HashSet源码解读

一：先看其实现了哪些接口和继承了哪些类

1.实现了Serializable接口，表明它支持序列化。 
2.实现了Cloneable接口，表明它支持克隆，可以调用超类的clone（）方法进行浅拷贝。

3.继承了AbstractSet抽象类，和ArrayList和LinkedList一样，在他们的抽象父类中，都提供了equals（）方法和hashCode（）方法。它们自身并不实现这两个方法

4.从JDK源码可以看出，底层并没有使用我们常规认为的利用hashcode（）方法求的值进行比较，而是通过调用AbstractCollection的containsAll（）方法，如果他们中元素完全相同（与顺序无关），则他们的equals（）方法的比较结果就为true。

    /**
     * Compares the specified object with this set for equality.  Returns
     * <tt>true</tt> if the given object is also a set, the two sets have
     * the same size, and every member of the given set is contained in
     * this set.  This ensures that the <tt>equals</tt> method works
     * properly across different implementations of the <tt>Set</tt>
     * interface.<p>
     *
     * This implementation first checks if the specified object is this
     * set; if so it returns <tt>true</tt>.  Then, it checks if the
     * specified object is a set whose size is identical to the size of
     * this set; if not, it returns false.  If so, it returns
     * <tt>containsAll((Collection) o)</tt>.
     *
     * @param o object to be compared for equality with this set
     * @return <tt>true</tt> if the specified object is equal to this set
     */
    public boolean equals(Object o) {
        if (o == this)
            return true;

        if (!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
　　　　 //保证个数相等
        if (c.size() != size())
            return false;
        try {
　　　　　　  //调用了AbstractCollection的方法。
            return containsAll(c);
        } catch (ClassCastException unused)   {
            return false;
        } catch (NullPointerException unused) {
            return false;
        }
    }

public boolean containsAll(Collection<?> c) {
    //只需要逐个判断集合是否包含其中的元素。
        for (Object e : c)
            if (!contains(e))
                return false;
        return true;
    }

4.实现了Set接口。

二：HashSet概述

1.HashSet是通过HashMap的键来存值，HashMap里面的所有值都为null;

2.学习这个之前先看HashMap;附上网址https://www.cnblogs.com/xhlwjy/p/11246618.html

三：HashSet的属性

　　private transient HashMap<E,Object> map;//用HashMap的Key来存值

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

四：构造方法

    /**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * default initial capacity (16) and load factor (0.75).
     */
　　//这个是初始化一个HashMap
    public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection.  The <tt>HashMap</tt> 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
     */
　　//初始化HashMap,把c集合中的所有元素添加到map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 <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
     */
　　//初始化HashMap的容量和加载因子
    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
     */
    //初始化HashMap的容量
    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
     */
　　//这个初始化使用的是LinkedHashMap
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

五：添加新元素

　　//底层仍然利用了HashMap键进行了元素的添加。
　　//在HashMap的put()方法中，该方法的返回值是对应HashMap中键值对中的值，而值总是PRESENT,
　　//该PRESENT一直都是private static final Object PRESENT = new Object();
　　//PRESENT只是初始化了，并不能改变，所以PRESENT的值一直为null。
　　//所以只要插入成功了，put（）方法返回的值总是null。
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

这段代码就不分析了，与HashMap的插入差不多，可以先去看HashMap

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

六：删除一个元素

    //该方法底层实现了仍然使用了map的remove（）方法。
    //map的remove（）方法的返回的是被删除键对应的值。（在HashSet的底层HashMap中的所有键值对的值都是PRESENT）
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

之后的代码就不粘贴出来了，自己可以去源码里面看

七：清空方法

 public void clear() {
        map.clear();
    }

八：克隆方法

　　底层仍然使用了Object的clone（）方法，得到的Object对象，并把它强制转化为HashSet<E>,然后把它的底层的HashMap也克隆一份（调用的HashMap的clone（）方法），并把它赋值给newSet，最后返回newSet即可。

  @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);
        }
    }

九：是否包含某个元素

底层使用了HashMap的containsKey（）

　　public boolean contains(Object o) {
        return map.containsKey(o);
    }

十：判读是不是空

调用HashMap的方法

　　public boolean isEmpty() {
        return map.isEmpty();
    }

十一：统计HashSet中包含元素的个数

　　public int size() {
        return map.size();
    }

十二：生成迭代器

    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

十三：序列化

    /**
     * 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
　　　　  //容量写入流
        s.writeInt(map.capacity());
　　　　 //加载因子写入流
        s.writeFloat(map.loadFactor());

        // Write out size
　　　　　//存放的数量写入流
        s.writeInt(map.size());

        // Write out all elements in the proper order.
　　　　 //把每个元素写入流
        for (E e : map.keySet())
            s.writeObject(e);
    }

十四：反序列化

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);

        //HashMap中构建哈希桶数组是在第一个元素被添加的时候才构建，所以在构建之前检查它,
        // 调用HashMap.tableSizeFor来计算实际分配的大小,
        // 检查Map.Entry []类，因为它是最接近的公共类型实际创建的内容。

        SharedSecrets.getJavaOISAccess()
                         .checkArray(s, Map.Entry[].class,HashMap.tableSizeFor(capacity));

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

        // 按写入流中的顺序再把元素依次读取出来放到map中。
        for (int i=0; i<size; i++) {
            @SuppressWarnings("unchecked")
                E e = (E) s.readObject();
            map.put(e, PRESENT);
        }
    }