本文的目录结构：
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，一个 HashMap 实例，用于存储元素 e，也即是 k，另外就是一个共用的 Object 对象，其实就是内部 HasMap 实例的 value。
// 内部使用的 HashMap 实例 private transient HashMap<E,Object> map; // 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; }