1.摘要
java 1.8优化了HashMap代码，主要有hash值计算方法，添加红黑树，resize顺序保证等，下面将一一进行阐述。
2.HashMap简介
HashMap和HashTable的区别在于hashtable是线程安全的，而且不允许有null值，而HashMap允许一个键为null，多个值为null。但是HashTable为遗留类，如果需要线程安全，可以使用性能更高的ConcurrentHashMap（使用了分段锁）。
HashMap还有LinkedHashMap和TreeHashMap两个子类，LinkedHashMap保存数据插入顺序，TreeHashMap会对插入的key进行排序。
HashMap是一个Node数组（哈希桶数组），每个数组的元素是一个链表或者红黑树，默认如果链表的长度超过8会自动转换为红黑树。
3.计算hash值
本质上为三个计算： 求hash值，高位运算，取模运算。
代码为：
//求hash值、高位运算
(h=key.hashCode()) ^ (h >>> 16)
//取模
i = h & (length-1) 
4. put方法
思路：哈希桶数组是否为空；通过key.hashCode得到数组下标，判断该哈希桶是否为空；判断该key值在哈希桶中是否存在。
1）判断哈希桶数组是否为空，若是，新建Node，直接跳到8；
2）获取下标，判断该哈希桶是否为空，若是，新建Node，直接跳到8；
3）新建缓存变量Node<K, V> e ；
4）判断该桶的key是否相等，将该Node赋值给e；
5）判断该桶是否为红黑树，如果是，插入e；
6）for循环遍历链表
6.1）没有相同的key，末端添加new Node；
6.2）有相同的key，break；
7）如果e非空，保存e中的旧值，赋值新值，并返回旧值；
8）size增加，并判断是否需要resize。
源码如下：
/**
* 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)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
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
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
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;
}5. resize方法
思路：计算newCap，newThr；新建容量为newCap的Node数组；将之前数组的值拷贝到新数组。
要强调的是
1. java 1.7拷贝数组的方法是挨个计算key的hash值对应的下标i，并将相应的Node放到tab[i]的头，这样会导致存放顺序相反。java 1.8中，因为扩容后newCap为oldCap的两倍，在hash值取模后，可能不一样的地方只在newCap的最高位，所以旧数组下标i的元素只可能被分配到i或者i+oldCap的下标中，故，只要新建两个链表，将对应不同下标的值赋给不同的链表，然后将两个链表在赋值给table[i]和table[i+oldCap]，就保证了Node的顺序一致性；
2. 在resize往新的链表中插入Node的时候，如果链表的长度大于8，会将链表转为红黑树。
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}参考
