类:
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
- 继承自AbstractMap 实现了Map,Cloneable,Serializable接口
- 可以被序列化
- 可以被Clone
//默认初始容量为16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30
//默认扩容因子
static final float DEFAULT_LOAD_FACTOR = 0.75f
//红黑树转链表的阀值
static final int TREEIFY_THRESHOLD = 8;
//链表转红黑树阀值
static final int UNTREEIFY_THRESHOLD = 6;
//存储方式由链表转成红黑树的容量的最小阈值
static final int MIN_TREEIFY_CAPACITY = 64;
初始化
Map<String, Object> map = new HashMap<>();
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
- new HashMap<>()的时候，会进入HashMap(int initialCapacity, float loadFactor)方法
- initialCapacity为0xB
- loadFactor默认为0.75
- 第一步判断initialCapacity的值是不是小于0或大于1 << 30,第二步判断loadFactor是不是大于0和是否为浮点数,第三步设置实例的loadFactor为0.75,第四步设置容量为16
tableSizeFor
//返回大于或等于cap且为2的幂的数值
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
- cap-1:如果cap不减去1,cap=2的幂时,结果为cap的2倍,不符合预期
hash方法
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
- hashCode的高位16bit与低16bit参与异或运算
- null放在第0位
Node对象
put方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
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)
//扩容,第一次扩容长度16
n = (tab = resize()).length;
//(n - 1) & hash的长度必定在0-n之间
//(n - 1) & hash等于hash % n,与运算比取模快
//n - 1的2进制为01111...,N为2的次方
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//hash和equals或=相同时,替换
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)//节点为树节点的时候,在红黑树上添加
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//链表
for (int binCount = 0; ; ++binCount) {
//添加在链表末尾
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//当binCount>=7的时候转换为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//已经存在
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//下一个对象
p = e;
}
}
//已经存在key
if (e != null) {
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
- 取模 x % 2^n == x & (2^n - 1)
扩容
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;
}