ConcurrentHashSet.java

package com.aiyou.utils.http;

import android.support.annotation.NonNull;

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<>();
    }

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

    @NonNull
    @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;
    }

}