HashMap源码解析
HashMap源码解析:
hash函数
JDK1.8hash函数:
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);//进行扰动
}JDK1.7hash函数
final int hash(int h) {
h ^= k.hashCode();
h ^= (h >>> 20) ^ (h >>> 12);//>>>是无符号右移,忽略符号位
return h ^ (h >>> 7) ^ (h >>> 4);
}
在.17中,进行了4次扰动,为什么要进行扰动?
因为hashmap,计算出hash值,插入的点为 (n-1)&hash,如果不进行扰动,如果n也较小,那么低位相同,高位不同的hash最后结果会全部相同.所有需要进行扰动,jdk1.7中扰动4次,jdk1.8中扰动1次,扰动次数少速度快一些.
在hashmap中,初始长度为n,n一定是2的幂次方,只有n为2的幂次方,(n-1)&hash = hash%n,在计算机中&运算比%运算快的多.
类属性
public class HashMap extends AbstractMap
implements Map, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;//序列号
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16 初始容量16
static final int MAXIMUM_CAPACITY = 1 << 30; //最大容量
static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认填充因子
static final int TREEIFY_THRESHOLD = 8; //链表转红黑树的阈值
static final int UNTREEIFY_THRESHOLD = 6; //红黑树转链表的阈值
static final int MIN_TREEIFY_CAPACITY = 64; //hash表容量>=64才运行转换为红黑树
transient Node[] table; //hash表数组
transient Set> entrySet; //存放具体元素的集
transient int size; //hashmap中的元素数目
transient int modCount; //扩容更改map结构的计数器
int threshold; //临界值,(容量*填充因子)>临界值就扩容
final float loadFactor; //加载因子
loadFactor加载因子
加载因子hashmap默认为0.75f.越接近1,hash表数组被用的越多,越加密集,越密,插入的时候越容易让链表长度增加.
加载因子不能太大,太大就很容易碰撞,太小数组利用率低,也占空间.
threshold临界值
threshold=capacity*loadFactor,如果hashmap元素数目>临界值,那么就需要扩容,也就是增大hash数组长度,然后重新分配元素.除非已经达到MAXIMUM_CAPACITY.
Node节点
static class Node implements Map.Entry {
final int hash;
final K key;
V value;
Node next;
Node(int hash, K key, V value, Node next) {//构造Node
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() {//重写了hashCode
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {//重写了equals
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;
}
} Node包含4个元素:
- hash值
- key值
- value值
- next,指向下一个节点
构造方法
有四种构造方法,可以自定义初始hash数组长度,和加载因子.也可以构造的时候,就包含另一个map.
设置的hash长度,会向上取到 2的幂次大小.
public HashMap(int initialCapacity, float loadFactor) {//自行设置容量和加载因子
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);
}
public HashMap(int initialCapacity) {//设置容量
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {//默认
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map m) {//包含另一个map
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}由于我们插入的元素可能非常多,可能进行多次扩容,而扩容又是很消耗时间的,所有如果我们确定会插入很多元素,那么可以构造的时候指定hashmap的hash数组长度,避免频繁扩容.
putMapEntries
final void putMapEntries(Map m, boolean evict) {
int s = m.size();
if (s > 0) {//table已经初始化
if (table == null) { // pre-size hash数组还没初始化,那么就初始化,下面计算threshold是否会大于阀值,如果大于就初始化阀值
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)//已经初始化了,那么扩容
resize();
for (Map.Entry e : m.entrySet()) {//将m的元素插入到新hashMap中
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}get方法
public V get(Object key) {
Node e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
} final Node getNode(int hash, Object key) {
Node[] tab; Node first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {//包含key,也就是找到的hash数组非空
if (first.hash == hash && // always check first node 头节点的key就是要取的key
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {//在链表里/树上
if (first instanceof TreeNode)//头节点是树节点,调用树查找
return ((TreeNode)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);//在链表里查找
}
}
return null;
} 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[] tab; Node p; int n, i;//tab是hash表数组,p用于记录链表节点,n是hash数组长度,i是一个索引值
if ((tab = table) == null || (n = tab.length) == 0)//hash表为空,就扩容
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)//通过(n-1)&hash找到目标hash坐标,发现空的就直接插入
tab[i] = newNode(hash, key, value, null);
else {//否则需要插入链表中/红黑树中
Node e; K k;//e是新增的节点,k是保存key的
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;//将第一个元素赋值给e,用e来记录
else if (p instanceof TreeNode)//是树节点,插入红黑树中
e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
else {//是链表节点
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {//到达末尾,要插入尾节点
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);//需要转换为红黑树
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))//找到key相同的节点,跳出
break;
p = e;
}
}
if (e != null) { // existing mapping for key 已存在key,覆盖value
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;//记录hashmap更改次数的计数器+1
if (++size > threshold)//需要hash数组扩容
resize();
afterNodeInsertion(evict);
return null;
} - 如果定位到的hash数组空的,那么直接插入元素
- 如果定位到的hash数组非空,就和第一个节点比较,如果相同就覆盖,如果是树节点那么久插入到树中,否则插入到链表(尾插)中,可能在链表中已经有需要插入的key,那么就覆盖.如果插入链表后链表长度>8,呢么就将链表转换为红黑树.
插入完了后如果size>阀值,就扩容
删除节点和插入是差不多的.
resize扩容
final Node[] resize() {
Node[] oldTab = table;//保存原来的hash数组
int oldCap = (oldTab == null) ? 0 : oldTab.length;//原长度
int oldThr = threshold; //原临界值
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {//达到最大,不扩容
threshold = Integer.MAX_VALUE; //临界值设置为int极限大
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold 扩容为原来2倍
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr; //新初始容量为原临界值
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//oldThr==0,使用默认值
}
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[] newTab = (Node[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {//遍历数组,将老数组的桶迁移到新的hash数组中
Node 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)e).split(this, newTab, j, oldCap);//红黑树特殊处理
else { // preserve order
Node loHead = null, loTail = null;
Node hiHead = null, hiTail = null;
Node 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;
}
扩容操作,需要重新分配hash表上所有的元素,非常消耗时间,我们在用hashmap的时候应该避免频繁扩容