Java容器——HashSet（Java8）源码解析

 HashSet是一种元素集合，实现了Set接口，是一种快速的，去重的集合对象，继承关系见图1所示。

                                                                                  图1 HashSet继承关系图

HashSet继承自Set接口，继承Set的还包括TreeSet，SortedSet等，他们都有一个基本的标准的特征，不包含重复的元素。HashSet从名称上容易看出，这是通过Hash来实现的元素去重以及对元素的各种快速操作。

HashSet是如何做到的呢？从成员变量和构造函数中就可以一窥端倪。

构造函数

    private transient HashMap map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

首先看着两个成员变量，HashSet中有一个HashMap，似乎已经预示着这两个者之间有一些关联，后面一个变量则印证了这个猜测，这个变量可能是要放置进HashMap中的。

    /**
     * Constructs a new, empty set; the backing HashMap instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection.  The HashMap 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 c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    /**
     * Constructs a new, empty set; the backing HashMap instance has
     * the specified initial capacity and the specified load factor.
     *
     * @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 HashMap instance has
     * the specified initial capacity and default load factor (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);
    }

看到这些HashSet的构造函数，是不是有些似曾相识的感觉，与HashMap的构造函数有些雷同，或者说，这就是在构造HashMap。因为实质上，HashSet就是由HashMap实现的，来看一下HashSet的关键函数就更清楚了。

关键函数

HashSet在构造之初，首先初始化了一个HashMap，并利用这个HashMap作为存储数据的容器。前面有介绍过，HashMap是一个键值对容器，节点元素以Node形式组织。HashMap使用Key计算Hash，使用Value存储对应的值。HashSet则只存储单个的元素，这样就可以只用HashMap的Key就可以实现。

HashSet关键的方法罗列如下，可以看到，所有的操作都是针对Key进行的。之前定义的PRESENT变量，就是为了填充到HashMap中Node节点的Value部分。

    /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator iterator() {
        return map.keySet().iterator();
    }

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() {
        return map.size();
    }

    /**
     * Returns true if this set contains no elements.
     *
     * @return true if this set contains no elements
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * Returns true if this set contains the specified element.
     * More formally, returns true if and only if this set
     * contains an element e such that
     * (o==null ? e==null : o.equals(e)).
     *
     * @param o element whose presence in this set is to be tested
     * @return true if this set contains the specified element
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /**
     * 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.
     *
     * @param e element to be added to this set
     * @return true if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element e such that
     * (o==null ? e==null : o.equals(e)),
     * if this set contains such an element.  Returns true 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 true if the set contained the specified element
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    /**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    public void clear() {
        map.clear();
    }

HashSet不需要重新实现这些函数，都直接借用HashMap的就好。包装在外层的HashSet实质上是一个代理，具体的操作都交由内部构造的HashMap实例实现。通过这样的方式，可以更加方便高效地实现HashSet这种数据结构，也避免了以几乎同样方式再实现HashSet的冗余。

小结

HashSet是一种支持快速操作的数据结构，理想情况下对HashSet的操作可达到常量级别，是存储不重复元素的常用集合。HashSet本身不是线程安全的，在多线程环境下使用可能出现并发访问问题，要在多线程环境下使用Set可以尝试使用ConcurrentHashMap构造一个类似的ConcurrentHashSet来实现。