HashSet源码解析

//底层使用了HashMap来存储元素，所以元素不可能出现重复
private transient HashMap<E,Object> map;
//用来填充底层数据结构HashMap中的value，因为HashSet只用key存储数据。
private static final Object PRESENT = new Object();

    //无参构造
    public HashSet() {
        map = new HashMap<>();
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }

    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }