HashMap
继承于
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
成员变量
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = 64;
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;
Q:为什么Map桶中个数超过8才转换为红黑树
红黑树的平均查找长度是log(n)，如果长度为8，平均查找长度为log(8)=3，链表的平均查找长度为n/2，当长度为8时，平均查找长度为8/2=4，这才有转换成树的必要；链表长度如果是小于等于6，6/2=3，而log(6)=2.6，虽然速度也很快的，但是转化为树结构和生成树的时间并不会太短。
构造函数
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);
}
//大于输入参数且最近的2的整数次幂的数
static final int tableSizeFor(int cap) {
int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
//Integer
public static int numberOfLeadingZeros(int i) {
// HD, Count leading 0's
if (i <= 0)
return i == 0 ? 32 : 0;
int n = 31;
if (i >= 1 << 16) { n -= 16; i >>>= 16; }
if (i >= 1 << 8) { n -= 8; i >>>= 8; }
if (i >= 1 << 4) { n -= 4; i >>>= 4; }
if (i >= 1 << 2) { n -= 2; i >>>= 2; }
return n - (i >>> 1);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
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)
resize();
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
数据操作
HashMap 通过 key 的 hashCode 经 过扰动函数处理过后得到 hash 值，然后通过 (n - 1) & hash 判断当前元素存放的位置（这里的 n 指的是数组的 长度），如果当前位置存在元素的话，就判断该元素与要存入的元素的 hash 值以及 key 是否相同，如果相同的 话，直接覆盖，不相同就通过拉链法解决冲突。 所谓扰动函数指的就是 HashMap 的 hash 方法。使用 hash 方法也就是扰动函数是为了防止一些实现比较差的 hashCode() 方法 换句话说使用扰动函数之后可以减少碰撞。
添加元素
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
//hash值是 【高16位 + （高16位^低16位）】 null的hash值位0
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
**
* 计算key的值 再计算下标
* 如果key未碰撞，直接放入桶里，有碰撞则以链表的形式往后加，如果链表的长度大于8 ，把链表转换为红黑树，如果红黑树长度低于6，则转回链表
* 如果节点已存在，则替换
* 桶满了 大于 threhold 则需扩容
* @param hash 由key计算出来的 hash值
* @param key 要存储的key
* @param value 要存储的value
* @param onlyIfAbsent 如果当前位置已存在一个值，是否替换，false是替换，true是不替换
* @param evict 表是否在创建模式，如果为false，则表是在创建模式。
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//如果集合为空，需要扩容生成Node<K,V> [] table 数组，默认大小是16
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//计算下标 获得元素p，如果p为null 即不存在，则存储该key value
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
//此下标有值，发生碰撞
else {
Node<K,V> e; K k;
//如果key value 与 原下标的桶内数值一样 保存该桶内值为e
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果p，即原桶内的非空node，是红黑树，则以红黑树的添加方式添加key value
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//如果p是链表
else {
//依次遍历该链表
for (int binCount = 0; ; ++binCount) {
//如果p的下一个元素e 为空 则添加，添加完后链表长度大于等于7 则需转换该链表为红黑树，结束循环
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//如果key value 与 链表该节点数值一样 保存该桶内值为e，结束循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//e不为null 说明集合内存在 key 返回旧value值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
//linkedHahMap
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//如果size 大于 threshold时 需扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
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) {
//如果旧容量大于最大容量值 下一次扩容阙值为最大整型 返回旧桶数组
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//新容量为 旧容量*2，如果小于最大容量 并且 旧容量大于初始默认容量
//就新阙值为 旧阙值*2
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//如果旧容量为0 旧阙值大于0 新容量为旧阙值
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
//如果旧容量为0 旧阙值为0 容量和阙值为默认的
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//新阙值为0 即新容量为旧阙值时，否则其他情况newThr都为0
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) {
//依次遍历桶数组 将旧桶内的值拷贝到新桶中
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//说明该桶内无冲突元素，计算新下标
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
//如果是红黑树，则另行拷贝，该方法主要处理链表时的情况
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
//如果为链表,resize()核心部分
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;
//说明 e.hash & (newCap -1)= j,下标不变
//jdk1.7之前是头插入法
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
//说明 e.hash & (newCap -1) = j + oldCap
//因为 newCap == oldCap*2
//假设 oldcap 为8 newcap为 16
//如果 e.hash 为 xxx1101, 则 e.hash & (odlCap -1) j== xxx0101
//e.hash & (newCap -1) == xxx1101 == j + oldCap
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;
}
有空补上红黑树
