HashSet 源码分析

package java.util;

import java.io.InvalidObjectException;

import jdk.internal.misc.SharedSecrets;

/**
* 1）HashSet 是基于 HashMap 的 Set 接口实现，元素的迭代是无序的，可以使用 null 元素。
*/
public class HashSet<E>
 extends AbstractSet<E>
 implements Set<E>, Cloneable, java.io.Serializable
{
 static final long serialVersionUID = -5024744406713321676L;

 // 支持此 HashSet 的 HashMap 实例
 private transient HashMap<E,Object> map;

 /**
 * Dummy value to associate with an Object in the backing Map
 * 与指定键关联的虚拟值。
 */
 private static final Object PRESENT = new Object();

 /**
 * Constructs a new, empty set; the backing {@code HashMap} instance has
 * default initial capacity (16) and load factor (0.75).
 * 创建一个空 HashSet，底层 HashMap 的初始容量为 16 加载因子为 0.75。
 */
 public HashSet() {
 map = new HashMap<>();
 }

 /**
 * Constructs a new set containing the elements in the specified
 * collection. The {@code HashMap} is created with default load factor
 * (0.75) and an initial capacity sufficient to contain the elements in
 * the specified collection.
 * 创建一个空 HashSet，底层 HashMap 的初始容量为 Math.max((int) (c.size()/.75f) + 1, 16) 加载因子为 0.75。
 * 集合中的元素顺序添加到 Set 中。
 */
 public HashSet(Collection<? extends E> c) {
 map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
 addAll(c);
 }

 /**
 * Constructs a new, empty set; the backing {@code HashMap} instance has
 * the specified initial capacity and the specified load factor.
 * 创建一个空 HashSet，底层 HashMap 的初始容量为 initialCapacity 加载因子为 loadFactor。
 */
 public HashSet(int initialCapacity, float loadFactor) {
 map = new HashMap<>(initialCapacity, loadFactor);
 }

 /**
 * Constructs a new, empty set; the backing {@code HashMap} instance has
 * the specified initial capacity and default load factor (0.75).
 * 创建一个空 HashSet，底层 HashMap 的初始容量为 initialCapacity 加载因子为 0.75。
 */
 public HashSet(int initialCapacity) {
 map = new HashMap<>(initialCapacity);
 }

 /**
 * Constructs a new, empty linked hash set. (This package private
 * constructor is only used by LinkedHashSet.) The backing
 * HashMap instance is a LinkedHashMap with the specified initial
 * capacity and the specified load factor.
 * 创建一个空 HashSet，底层 HashMap 的初始容量为 initialCapacity 加载因子为 loadFactor。
 */
 HashSet(int initialCapacity, float loadFactor, boolean dummy) {
 map = new LinkedHashMap<>(initialCapacity, loadFactor);
 }

 /**
 * Returns an iterator over the elements in this set. The elements
 * are returned in no particular order.
 * 获取 HashSet 中所有元素的迭代器
 */
 public Iterator<E> iterator() {
 return map.keySet().iterator();
 }

 /**
 * Returns the number of elements in this set (its cardinality).
 * 获取 HashSet 中元素的总数
 */
 public int size() {
 return map.size();
 }

 /**
 * Returns {@code true} if this set contains no elements.
 * HashSet 是否为空
 */
 public boolean isEmpty() {
 return map.isEmpty();
 }

 /**
 * Returns {@code true} if this set contains the specified element.
 * More formally, returns {@code true} if and only if this set
 * contains an element {@code e} such that
 * {@code Objects.equals(o, e)}.
 * 判断此 HashSet 中是否包含指定的值
 */
 public boolean contains(Object o) {
 return map.containsKey(o);
 }

 /**
 * Adds the specified element to this set if it is not already present.
 * More formally, adds the specified element {@code e} to this set if
 * this set contains no element {@code e2} such that
 * {@code Objects.equals(e, e2)}.
 * If this set already contains the element, the call leaves the set
 * unchanged and returns {@code false}.
 * 如果元素不存在，则新增【返回值为 null】，返回值为 true;
 * 否则不对 HashSet 做修改，返回值为 false【PRESENT == null】。
 */
 public boolean add(E e) {
 return map.put(e, PRESENT)==null;
 }

 /**
 * Removes the specified element from this set if it is present.
 * More formally, removes an element {@code e} such that
 * {@code Objects.equals(o, e)},
 * if this set contains such an element. Returns {@code true} if
 * this set contained the element (or equivalently, if this set
 * changed as a result of the call). (This set will not contain the
 * element once the call returns.)
 * 从 HashSet 中移除指定的元素
 */
 public boolean remove(Object o) {
 return map.remove(o)==PRESENT;
 }

 /**
 * Removes all of the elements from this set.
 * 清空 HashSet
 */
 public void clear() {
 map.clear();
 }

 /**
 * Returns a shallow copy of this {@code HashSet} instance: the elements
 * themselves are not cloned.
 */
 @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 {@code HashSet} instance to a stream (that is,
 * serialize it).
 */
 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 {@code HashSet} 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);

 // Constructing the backing map will lazily create an array when the first element is
 // added, so check it before construction. Call HashMap.tableSizeFor to compute the
 // actual allocation size. Check Map.Entry[].class since it's the nearest public type to
 // what is actually created.
 SharedSecrets.getJavaObjectInputStreamAccess()
 .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));

 // Create backing HashMap
 map = (((HashSet<?>)this) instanceof LinkedHashSet ?
 new LinkedHashMap<>(capacity, loadFactor) :
 new HashMap<>(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 <a href="Spliterator.html#binding">late-binding</a>
 * and fail-fast {@link Spliterator} over the elements in this
 * set.
 *
 * 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
 */
 public Spliterator<E> spliterator() {
 return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
 }
}

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