HashMap源码解析（2）

    万里长征，翻译完HashMap注释居然花了一周时间。看来解析源码也要好久，先把源码copy过来咯

   HashMap继承AbstractMap,实现了接口：Map<K, V>, Cloneable, Serializable。

public class HashMap<K,V>    extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable{    /**     * The default initial capacity - MUST be a power of two. 默认初始容量，值必须是2的幂。     */    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.  最大容量，初始值时2^30     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * The load factor used when none specified in constructor. 默认加载因子，默认值为0.75     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     * The table, resized as necessary. Length MUST Always be a power of two.Entry数组，大小必须为2的幂。     */    transient Entry<K,V>[] table;    /**     * The number of key-value mappings contained in this map.已存元素的个数，     */    transient int size;    /**     * The next size value at which to resize (capacity * load factor). 阈值，当size>=threshold就会扩容     * @serial     */    int threshold;    /**     * The load factor for the hash table.加载因子     *     * @serial     */    final float loadFactor;    /**     * The number of times this HashMap has been structurally modified     * Structural modifications are those that change the number of mappings in     * the HashMap or otherwise modify its internal structure (e.g.,     * rehash).  This field is used to make iterators on Collection-views of     * the HashMap fail-fast.  (See ConcurrentModificationException).hashmap结构性修改的次数，     结构性修改是指改变映射对的数量或内部结构的修改。这个变量是为了辅助hashmap快速修改机制。
     */
    transient int modCount;
     */

此时我们介绍下构造函数,转自（http://blog.csdn.net/ghsau/article/details/16890151）HashMap一共重载了4个构造方法，分别为：构造方法摘要HashMap()           构造一个具有默认初始容量 (16) 和默认加载因子 (0.75) 的空 HashMap。HashMap(int initialCapacity)           构造一个带指定初始容量和默认加载因子 (0.75) 的空 HashMap。HashMap(int initialCapacity, float loadFactor)           构造一个带指定初始容量和加载因子的空 HashMap。HashMap(Map<? extendsK,? extendsV> m)           构造一个映射关系与指定 Map 相同的 HashMap。

我们看下第三个构造方法，如下源码所示其他构造方法都是引用的这个构造方法。

    /**     * 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)//参数合规性检查            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;//容量设置为2的幂        while (capacity < initialCapacity)            capacity <<= 1;        //设置加载因子        this.loadFactor = loadFactor;        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        table = new Entry[capacity];//初始化hash表        useAltHashing = sun.misc.VM.isBooted() &&                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);        init();    }    /**     * Constructs an empty <tt>HashMap</tt> 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);    }    /**     * Constructs an empty <tt>HashMap</tt> with the default initial capacity     * (16) and the default load factor (0.75).     */    public HashMap() {        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);    }    /**     * Constructs a new <tt>HashMap</tt> with the same mappings as the     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with     * default load factor (0.75) and an initial capacity sufficient to     * hold the mappings in the specified <tt>Map</tt>.     *     * @param   m the map whose mappings are to be placed in this map     * @throws  NullPointerException if the specified map is null     */    public HashMap(Map<? extends K, ? extends V> m) {        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);        putAllForCreate(m);    }    // internal utilities    /**     * Initialization hook for subclasses. This method is called     * in all constructors and pseudo-constructors (clone, readObject)     * after HashMap has been initialized but before any entries have     * been inserted.  (In the absence of this method, readObject would     * require explicit knowledge of subclasses.)     */    void init() {    }    /**     * Retrieve object hash code and applies a supplemental hash function to the     * result hash, 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.     */    final int hash(Object k) {        int h = 0;        if (useAltHashing) {            if (k instanceof String) {                return sun.misc.Hashing.stringHash32((String) k);            }            h = hashSeed;        }        h ^= k.hashCode();        // 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);    }

HashMap put方法，允许加入key value 为null。 

public V put(K key, V value) {      // 处理key为null，HashMap允许key和value为null      if (key == null)          return putForNullKey(value);      // 得到key的哈希码      int hash = hash(key);      // 通过哈希码计算出bucketIndex      int i = indexFor(hash, table.length);      // 取出bucketIndex位置上的元素，并循环单链表，判断key是否已存在      for (Entry<K,V> e = table[i]; e != null; e = e.next) {          Object k;          // 哈希码相同并且对象相同时          if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {              // 新值替换旧值，并返回旧值              V oldValue = e.value;              e.value = value;              e.recordAccess(this);              return oldValue;          }      }        // key不存在时，加入新元素      modCount++;      addEntry(hash, key, value, i);      return null;  }  

    void addEntry(int hash, K key, V value, int bucketIndex) {        if ((size >= threshold) && (null != table[bucketIndex])) {//判断是否需要扩容            resize(2 * table.length);//resize函数所做的就是将旧table rehash到新的newtable            hash = (null != key) ? hash(key) : 0;            bucketIndex = indexFor(hash, table.length);        }        createEntry(hash, key, value, bucketIndex);    }    /**     * Like addEntry except that this version is used when creating entries     * as part of Map construction or "pseudo-construction" (cloning,     * deserialization).  This version needn't worry about resizing the table.     *     * Subclass overrides this to alter the behavior of HashMap(Map),     * clone, and readObject.     */    void createEntry(int hash, K key, V value, int bucketIndex) {        Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<>(hash, key, value, e);        size++;    }

最后标记下内部类，Entry

static class Entry<K,V> implements Map.Entry<K,V> {    final K key;    V value;    Entry<K,V> next;    int hash;    /**     * Creates new entry.     */    Entry(int h, K k, V v, Entry<K,V> n) {        value = v;        next = n;        key = k;        hash = h;    }    public final K getKey() {        return key;    }    public final V getValue() {        return value;    }    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());    }    public final String toString() {        return getKey() + "=" + getValue();    }    /**     * This method is invoked whenever the value in an entry is     * overwritten by an invocation of put(k,v) for a key k that's already     * in the HashMap.     */    void recordAccess(HashMap<K,V> m) {    }    /**     * This method is invoked whenever the entry is     * removed from the table.     */    void recordRemoval(HashMap<K,V> m) {    }}