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：

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。

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。

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

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;
}