Hashmap in java

1. HashMap要点：

1.1 基本数据结构： 

采用 数组+链表/平衡二叉查找树 的组合形式，所有键值对都以Entry<K,V>形式存储（每put进一个键值对，就会实例化一个Entry<K, V>）。

• 数组：Entry<K,V>数组，以hash( key.hashCode() ) 为数组索引。即计算键值的hash值，以此为索引存储键值对Entry<K, V>。数组长度总是2的n次方（这与hash有关，后边会讲）
• 链表：如果hash()方法算出的hash值相同，在该索引处，建立链表，存储hash值相同的键值对。
• 平衡二叉查找树：如果该当前索引处是一个链表，且插入当前键值对后，该链表处元素个数超过阈值（8），则把当前索引处的链表转换为一个平衡二叉查找树；如果当前索引处是一个树，则直接直接插入当前键值对

JDK1.8版本中table的定义（Node<K, V>实现了接口Entry<K,V>）：

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;　　//static class Node<K,V> implements Map.Entry<K,V>

table初值为16，扩充时，每次扩原来的两倍。下边是对table初值的定义（java中基本数据类型的字长是固定的，平台独立。如int总是4bytes的，因此table索引最大是整型的最大值。）：

  /**
      * The default initial capacity - MUST be a power of two.
      */
     static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 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 bin count threshold for using a tree rather than list for a
      * bin.  Bins are converted to trees when adding an element to a
      * bin with at least this many nodes. The value must be greater
      * than 2 and should be at least 8 to mesh with assumptions in
      * tree removal about conversion back to plain bins upon
      * shrinkage.
      */
     static final int TREEIFY_THRESHOLD = 8;　　//每个索引处链表元素个数的阈值，超过则用二叉查找树存

1.2 哈希算法

JDK1.8版本的源码如下：

    /**
     * Computes key.hashCode() and spreads (XORs) higher bits of hash
     * to lower.  Because the table uses power-of-two masking, sets of
     * hashes that vary only in bits above the current mask will
     * always collide. (Among known examples are sets of Float keys
     * holding consecutive whole numbers in small tables.)  So we
     * apply a transform that spreads the impact of higher bits
     * downward. There is a tradeoff between speed, utility, and
     * quality of bit-spreading. Because many common sets of hashes
     * are already reasonably distributed (so don't benefit from
     * spreading), and because we use trees to handle large sets of
     * collisions in bins, we just XOR some shifted bits in the
     * cheapest possible way to reduce systematic lossage, as well as
     * to incorporate impact of the highest bits that would otherwise
     * never be used in index calculations because of table bounds.
     */
    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

即将key.hashCode()的高16位与低16位按位异或的结果作为键值的hash值。

有几点：

1. key.hashCode()：hashCode()是Object类的本地方法。键值对象类可以重写自己的hashCode()方法，例如Integer类的hashCode()方法仅返回其int值。
2. hash()：为避免key.hashCode()获得的hash值重复太多，如Integer类的就很简单，所以就又加了一层hash。但是也可能原来的hashCode()方法就已经很好，所以衡量效率和效果之后，采用尽量简单的方法来获取新的hash值。
3. h>>>16：这里hash值的结果是将高16位与低16位异或，并存储在低16位中。即hash值是16位bit值，这与1.1中介绍的table长度是相关的，因为table索引就是hash值。

1.3 主要基本操作

1.3.1 put()

• 索引处没有键值对，直接插入为链表
• 否则，如果树中或链表中有相同key值，则更新原来的value值，并返回旧value值
• 否则，如果是链表则插入链表，插入后超过长度阈值，则转为二叉查找树；如果是树，则插入树

JDK1.8版本源码如下：

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

     /**
      * Implements Map.put and related methods
      *
      * @param hash hash for key
      * @param key the key
      * @param value the value to put
      * @param onlyIfAbsent if true, don't change existing value
      * @param evict if false, the table is in creation mode.
      * @return previous value, or null if none
      */
     final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                    boolean evict) {
         Node<K,V>[] tab; Node<K,V> p; int n, i;
　　　// 如果目前table为空或0，就对table初始化resize()，
         if ((tab = table) == null || (n = tab.length) == 0)
             n = (tab = resize()).length;
     //如果当前key值的hash索引处没有值，直接在该索引处存储键值对
         if ((p = tab[i = (n - 1) & hash]) == null)
             tab[i] = newNode(hash, key, value, null);
     //如果当前key值的hash索引处有值
         else {
             Node<K,V> e; K k;
     //如果当前索引处的key值和待插入的key值相同（即待插入键值对与bin内第一个键值对的键值相同）----即使是不同对象，值相同就可以（因为采用了equals()方法判断）
             if (p.hash == hash &&
                 ((k = p.key) == key || (key != null && key.equals(k))))
                 e = p;
     //否则，如果当前bin内是以树节点的形式存储，把该节点插入到这个树中
             else if (p instanceof TreeNode)
                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);　　//如果树中已经有这个节点，返回这个节点之前的value；如果没有返回null
　　　//否则，即当前bin内以链表形式存储，遍历链表，插入该节点
             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　　//如果插入节点之后，元素个数超过阈值，就把链表转换为二叉查找树
                             treeifyBin(tab, hash);
                         break;
                     }
                     if (e.hash == hash &&
                         ((k = e.key) == key || (key != null && key.equals(k))))
                         break;
                     p = e;
                 }
             }
　　　　//如果该索引处有键值相同的键值对，那么更新原有键值对的value为新的value，返回旧的value
             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;
     }

1.3.2 pop

1. 如果hashmap中有当前的key，返回其对应的value，否则返回null

JDK1.8版本源码如下：

     /**
      * 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) {
         Node<K,V> e;
         return (e = getNode(hash(key), key)) == null ? null : e.value;
     }