ConcurrentIterator.java

package test;

import java.util.ArrayList;
import java.util.ConcurrentModificationException;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

public class ConcurrentIterator<E> implements Iterator<E> {

	private final Iterable<E> iterable;
	private Iterator<E> iterator;
	private Set<E> seen = null;
	private List<E> seenList;
	private E next = null;
	
	public ConcurrentIterator(Iterable<E> iterable) {
		this.iterable = iterable;
		this.iterator = iterable.iterator();
		this.seenList = new ArrayList<E>(); // TODO second constructor that takes a Collection to init the size here
	}
	
	@Override
	public boolean hasNext() {
		while (true) {
			try {
				while (iterator.hasNext()) {
					next = iterator.next();
					if (seen == null) {
						seenList.add(next);
						return true;
					} else if (!seen.contains(next)) {
						seen.add(next);
						return true;
					}
				}
				return false;
			} catch (ConcurrentModificationException e) {
				iterator = iterable.iterator();
				if (seen == null) {
					seen = new HashSet<E>(seenList);
					seenList = null;
				}
			}
		}
	}

	@Override
	public E next() {
		return next;
	}

}