1. 概述
This class implements the Set interface, backed by a hash table (actually a HashMap instance).
It makes no guarantees as to the iteration order of the set; in particular,
it does not guarantee that the order will remain constant over time.
This class permits the null element.This class implements the Set interface, backed by a hash table (actually a HashMap instance).
It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time.
This class permits the null element.
HashSet是基于HashMap来实现的，操作很简单，更像是对HashMap做了一次“封装”，而且只使用了HashMap的key来实现各种特性，我们先来感性的认识一下这个结构：
HashSet<String> set = new HashSet<String>();
set.add("语文");
set.add("数学");
set.add("英语");
set.add("历史");
set.add("政治");
set.add("地理");
set.add("生物");
set.add("化学");
其大致的结构是这样的：
private transient HashMap<E,Object> map;
// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();
map是整个HashSet的核心，而PRESENT则是用来造一个假的value来用的。Map有键和值，HashSet相当于只有键，值都是相同的固定值，即PRESENT。
2. 基本操作
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
public boolean contains(Object o) {
return map.containsKey(o);
}
public int size() {
return map.size();
}基本操作也非常简单，就是调用HashMap的相关方法，其中value就是之前那个dummy的Object。
Refer:
http://yikun.github.io/2015/04/08/Java-HashSet%E5%B7%A5%E4%BD%9C%E5%8E%9F%E7%90%86%E5%8F%8A%E5%AE%9E%E7%8E%B0/