import java.util.AbstractSet;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
public class ConcurrentHashSet<E> extends AbstractSet<E> implements Set<E>{
private final ConcurrentMap<E, Object> theMap;
private static final Object dummy = new Object();
public ConcurrentHashSet(){
theMap = new ConcurrentHashMap<E, Object>();
}
@Override
public int size() {
return theMap.size();
}
@Override
public Iterator<E> iterator(){
return theMap.keySet().iterator();
}
@Override
public boolean isEmpty(){
return theMap.isEmpty();
}
@Override
public boolean add(final E o){
return theMap.put(o, ConcurrentHashSet.dummy) == null;
}
@Override
public boolean contains(final Object o){
return theMap.containsKey(o);
}
@Override
public void clear(){
theMap.clear();
}
@Override
public boolean remove(final Object o){
return theMap.remove(o) == ConcurrentHashSet.dummy;
}
public boolean addIfAbsent(final E o){
Object obj = theMap.putIfAbsent(o, ConcurrentHashSet.dummy);
return obj == null;
}
}