简单实现java中的HashSet

package com.cym.collection;


import java.util.HashMap;
import java.util.Map;




public class MyHashSet {
	private Map<Object, Object> map;
	private static final Object PRESENT = new Object();
	private int size;
	public MyHashSet() {
		map = new HashMap<Object, Object>();
	}


	public void add(Object obj){
		map.put(obj, PRESENT);
	}
	public boolean remove(Object key) {
		return map.remove(key)==PRESENT;
	}
	public int size(){
		return map.size();
	}
	
}