.ArrayList;
import java.util.Iterator;


public class ItIterator<T> implements Iterator<T> {

	private Iterator<T> currIterator=null;
	private ArrayList<Iterator<T>> arr;
	private Iterator<Iterator<T>> arrIterator;
	
	public ItIterator()
	{
		arr=new ArrayList<Iterator<T>>();
		arrIterator=arr.iterator();
	}
	
	public void add(Iterator<T> e)
	{
		arr.add(e);
	}
	
	
	@Override
	public boolean hasNext() 
	{
		if(currIterator!=null && currIterator.hasNext())
			return true;
		
		while(arrIterator.hasNext())
		{
			currIterator=arrIterator.next();
			if(currIterator.hasNext())
				return true;
		}
		return false;
	}

	@Override
	public T next() 
	{
		if(currIterator!=null && currIterator.hasNext())
			return currIterator.next();
		
		while(arrIterator.hasNext())
		{
			currIterator=arrIterator.next();
			if(currIterator.hasNext())
				return currIterator.next();
		}
		return null;
	}

	@Override
	public void remove() 
	{
		if(currIterator!=null && currIterator.hasNext())
			currIterator.remove();
		
		while(arrIterator.hasNext())
		{
			currIterator=arrIterator.next();
			if(currIterator.hasNext())
			{
				currIterator.remove();
				return;
			}
		}
		
	}

}

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;


public class Test 
{
	
	
	
	public static void main(String args[])
	{
		ItIterator<Integer> i=new ItIterator<Integer>();
		
		ArrayList<Integer> a=new ArrayList<Integer>();
		a.add(1);
		a.add(2);
		a.add(3);
		ArrayList<Integer> b=new ArrayList<Integer>();
		b.add(4);
		b.add(5);
		b.add(6);
		
		i.add(a.iterator());
		i.add(b.iterator());
		
		while(i.hasNext())
			System.out.println(i.next().toString());
		
	}
	

}

Te above code complies without any error but gives a runtime exception as follows:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
	at java.util.AbstractList$Itr.next(Unknown Source)
	at ItIterator.hasNext(ItIterator.java:31)
	at Test.main(Test.java:28)

If someone could help me out solving this exception that would be great.