HashMap类

简要说明

HashMap内部存储采用的是一个一维数组，数组长度为2的倍数。数组默认大小为16，如果指定初始化大小为N，则数组长度为小于等于N的最大偶数（2的倍数）。通过key将value映射到数据中的某个位置，发生位置冲突时，使用数组元素的next指针（引用）将发生冲突的元素连接起来，构成一个链表结构，最新存入的元素放在链表最前面。

HashMap存储结构

HashMap存储元素的数组定义如下：

/** * The table, resized as necessary. Length MUST Always be a power of two. */ transient Entry[] table;

Entry包含四个字段：key，value，hash，next。

• key：存入value时使用的关键字对象；
• value：要存储的对象；
• hash：HashMap内存计算出来的一个hash值；
• next：指向下一个元素的指针（引用），发生位置冲突时候把映射位置相同的元素链接起来。

Entry类定义如下（省略掉一些普通方法）：

    static class Entry<K,V> implements Map.Entry<K,V> {        final K key;        V value;        Entry<K,V> next;        final int hash;        public final V setValue(V newValue) {        V oldValue = value;            value = newValue;            return oldValue;        }        public final boolean equals(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry e = (Map.Entry)o;            Object k1 = getKey();            Object k2 = e.getKey();            if (k1 == k2 || (k1 != null && k1.equals(k2))) {                Object v1 = getValue();                Object v2 = e.getValue();                if (v1 == v2 || (v1 != null && v1.equals(v2)))                    return true;            }            return false;        }        public final int hashCode() {            return (key==null   ? 0 : key.hashCode()) ^                   (value==null ? 0 : value.hashCode());        }    }

HashMap初始化

数组默认大小为16，如果指定初始化大小为N，则数组长度为小于等于N的最大偶数（2的倍数）。数组大小会根据元素数量进行扩容，当元素个数大于某个阈值（threshold）时，就会触发扩容。当容量达到最大(MAXIMUM_CAPACITY)时，HashMap就不会再进行扩容。threshold的值与加载因子(loadFactor)有关：threshold = (int)(capacity * loadFactor)，loadFactor默认值为0.75，capacity为数组长度。
默认值代码：

    /**     * The default initial capacity - MUST be a power of two.     */    static final int DEFAULT_INITIAL_CAPACITY = 16;    /**     * The maximum capacity, used if a higher value is implicitly specified     * by either of the constructors with arguments.     * MUST be a power of two <= 1<<30.     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * The load factor used when none specified in constructor.     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;

初始化代码：

    /**     * Constructs an empty <tt>HashMap</tt> with the specified initial     * capacity and load factor.     *     * @param  initialCapacity the initial capacity     * @param  loadFactor      the load factor     * @throws IllegalArgumentException if the initial capacity is negative     *         or the load factor is nonpositive     */    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);        // Find a power of 2 >= initialCapacity        int capacity = 1;        while (capacity < initialCapacity)            capacity <<= 1;        this.loadFactor = loadFactor;        threshold = (int)(capacity * loadFactor);        table = new Entry[capacity];        init();    }

HashMap存储位置映射算法

HashMap关键的算法，以get方法为例：
1. 调用key.hashCode()方法获取key的hashCode，然后对hashCode做一些计算，最后的到一个hash值。
2. 使用hash值，计算出元素应该储存的位置index。
3. 取出index位置的元素链表，查找目标元素e。查找过程中比较方法：hash == e.hash && (e.key == key || key.equals(e.k))，找到后，返回e.value。

get方法代码如下：

    /**     * Returns the value to which the specified key is mapped,     * or {@code null} if this map contains no mapping for the key.     *     * <p>More formally, if this map contains a mapping from a key     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :     * key.equals(k))}, then this method returns {@code v}; otherwise     * it returns {@code null}.  (There can be at most one such mapping.)     *     * <p>A return value of {@code null} does not <i>necessarily</i>     * indicate that the map contains no mapping for the key; it's also     * possible that the map explicitly maps the key to {@code null}.     * The {@link #containsKey containsKey} operation may be used to     * distinguish these two cases.     *     * @see #put(Object, Object)     */    public V get(Object key) {        if (key == null)            return getForNullKey();        int hash = hash(key.hashCode());        for (Entry<K,V> e = table[indexFor(hash, table.length)];             e != null;             e = e.next) {            Object k;            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))                return e.value;        }        return null;    }

计算key的hash值的hash方法如下：

    /**     * Applies a supplemental hash function to a given hashCode, which     * defends against poor quality hash functions.  This is critical     * because HashMap uses power-of-two length hash tables, that     * otherwise encounter collisions for hashCodes that do not differ     * in lower bits. Note: Null keys always map to hash 0, thus index 0.     */    static int hash(int h) {        // This function ensures that hashCodes that differ only by        // constant multiples at each bit position have a bounded        // number of collisions (approximately 8 at default load factor).        h ^= (h >>> 20) ^ (h >>> 12);        return h ^ (h >>> 7) ^ (h >>> 4);    }

根据hash值计算存储位置的indexFor方法如下：

    /**     * Returns index for hash code h.     */    static int indexFor(int h, int length) {        return h & (length-1);    }

HashMap扩容处理

添加元素（键值对）时，会检测是否需要扩容：当HashMap中元素个数大于等于threshold时，就会触发扩容。
threshold = (int)(capacity * loadFactor)
扩容原理：重新分配一个数组table，然后把原数组中的元素逐个重新映射到新数组中，而非直接拷贝进去，因为数组长度发生了变化，映射位置也会发生变化。从上面的indexFor方法可以看出这一点：映射位置 ＝ h & (length-1)，h是根据key的hashCode计算出来的一个哈希值，length是数组长度。
addEntry方法代码如下：

    /**     * Adds a new entry with the specified key, value and hash code to     * the specified bucket.  It is the responsibility of this     * method to resize the table if appropriate.     *     * Subclass overrides this to alter the behavior of put method.     */    void addEntry(int hash, K key, V value, int bucketIndex) {    Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);        if (size++ >= threshold)            resize(2 * table.length);    }

负责扩容的resize方法代码如下：

    /**     * Rehashes the contents of this map into a new array with a     * larger capacity.  This method is called automatically when the     * number of keys in this map reaches its threshold.     *     * If current capacity is MAXIMUM_CAPACITY, this method does not     * resize the map, but sets threshold to Integer.MAX_VALUE.     * This has the effect of preventing future calls.     *     * @param newCapacity the new capacity, MUST be a power of two;     *        must be greater than current capacity unless current     *        capacity is MAXIMUM_CAPACITY (in which case value     *        is irrelevant).     */    void resize(int newCapacity) {        Entry[] oldTable = table;        int oldCapacity = oldTable.length;        if (oldCapacity == MAXIMUM_CAPACITY) {            threshold = Integer.MAX_VALUE;            return;        }        Entry[] newTable = new Entry[newCapacity];        transfer(newTable);        table = newTable;        threshold = (int)(newCapacity * loadFactor);    }    /**     * Transfers all entries from current table to newTable.     */    void transfer(Entry[] newTable) {        Entry[] src = table;        int newCapacity = newTable.length;        for (int j = 0; j < src.length; j++) {            Entry<K,V> e = src[j];            if (e != null) {                src[j] = null;                do {                    Entry<K,V> next = e.next;                    int i = indexFor(e.hash, newCapacity);                    e.next = newTable[i];                    newTable[i] = e;                    e = next;                } while (e != null);            }        }    }