二、HashMap、HashSet和Hashtable

所有集合基于jdk1.8，对源码稍做调整。

HashMap

主要变量

    // 默认容量
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
    // 最大容量
    static final int MAXIMUM_CAPACITY = 1 << 30;
    // 默认加载因子
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    // 树化临界值
    static final int TREEIFY_THRESHOLD = 8;
    // 非树化临界值
    static final int UNTREEIFY_THRESHOLD = 6;
    //最小树化容量
    static final int MIN_TREEIFY_CAPACITY = 64;
    // HashMap数据表
    transient Node[] table;
    // Entry集
    transient Set> entrySet;
    // 临界值
    int threshold;
    // 加载因子
    final float loadFactor;
    // 元素个数
    transient int size;

构造方法

    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
        this.loadFactor = loadFactor;
        // 此处得到的是容量，在put时会再做处理
        this.threshold = tableSizeFor(initialCapacity);
    }

    /**
     * 将数组大小处理为2的N次方
     * @return
     */
    private int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

    /**
     * Constructs an empty HashMap with the specified initial capacity
     * and the default load factor (0.75).
     *
     * @param initialCapacity
     *            the initial capacity.
     * @throws IllegalArgumentException
     *             if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    

put

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

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node[] tab; Node p; int n, i;
        //当HashMap还没有元素时扩容
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //要插入位置无值直接创建新节点
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node e; K k;
            //key值和获取的桶第一个元素相同时，直接赋值
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //当p是树化后的节后
            else if (p instanceof TreeNode)
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else {
                //遍历桶中元素
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //当达到树化临界值时，树化
                        if (binCount >= TREEIFY_THRESHOLD - 1)
                            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;
    }

get

    public V get(Object key) {
        Node e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    final Node getNode(int hash, Object key) {
        Node[] tab; Node first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            //先判断桶中第一个元素
            if (first.hash == hash &&
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            //遍历桶后面的元素
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)
                    return ((TreeNode)first).getTreeNode(hash, key);
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

HashSet

HashSet内部维护了一个HashMap，元素不重复也是依靠HashMap的key值不重复实现的。

主要变量

    //实际元素
    private transient HashMap map;
    //充当每个key的对应value
    private static final Object PRESENT = new Object();

Hashtable

Hashtable和HashMap相似，只是方法都用synchronized修饰来保证线程安全，而且Hashtable并没有树化的概念。