http://momsbaby1986.iteye.com/blog/1452022

java没有提供ConcurrentHashSet, 可以通过ConcurrentHashMap来间接实现

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

 public class ConcurrentHashSet<E> extends AbstractSet<E> implements Set<E>, java.io.Serializable {

     private static final long serialVersionUID = -8672117787651310382L;

     private static final Object PRESENT = new Object();

     private final ConcurrentHashMap<E, Object> map;

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

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

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

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

     public boolean isEmpty() {
         return map.isEmpty();
     }

     public boolean contains(Object o) {
         return map.containsKey(o);
     }

     public boolean add(E e) {
         return map.put(e, PRESENT) == null;
     }

     public boolean remove(Object o) {
         return map.remove(o) == PRESENT;
     }

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