Set之HashSet源码解析

Posted 2023-01-15 夜宿山寺

对于HashSet来说，源码相对简单，没有太多东西是自己实现的，基本上都是基于HashMap实现的，这就是他为什么不提供随机访问的原因，因为HashMap要遍历元素的话就只有迭代的方式，

/*
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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

import java.io.InvalidObjectException;

//hashSet是一个实现起来非常简单的集合，代码也不多，就二百行左右，去掉了注释估计也没有多少了，
//因为他们身只是一个是借助HashMap来实现的。可见作者是有多么懒啊,只不过这个集合也是一个常用的集合,其实我觉得这类完全可以优化一下的
//因为它并不使用HashMap的所有功能。
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;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

	//构建一个HashMap集合
    public HashSet() 
        map = new HashMap<>();
    

	//构造的时候动态添加集合元素
    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 <tt>HashMap</tt> 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, empty set; the backing <tt>HashMap</tt> 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 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
     */
	 //采用LinkedHashMap作为底层实现
    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.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator<E> iterator() 
        return map.keySet().iterator();
    

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() 
        return map.size();
    

    /**
     * Returns <tt>true</tt> if this set contains no elements.
     *
     * @return <tt>true</tt> if this set contains no elements
     */
    public boolean isEmpty() 
        return map.isEmpty();
    

    /**
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null ? e==null : o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    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 <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    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 <tt>e</tt> such that
     * <tt>(o==null ? e==null : o.equals(e))</tt>,
     * if this set contains such an element.  Returns <tt>true</tt> 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 <tt>true</tt> if the set contained the specified element
     */
    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.
     */
    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
     */
    public Spliterator<E> spliterator() 
        return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);