简述 HashSet 实现原理

底层结构是 HashMap 。value都是存放同一个虚拟对象。

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
    static final long serialVersionUID = -5024744406713321676L;
 
    private transient HashMap<E,Object> map;
 
    // 虚拟对象
    private static final Object PRESENT = new Object();
 
    // 无参构造
    public HashSet() {
        map = new HashMap<>();
    }
 
    // 集合构造
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }
 
    // 初始化容量及负载因子构造
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }
 
    // 初始化容量构造
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
 
    // 由于默认修饰符，同包可访问，我们用不到，在LinkedLinkedHashMap中有具体应用，后期会讲解
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }
}