HashSet源码分析

HashSet是基于HashMap来实现的，因为它的底层是通过HashMap来保存元素的。

1.属性

    // 用于存储存放到HashSet中的元素
    private transient HashMap<E,Object> map;
    
    // HashSet中存放的是一个元素，而HashMap存放的是key-value键值对，此PRESENT对象就是value值
    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

2.构造器

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     * 构造一个空的HashSet，HashMap默认的初始容量是16，加载因子是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
     * 构造一个包含指定集合元素的Set
     */
    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.
     *
     * @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
     * 构造一个空的HashSet，底层HashMap是根据指定的参数初始化容量和加载因子来构造的
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

3.方法

3.1 boolean add（E e）：如果此 set 中尚未包含指定元素，则添加指定元素

   /**
     * 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&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;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
     * 添加指定元素到set中，如果set中已经包含了该元素，那么不会重复添加，并且返回false；否则，返回true
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

3.2 boolean remove(Object o):如果指定元素存在于此 set 中，则将其移除。

   /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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
     * 如果此set包含指定的元素，那么移除然后返回true
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

3.3 boolean contains(Object o):如果此 set 包含指定元素，则返回 true。

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

总结：HashSet其实比较简单，它不包含重复的元素，它的底层就是HashMap，HashSet中的元素就是HashMap中的key，HashMap中的value值则是一个静态的Object对象。