UniqueExtendedIterator.java

package net.enilink.commons.iterator;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

/**
 * A variant on the closable/extended iterator that filters out duplicate
 * values. There is one complication that the value which filtering is done on
 * might not be the actual value to be returned by the iterator.
 * 
 * @author <a href="mailto:der@hplb.hpl.hp.com">Dave Reynolds</a>
 * @version $Revision: 1.16 $ on $Date: 2008/01/02 12:07:36 $
 */
public class UniqueExtendedIterator<T> extends WrappedIterator<T> {
	/** The set of objects already seen */
	protected Set<T> seen = new HashSet<T>();

	/** One level lookahead */
	protected T next = null;

	/**
	 * Constructor. Note the use of {@link #create} as reliable means of
	 * creating a unique iterator without double-wrapping iterators that are
	 * already unique iterators.
	 */
	public UniqueExtendedIterator(Iterator<T> underlying) {
		super(underlying, true);
	}

	/**
	 * Factory method for generating an iterator that is guaranteed only to
	 * return one instance of every result from the wrapped iterator
	 * <code>it</code>.
	 * 
	 * @param it
	 *            An iterator to wrap
	 * @return A iterator that returns the elements of the wrapped iterator
	 *         exactly once. If <code>it</code> is already a unique extended
	 *         iteator, it is not further wrapped.
	 */
	public static <T> IExtendedIterator<T> create(Iterator<T> it) {
		return (it instanceof UniqueExtendedIterator<?>) ? ((UniqueExtendedIterator<T>) it)
				: new UniqueExtendedIterator<T>(it);
	}

	/**
	 * Fetch the next object to be returned, only if not already seen.
	 * Subclasses which need to filter on different objects than the return
	 * values should override this method.
	 * 
	 * @return the object to be returned or null if the object has been
	 *         filtered.
	 */
	protected T nextIfNew() {
		T value = super.next();
		return seen.add(value) ? value : null;
	}

	/**
	 * @see Iterator#hasNext()
	 */
	public boolean hasNext() {
		while (next == null && super.hasNext())
			next = nextIfNew();
		return next != null;
	}

	/**
	 * @see Iterator#next()
	 */
	public T next() {
		ensureHasNext();
		T result = next;
		next = null;
		return result;
	}
}

/*
 * (c) Copyright 2003, 2004, 2005, 2006, 2007, 2008 Hewlett-Packard Development
 * Company, LP All rights reserved.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 1. Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer. 2. Redistributions in
 * binary form must reproduce the above copyright notice, this list of
 * conditions and the following disclaimer in the documentation and/or other
 * materials provided with the distribution. 3. The name of the author may not
 * be used to endorse or promote products derived from this software without
 * specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
 * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */