HashSet.java

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package java.util;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import jdk.internal.misc.SharedSecrets;

/**
 * This class implements the {@code Set} interface, backed by a hash table
 * (actually a {@code HashMap} instance).  It makes no guarantees as to the
 * iteration order of the set; in particular, it does not guarantee that the
 * order will remain constant over time.  This class permits the {@code null}
 * element.
 *
 * <p>This class offers constant time performance for the basic operations
 * ({@code add}, {@code remove}, {@code contains} and {@code size}),
 * assuming the hash function disperses the elements properly among the
 * buckets.  Iterating over this set requires time proportional to the sum of
 * the {@code HashSet} instance's size (the number of elements) plus the
 * "capacity" of the backing {@code HashMap} instance (the number of
 * buckets).  Thus, it's very important not to set the initial capacity too
 * high (or the load factor too low) if iteration performance is important.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a hash set concurrently, and at least one of
 * the threads modifies the set, it <i>must</i> be synchronized externally.
 * This is typically accomplished by synchronizing on some object that
 * naturally encapsulates the set.
 *
 * If no such object exists, the set should be "wrapped" using the
 * {@link Collections#synchronizedSet Collections.synchronizedSet}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the set:<pre>
 *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
 *
 * <p>The iterators returned by this class's {@code iterator} method are
 * <i>fail-fast</i>: if the set is modified at any time after the iterator is
 * created, in any way except through the iterator's own {@code remove}
 * method, the Iterator throws a {@link ConcurrentModificationException}.
 * Thus, in the face of concurrent modification, the iterator fails quickly
 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
 * an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness: <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements maintained by this set
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see Set
 * @see TreeSet
 * @see HashMap
 * @since 1.2
 */
// HashSet是无序Set，内部通过HashMap或LinkedHashMap来完成其固有操作
public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, Serializable {
    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();
    
    private transient HashMap<E, Object> map;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructs a new, empty set; the backing {@code HashMap} instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }
    
    /**
     * Constructs a new, empty set; the backing {@code HashMap} instance has
     * the specified initial capacity and default load factor (0.75).
     *
     * @param initialCapacity the initial capacity of the hash table
     *
     * @throws IllegalArgumentException if the initial capacity is less
     *                                  than zero
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }
    
    /**
     * Constructs a new, empty set; the backing {@code HashMap} instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param initialCapacity the initial capacity of the hash map
     * @param loadFactor      the load factor of the hash map
     *
     * @throws IllegalArgumentException if the initial capacity is less
     *                                  than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }
    
    /**
     * 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.
     *
     * @param c the collection whose elements are to be placed into this set
     *
     * @throws NullPointerException if the specified collection is null
     */
    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size() / .75f) + 1, 16));
        addAll(c);
    }
    
    
    /**
     * 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.
     *
     * @param initialCapacity the initial capacity of the hash map
     * @param loadFactor      the load factor of the hash map
     * @param dummy           ignored (distinguishes this
     *                        constructor from other int, float constructor.)
     *
     * @throws IllegalArgumentException if the initial capacity is less
     *                                  than zero, or if the load factor is nonpositive
     */
    // 内部使用了LinkedHashMap
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 存值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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}.
     *
     * @param e element to be added to this set
     *
     * @return {@code true} if this set did not already contain the specified
     * element
     */
    // 向Set中添加元素
    public boolean add(E e) {
        return map.put(e, PRESENT) == null;
    }
    
    /*▲ 存值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 移除 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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.)
     *
     * @param o object to be removed from this set, if present
     *
     * @return {@code true} if the set contained the specified element
     */
    // 移除指定的元素，返回值指示是否移除成功
    public boolean remove(Object o) {
        return map.remove(o) == PRESENT;
    }
    
    /**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    // 清空当前集合中所有元素
    public void clear() {
        map.clear();
    }
    
    /*▲ 移除 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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)}.
     *
     * @param o element whose presence in this set is to be tested
     *
     * @return {@code true} if this set contains the specified element
     */
    // 判断当前集合中是否包含元素o
    public boolean contains(Object o) {
        return map.containsKey(o);
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 迭代 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     *
     * @see ConcurrentModificationException
     */
    // 返回当前集合的迭代器
    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }
    
    /**
     * 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
     */
    // 返回描述此集合中元素的Spliterator
    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
    }
    
    /*▲ 迭代 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    // 返回当前集合的元素数量
    public int size() {
        return map.size();
    }
    
    /**
     * Returns {@code true} if this set contains no elements.
     *
     * @return {@code true} if this set contains no elements
     */
    // 判断当前集合是否为空
    public boolean isEmpty() {
        return map.isEmpty();
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 序列化 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    static final long serialVersionUID = -5024744406713321676L;
    
    /**
     * Save the state of this {@code HashSet} instance to a stream (that is,
     * serialize it).
     *
     * @serialData The capacity of the backing {@code HashMap} 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(ObjectOutputStream s) throws 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(ObjectInputStream s) throws 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 = (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);
        }
    }
    
    /*▲ 序列化 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * Returns a shallow copy of this {@code HashSet} 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);
        }
    }
    
}