HashMap源码解析(java1.8.0)

//HashMap<String,String> hashMap = new HashMap<String, String>(11);/**
* Returns a power of two size for the given target capacity.
**/
static final int tableSizeFor(int cap) {
int n = cap - 1; //10 防止在cap已经是2的n次幂的情况下
// >>> 表示不关心符号位，对数据的二进制形式进行右移 |表示或运算
n |= n >>> 1; //15
n |= n >>> 2; //15
n |= n >>> 4; //15
n |= n >>> 8; //15
n |= n >>> 16; //15
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1; //16
}

//hashMap.put("2020", "good luck"); /**
* 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hashtable没有初始化，则初始化该table数组
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//通过位运算找到数组中的下标位置，如果数组中对应下标为空，则可以直接存放下去
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//数组元素对应的位置已经有元素，产生碰撞
Node<K,V> e; K k;
//如果插入的元素key是已经存在的，则将新的value替换掉原来的旧值
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果此时table数组对应的位置是红黑树结构，则将该节点插入红黑树中
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//如果此时table数组对应的位置是链表结构
for (int binCount = 0; ; ++binCount) {
//遍历到数组尾端，没有与插入键值对相同的key，则将新的键值对插入链表尾部
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//链表过长，将链表转化为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//发现链表中的某个节点有与插入键值对相同的key，则跳出循环，在循环外部重新赋值
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//该key在hashmap已存在，更新与在链表跳出循环节点对应的值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//超过阈值则更新
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}

/**
* 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;
//table数组不为空，且对应的下标位置也不为空。
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//如果第一个位置是对应的key，则返回
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//遍历其他元素
if ((e = first.next) != null) {
//红黑树
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
//链表
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}

/**
* 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;
//table不为空，且容量大于0
if (oldCap > 0) {
//如果旧的容量到达阈值，则不再扩容，阈值直接设置为最大值
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//如果旧的容量没有到达阈值，直接操作
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//阈值大于0，直接使用旧的阈值
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
//如果阈值为零，则使用默认的初始化值
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;
//更新数组桶
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//将之前旧数组桶的数据重新移到新数组桶中
if (oldTab != null) {
//依次遍历旧table中每个数组桶的元素
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
//如果数组桶中含有元素
if ((e = oldTab[j]) != null) {
//将下标数据清空
oldTab[j] = null;
//如果元组的某一桶中只有一个元素，则直接将该元素移到新的位置去
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
//如果是红黑树结构
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//链表 -- 对旧桶里链表中的每一个元素重新计算哈希值得到下标
else { // preserve order
//将原先桶中的链表分为两个链表
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
/*
* e.hash & oldCap 对hash取模运算，
* 虽然数组大小扩大了一倍，
* 但是同一个key在新旧table中对应的index却存在一定联系：
* 要么一致，要么相差一个 oldCap。
*/
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;
}