本文关于HashMap的底层实现若无特殊说明都是基于JDK1.8。
HashMap的底层数据结构是数组+链表（红黑树），它是基于hash算法实现的，通过put(key, value) 和 get(key) 方法存储和获取对象。
我们一般是这么使用HashMap的
Map<String, Object> map = new HashMap<>();
map.put("a", "first");
当调用HashMap的无参构造方法时，HashMap的底层数组是没有初始化的。
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
无参构造方法中只是对加载因子loadFactor初始化为默认值0.75，并没有对Node<K,V>[] table数组初始化。 （可以思考下HashMap中的加载因子为什么是0.75？）
put(key, value)方法存储元素
当我们第一次调用put(key, value) 方法时，key-value在HashMap内部的数组和链表中是如何存储的呢？ put方法内部首先根据key计算hash值，hash函数是如何计算的呢？是直接返回key的hashCode吗，当然不是啦！ HashMap中的hash算法：
/**
* 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);
}
key的hash值已经知道了，那么计算 hash & (table.length - 1) 结果就是key的hash值在数组table中的位置bucket。 等等，数组table还没有初始化呢，那么在计算bucket之前应该先将table[]数组进行初始化， 既然要初始化一个数组，那么数组的长度应该是多少呢？
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
默认的初始容量16，数组创建方式如下
newCap = DEFAULT_INITIAL_CAPACITY;
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
Node[] 数组就这么被创建好了，是不是很简单（具体看源码中的resize()方法）。
数组创建好了，key在数组中对应的位置bucket也找到了，那么现在就该将key-value放入数组中了，该如何放入呢？ 数组的类型为Node，那么需要将key-value封装成数组的元素类型Node。
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
// 省略其他部分代码
}
Node类中包含4个属性，K key、V value、int hash、Node<K,V> next。 key、value就是我们要放入HashMap中的数据，hash就是我们上面计算出来的key的hash值，这个Node类型的next是干嘛的呢？ 还记得HashMap底层的数据结构吗？数组 + 链表，next这个地方就是链表的实现。next指向与key的hash值相同的新Node。
根据key在数组中对应的位置bucket，获取bucket位置上的元素，如果该位置上没有元素，则直接将key-value封装成的Node放入数组中
tab = table
tab[i] = newNode(hash, key, value, null);
如果该位置上有元素，则比较key的值是否相等，有两种情况：
1、如果key的值相等，则要更新key对应的vaule，将新的value覆盖旧的value；
2、如果key的值不相等，则说明发生了hash冲突。也就是说不同的key计算出的hash值相等，说明它们在table[]中在同一个位置， 这个时候就需要使用链表了，遍历链表，比较key值是否相等，直到链表的最后一个节点，若未找到则将新的元素插入到链表的尾部（尾插法）。
JDK1.8中put方法源码
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
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;
if ((tab = table) == null || (n = tab.length) == 0)
// 初始化table
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
// 目标位置上没有元素，直接将key-value封装成的Node放入数组
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 目标位置上有元素，则比较key值是否相等
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
// key值相等
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// key值不相等
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// 遍历链表，找到链表的最后一个节点，将新的元素插入到链表的尾部（尾插法）
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
// 判断是否需要将链表转成红黑树
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
// 链表继续遍历
p = e;
}
}
if (e != null) { // existing mapping for key
// 目标位置上有key值相等的元素，将新的value覆盖旧的value
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
// 数组扩容
resize();
afterNodeInsertion(evict);
return null;
}
get(key) 方法获取对象
当获取对象时，首先跟put方法存储元素一样，也是先调用hash函数计算key的hash值， 然后计算hash & (table.length - 1) 的结果就是key的hash值在数组table中的位置bucket， 根据bucket获取该位置上的元素，判断key值是否相等，如果相等直接返回对象的value值。 如果不相等，则遍历链表，比较key值，直到找到key值相等的节点，如果没有找到则返回null。
JDK1.8中get方法源码
/**
* 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;
}
/**
* Implements Map.get and related methods.
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
// 找到hash值在数组位置上的元素
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
// 比较key值是否相等
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
// 遍历链表
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
本文主要分析了HashMap存储对象和获取对象的过程，重点介绍了数组和链表在HashMap底层的使用。
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