CollectionsIterator.java

package com.designingmicroservices;

import java.util.Collection;
import java.util.Iterator;

/**
 * Implements the Iterator on Collection of Collections.
 * 
 * @author DesigningMicroservices.com
 *
 * @param <E>
 */
public class CollectionsIterator<E> implements Iterator<E> {

	private Iterator<? extends Collection<E>> topIterator;
	private Iterator<E> innerIterator;
	private E next;

	CollectionsIterator(Collection<? extends Collection<E>> collection) {
		this.topIterator = collection.iterator();
		prepareNext();
	}

	private void prepareNext() {

		do {
			if (innerIterator == null || !innerIterator.hasNext()) {
				if (!topIterator.hasNext()) {
					this.next = null;
					return;
				} else {
					this.innerIterator = this.topIterator.next().iterator();
					this.next = innerIterator.next();
				}
			} else {
				this.next = innerIterator.next();
			}

		} while (next == null && topIterator.hasNext());

	}

	public boolean hasNext() {

		return (next != null);
	}

	public E next() {

		E previousNext = this.next;

		prepareNext();

		return previousNext;
	}

}