java数据结构之HashSet和TreeSet以及LinkedHashSet

一、HashSet源码注释

 

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    //HashSet底层是HashMap
    private transient HashMap<E,Object> map;

    // 用来在HashMap中所有的key对应的value值指向这个对象
    private static final Object PRESENT = new Object();

    /**
     * 创建空的HashSet,实际上是创建了一个默认实现的HashMap
     */
    public HashSet() {
        map = new HashMap<>();
    }

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

    /**
     * 手动指定容量和加载因子创建HashSet
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    /**
     * 创建指定容量的HashSet
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /**
     * 底层是LinkedHashSet
     */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    /**
     * 获取迭代器
     */
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    /**
     * 获取元素数量
     */
    public int size() {
        return map.size();
    }

    /**
     * 判断是否为空
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * 判断HashSet是否存在该 对象
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /**
     * 新增一个元素，这个元素在HashMap中作为Key，而value则是一个公共的Object
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    /**
     * 删除一个元素
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    /**
     * 清空整个集合
     */
    public void clear() {
        map.clear();
    }

    /**
     * 浅克隆
     */
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet<E> newSet = (HashSet<E>) super.clone();
            newSet.map = (HashMap<E, Object>) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * 将HashSet写入流中
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out HashMap capacity and load factor
        s.writeInt(map.capacity());
        s.writeFloat(map.loadFactor());

        // Write out size
        s.writeInt(map.size());

        // Write out all elements in the proper order.
        for (E e : map.keySet())
            s.writeObject(e);
    }

    /**
     * 从流中读取HashSet的元素
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read capacity and verify non-negative.
        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }

        // Read load factor and verify positive and non NaN.
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }

        // Read size and verify non-negative.
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }

        // Set the capacity according to the size and load factor ensuring that
        // the HashMap is at least 25% full but clamping to maximum capacity.
        capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                HashMap.MAXIMUM_CAPACITY);

        // Create backing HashMap
        map = (((HashSet<?>)this) instanceof LinkedHashSet ?
               new LinkedHashMap<E,Object>(capacity, loadFactor) :
               new HashMap<E,Object>(capacity, loadFactor));

        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }

    /**
     * 分割迭代器
     */
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }
}

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