本文的目录结构：
- 一、HashSet 的 Javadoc 文档注释和简要说明
- 二、HashSet 的内部实现：内部属性和构造函数
- 三、HashSet 的 add 操作和扩容
- 四、HashSet 的 remove 操作
一、HashSet 的 Javadoc 文档注释和简要说明
截个图，然后来观摩 HashSet 的javadoc，照样是几点总结摘抄：
- Set 接口的实现类，内部使用了一个 HashMap 实例；不保证 set 的迭代顺序（无序）；允许存储 null
- 通常情况下（假如 hash 分布比较均匀），基本操作（add, remove, contains 和 size）可以看成是 O(1) 的；迭代/遍历的时间和 HashSet 的元素数量以及内部 HashMap 实例的的内部数组大小有关，因此对于迭代遍历性能有要求的， HashSet 的初始容量不能设置太大或者负载因子不能太小。
- HashSet 也是非线程安全的，需要其他的工具类来保证线程安全
- HashSet 也是 fail-fast；同样也并不保证出现有并发修改就百分百抛出 ConcurrentModificationException，
二、HashSet 的内部实现：内部属性和构造函数
这个就比较简洁了，只有一个共用的 Object 对象，其实就是内部 HasMap 的 value。
// Dummy value to associate with an Object in the backing Map private static final Object PRESENT = new Object();
再来看一看构造函数，都似曾相识，其实基本就是 HashMap 的构造函数调用一遍，还有一个包级私有的构造函数，内部创建了 LinkedHashMap，这个是给 LinkedHashSet 用的，双链表保证有序。
/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* default initial capacity (16) and load factor (0.75).
* 默认初始容量 16 和 负载因子 0.75
*/
public HashSet() {
map = new HashMap<>();
}
/**
* Constructs a new set containing the elements in the specified
* collection. The <tt>HashMap</tt> is created with default load factor
* (0.75) and an initial capacity sufficient to contain the elements in
* the specified collection.
*
* @param c the collection whose elements are to be placed into this set
* @throws NullPointerException if the specified collection is null
*/
public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}
/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* the specified initial capacity and the specified load factor.
* 指定 初始容量 和 负载因子
* @param initialCapacity the initial capacity of the hash map
* @param loadFactor the load factor of the hash map
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* the specified initial capacity and default load factor (0.75).
* 初定 初始容量，默认负载因子 0.75
* @param initialCapacity the initial capacity of the hash table
* @throws IllegalArgumentException if the initial capacity is less
* than zero
*/
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
/**
* Constructs a new, empty linked hash set. (This package private
* constructor is only used by LinkedHashSet.) The backing
* HashMap instance is a LinkedHashMap with the specified initial
* capacity and the specified load factor.
* 给 LinkedHashSet 用的构造函数，其实和 HashMap 套路类似，只是 LinkedHashMap 有序
* @param initialCapacity the initial capacity of the hash map
* @param loadFactor the load factor of the hash map
* @param dummy ignored (distinguishes this
* constructor from other int, float constructor.)
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive
*/
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
三、HashSet 的 add 操作和扩容
也比较简单直接，就是调用内部 HashMap 实例的 put 方法。当然扩容也是 HashMap 本身的扩容。
这里还是摘抄下 javadoc，大致就是说 set 中不会有重复的 key，e 和 e2 重复的判断条件是 (e==null ? e2==null : e.equals(e2))。 e 存在的时候该方法返回 false。
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.
/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element <tt>e</tt> to this set if
* this set contains no element <tt>e2</tt> such that
* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>.
*
* @param e element to be added to this set
* @return <tt>true</tt> if this set did not already contain the specified
* element
*/
public boolean add(E e) {
return map.put(e, PRESENT)==null;
四、HashSet 的 remove 操作
remove 移除元素，内部判断存在的条件是 o==null ? e==null : o.equals(e)
/**
* Removes the specified element from this set if it is present.
* More formally, removes an element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>,
* if this set contains such an element. Returns <tt>true</tt> if
* this set contained the element (or equivalently, if this set
* changed as a result of the call). (This set will not contain the
* element once the call returns.)
*
* @param o object to be removed from this set, if present
* @return <tt>true</tt> if the set contained the specified element
*/
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}