HashSet.java

package java.util;

import java.io.InvalidObjectException;

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable{

    //序列化ID
    static final long serialVersionUID = -5024744406713321676L;

    //HashSet 底层用 HashMap 来存放数据
    //Key值由外部传入，Value则由 HashSet 内部来维护
    private transient HashMap<E,Object> map;

    //HashMap 中所有键值对都共享同一个值
    //即所有存入 HashMap 的键值对都是使用这个对象作为值
    private static final Object PRESENT = new Object();

    //无参构造函数，HashMap 使用默认的初始化大小和装载因子
    public HashSet() {
        map = new HashMap<>();
    }

    //使用默认的装载因子，并以此来计算 HashMap 的初始化大小
    //+1 是为了弥补精度损失
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    //为 HashMap 自定义初始化大小和装载因子
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    //为 HashMap 自定义初始化大小
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    //此构造函数为包访问权限，只用于对 LinkedHashSet 的支持
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    //将对 HashSet 的迭代转换为对 HashMap 的 Key 值的迭代
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

    //获取集合中的元素数量
    public int size() {
        return map.size();
    }

    //判断集合是否为空
    public boolean isEmpty() {
        return map.isEmpty();
    }

    //判断集合是否包含指定元素
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    //如果 HashSet 中不包含元素 e，则添加该元素，并返回 true
    //如果 HashSet 中包含元素 e，则不会影响 HashSet ，并返回 false
    //该方法将向 HashSet 添加元素 e 的操作转换为向 HashMap 添加键值对
    //如果 HashMap 中包含 key 值与 e 相等的结点（hashCode() 方法返回值相等，通过 equals() 方法比较也返回 true）
    //则新添加的结点的 value 会覆盖原有数据，但 key 不会有所改变
    //因此如果向 HashSet 添加一个已存在的元素时，元素不会被存入 HashMap 中
    //从而实现了 HashSet 元素不重复的特征
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    //移除集合中的元素 o
    //如果集合不包含元素 o，则返回 false
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    //清空集合中的元素
    public void clear() {
        map.clear();
    }

    /**
     * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
     * themselves are not cloned.
     *
     * @return a shallow copy of this set
     */
    @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);
        }
    }

    /**
     * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
     * serialize it).
     *
     * @serialData The capacity of the backing <tt>HashMap</tt> instance
     *             (int), and its load factor (float) are emitted, followed by
     *             the size of the set (the number of elements it contains)
     *             (int), followed by all of its elements (each an Object) in
     *             no particular order.
     */
    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);
    }

    /**
     * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
     * deserialize it).
     */
    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);
        }
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#DISTINCT}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    //为了并行遍历数据源中的元素而设计的迭代器
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
    }

}