ConcurrentHashSet.java

/**
 * 
 */
package ecologylab.collections;

import java.util.AbstractSet;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Low locking ConcurrentHashSet uses ConcurrentHashMap to lock only when add actually adds.
 * 
 * @author andruid
 */
public class ConcurrentHashSet<E extends Object> extends AbstractSet<E>
{
	private ConcurrentHashMap<E, E>	map;
	
	public ConcurrentHashSet()
	{
		map = new ConcurrentHashMap<E, E>();
	}

	public ConcurrentHashSet(int capacity)
	{
		map = new ConcurrentHashMap<E, E>(capacity);
	}

	public ConcurrentHashSet(Set<E> set)
	{
		for (E element: set)
			map.put(element, element);
	}
	/**
	 * Add element to set if it wasn't there already.
	 * 
	 * @param	element	Element to test for membership and add if it was absent.
	 * 
	 * @return	true if the set changed, that is, if the element was new.
	 */
	@Override
	public boolean add(E element)
	{
		E fromCollection = map.putIfAbsent(element, element);
		return fromCollection == null;	// previous value is null means that the collection changed.
	}

	@Override
	public void clear()
	{
		map.clear();
	}

	@Override
	public boolean contains(Object obj)
	{
		return map.containsKey(obj);
	}

	@Override
	public Iterator<E> iterator()
	{
		return map.keySet().iterator();
	}

	@Override
	public boolean remove(Object obj)
	{
		return map.remove(obj) != null;
	}

	@Override
	public int size()
	{
		return map.size();
	}
}