Java HashSet

HashSet继承自AbstractSet，实现了Set接口、Cloneable、Serializable接口。

构造函数

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

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

通过hashmap实现Set。

add方法

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

其中添加的e在map中是作为Key，所以没有重复。具体是根据HashMap的put方法