总览
变量
- serialVersionUID：序列化
- DEFAULT_INITIAL_CAPACITY：默认容量16
- MAXIMUM_CAPACITY：最大容量2^30(integer最大值2^31-1,最小值-2^31)
- DEFAULT_LOAD_FACTOR：默认负载因子0.75
- TREEIFY_THRESHOLD：8(链表转红黑树阈值)
- UNTREEIFY_THRESHOLD：6(红黑树转链表阈值)
- MIN_TREEIFY_CAPACITY：64(最低开启红黑树容量阈值)
- table：存放数据
- entrySet：Holds cached entrySet()
- size：map当前大小
- modCount：修改次数
- threshold：The next size value at which to resize (capacity * load factor)
- loadFactor：负载因子
内部类
- 迭代器 HashIterator KeyIterator ValueIterator EntryIterator
- 并行迭代器 HashMapSpliterator KeySpliterator ValueSpliterator EntrySpliterator
方法
初始化
- HashMap(Map<? extends K, ? extends V> m)
- HashMap()
- HashMap(int initialCapacity)
- HashMap(int initialCapacity, float loadFactor)
//找到最小2^x > cap
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1; //保证前1位是1
n |= n >>> 2; //保证前2位是1
n |= n >>> 4; //保证前4位是1
n |= n >>> 8; //保证前8位是1
n |= n >>> 16; //保证前16位是1
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
插入
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
hash函数
static final int hash(Object key) {
int h;
//高16位和低16位异或（传说可以均匀hash，自己测试下来和不异或没啥区别）
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
其实是通过putVal方法
/**
* 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(这里可以注意下put 和 putIfAbsent的区别)
* @param evict if false, 创建模式.
* @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)
//table为null 或 table没有数据 初始化table
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//i = (n - 1) & hash 计算坑位 注意这里使用的是与运算，而不是求余（为啥容量要为2^x）
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))))
//key 存在
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);
//注意是从0开始数的，所以是8的时候转换为树
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))))
//key 存在
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
//onlyifavsent = true 不改变原始值
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
//是否需要扩容
resize();
afterNodeInsertion(evict);
return null;
}
重点函数resize()
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) {
//已经是最大容量了，无法再扩容
threshold = Integer.MAX_VALUE;
return oldTab;
}
//小于最大容量 && 大于16
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
//阈值*2
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
//初始化没有容量，将阈值设为容量 ？？
newCap = oldThr;
else { // zero initial threshold signifies using defaults
//初始化 设置容量 和 阈值
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
//给新阈值 设值
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
//新阈值赋值给 阈值变量
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) {
//清理引用，避免内存泄漏
oldTab[j] = null;
if (e.next == null)
//不是链表直接迁移
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
//是树节点
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
//当前位置，还是新位置 j + oldCap
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
//010000 16
//100000 32
//16容量都在3，32容量 19=3+16，3
//hash 10011
//hash 00011
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;
}
红黑树的拆分
//和链表一样将节点分到当前位置j 或 j + oldCap
//多了树变链表判断 阈值为6， 或建树
final void split(HashMap<K,V> map, Node<K,V>[] tab, int index, int bit) {
TreeNode<K,V> b = this;
// Relink into lo and hi lists, preserving order
TreeNode<K,V> loHead = null, loTail = null;
TreeNode<K,V> hiHead = null, hiTail = null;
int lc = 0, hc = 0;
for (TreeNode<K,V> e = b, next; e != null; e = next) {
next = (TreeNode<K,V>)e.next;
e.next = null;
if ((e.hash & bit) == 0) {
if ((e.prev = loTail) == null)
loHead = e;
else
loTail.next = e;
loTail = e;
++lc;
}
else {
if ((e.prev = hiTail) == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
++hc;
}
}
if (loHead != null) {
if (lc <= UNTREEIFY_THRESHOLD)
tab[index] = loHead.untreeify(map);
else {
tab[index] = loHead;
if (hiHead != null) // (else is already treeified)
loHead.treeify(tab);
}
}
if (hiHead != null) {
if (hc <= UNTREEIFY_THRESHOLD)
tab[index + bit] = hiHead.untreeify(map);
else {
tab[index + bit] = hiHead;
if (loHead != null)
hiHead.treeify(tab);
}
}
}
