hashmap的源码分析，基于jdk8，关于树的方法，不展开
1、hashmap的一些属性
//默认容量，位运算
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量，位运算
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//链表转树界限
static final int TREEIFY_THRESHOLD = 8;
//树转链表界限
static final int UNTREEIFY_THRESHOLD = 6;
//链表转树的容量最小限制
static final int MIN_TREEIFY_CAPACITY = 64;
//hashmap的数组，1.8的hashmap是数组+链表和红黑树的结构，每个数组元素保存链表的头节点或者红黑树的根节点
transient Node<K,V>[] table;
transient int size;
int threshold;
//负载因子
final float loadFactor;
2、构造方法
//无参构造方法，将负载因子设为默认的负载因子，初始化hashmap不在这里，在put方法
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//设置容量和负载因子，容量最大为最大容量，
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);//保证容量是2的幂
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
3、容量控制方法
//控制容量是2的幂
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;
}
4、put方法
//计算key的hash，并调用真实设值方法
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
//真实的设值方法
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
//hashmap的临时变量
Node<K,V>[] tab; Node<K,V> p;
//n,数组长度；i，key在数组的下标
int n, i;
//如果hashmap没有初始化则初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//如果对应数组元素没有值，则直接设值
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {//对应数组元素有值，需要区分，是链表还是红黑树，还是直接覆盖
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果是树，则走树的方法
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//走链表的方法
else {
//遍历链表，直到找到链表末端，将Node放入链表末端，判断是否是否应该树化
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;
}
}
//覆盖原来Node的value
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;//修改次数
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
5、树化方法
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
//当容量小于最小树化阈值(64)时先扩容
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
//树化，不展开
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
6、扩容方法
final Node<K,V>[] resize() {
//保存到临时变量
Node<K,V>[] oldTab = table;
//旧的容量
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//旧的扩容阈值
int oldThr = threshold;
//新的容量，新的扩容阈值
int newCap, newThr = 0;
//hashmap不为空
if (oldCap > 0) {
//如果容量已达上限，将扩容阈值设为比容量上限更大的值，避免再扩容
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//旧容量*2小于最大容量并且旧容量大于等于默认容量，新容量设为旧容量的两倍
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
//初始化
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);
}
//设置新的扩容阈值
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
//初始化一个新的空数组 容量原来的两倍，为扩容后的hashmap数据结构
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//hashmap原来有值，进行扩容
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)
//原下标只有一个元素，将其放入新数组的对应下标
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
//原下标是树，走树的拆分方法
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
//链表高低链，新数组的前半部分是低链，后半部分是高链，原数组的元素会被拆分到新数组，尾插法
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
//将链表才分为两个链表，下面分析
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;
}
7、get方法
//计算hash并调用getNode方法
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
//真正执行get的方法
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//hashmap的数组初始化了并且hash对应数组下标有值
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//数组下标对应元素就是要寻找的元素，直接返回
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//查找树或者遍历链表，获取被查找对象
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
8、hashmap的Node
//Node，静态内部类
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;//node的hash
final K key;//Node的key值
V value;//Node的value
Node<K,V> next;//下一个节点
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
//设置值，返回旧的值
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
9、链表的拆分原理
链表的拆分根据Node的hash进行拆分，hash按位与就数组容量，为0，放在低链，不为0，放在高链
比如旧的容量是16，新的容量是32
旧的容量的二进制是 00000000 000000000 00000000 00010000
Node1的hash是xxxxxxxx xxxxxxxx xxxxxxxx xxx01xxx
按位与后的结果 00000000 00000000 00000000 00000000 为0
Node2的hash是xxxxxxxx xxxxxxxx xxxxxxxx xxx11xxx
按位与后的结果 00000000 00000000 00000000 00010000 不为0
但是这两个在旧的hashmap的数组存放数据时计算hash是和(旧容量-1)按位与，结果是一样的
(旧容量-1) = 16-1 = 15 = 00000000 00000000 00000000 00001111
Node1和(旧容量-1)按位与结果是 00000000 00000000 00000000 00001000
Node2和(旧容量-1)按位与结果是 00000000 00000000 00000000 00001000
因此这两个都会放在相同的数组位置
10、图解
11、hashmap的put操作流程
1）、如果hashmap没有初始化，则进行初始化
2）、对key求hash然后计算下标
3）、如果数组下标没有元素，则直接放入数组下标
4）、如果数组下标有元素，则以链表形式放入下标
5）、如果链表长度达到阈值（8），则将链表转化为红黑树（容量需要达到64，否则进行扩容）
6）、如果达到扩容阈值，就扩容
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