HashMap
2020-11-17 本文已影响0人
神的漾
1.HashMap简介
HashMap是基于哈希表的Map接口实现,是以key-value存储数据.除了不是同步的和允许使用null以外,HashMap和HashTable大致相同.如果想使HashMap同步,可以使用Collections.synchronizedMap(Map)使HashMap具有同步能力,或者使用ConcurrentHashMap本身就具有同步的能力.
HashMap的实现不是同步的,所以是线程不安全的,它的key,value都可以为null.此外在JDK1.8中,HashMap是由数组+链表+红黑树构成的,新增了红黑树作为底层数据结构,结构变复杂了,但是效率也变的更高效了.
当一个值中要存储到Map的时候会根据Key的值来计算出他的hash,通过hash来确认到数组的位置,如果发生哈希碰撞,就以链表的形式存储,如果链表长度超过8时,HashMap就会把这个链表转换成红黑树来存储.因为链表很短的时候,即使遍历,速度也非常快,但是当链表长度不断变长,肯定会对查询性能有一定的影响.所以转成红黑树.每次resize会把HashMap的size扩大一倍.
树形化时会判断是否大于MIN_TREEIFY_CAPACITY大于才会树形化,小于的话会直接扩容.
数据结构
HashMap数据结构
上面是HashMap的数据结构.当一个值要存储到Map的时候会根据Key的值来计算出他的Hash值,通Hash值来确认到数组的位置,如果发生哈希碰撞就以链表的形式存储,如果链表长度超过8,就会把这个链表置换成红黑树来存储.
类结构
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
......
}
是继承了AbstractMap<K,V>和实现Map<K,V>,Cloneable,Serializable
AbstactMap已经实现了Map,据java集合框架创始人josh Bloch描述,继承map的写法是一个失误.
- CloneAble 空接口,表示可以克隆
- Serializable 序列化
- AbstractMap 提供Map实现接口
属性意义
/**
* The default initial capacity - MUST be a power of two.
* 默认初始容量,必须是2的n次方,默认是16,1向右移4位就是2的4次幂
*/
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.
* 最大容量 2的30次幂,值为1 073 741 824,这么写简单,运算效率也高.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 默认加载因子.0.75
*/
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.
* 树形化阀值值=8,要大于2并至少是8,使整个链表转化为红黑树.
*/
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.
* 取消树形化阀值 =6,要小于TREEIFY_THRESHOLD,
* 指从黑红树转换为链表的阀值
*/
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.
* 最小树形化阀值,当hash表中容量大于该值时才进行树形化.
* 否则直接扩容.为了避免扩容和树形化冲突,
* 这个值不能小于4 * TREEIFY_THRESHOLD
*/
static final int MIN_TREEIFY_CAPACITY = 64;
变量意义
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
* ransient代表序列化时不会存储
* 数组表,第一次使用时初始化,并调整为必要的大小,长度总是2的n次幂.
* 某些情况也允许长度为0的.
*/
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
* 用来存放缓存
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
* HashMap中存储数据的数量
*/
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).
* 用来记录HashMap的修改次数.
*/
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.)
// 要调整大小的下一个大小值(容量*负载因子)
int threshold;
/**
* The load factor for the hash table.
*
* @serial
* 哈希表负载因子
*/
final float loadFactor;
构造方法
put方法
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
* 将此映射中的指定值与指定键关联.如果以前的映射包含键的映射,则旧的值被
* 替换.
* return 会返回旧的关联的键值,,如果无映射值,会返回null,也有可能是原键值映
* 射的就是null值.
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
put方法实际调用的是putVal方法来进入数据插入,这里调用了hash(key)方法,来看看hash方法如何实现的.
/**
* Computes key.hashCode() and spreads (XORs) higher bits of hash
* to lower. Because the table uses power-of-two masking, sets of
* hashes that vary only in bits above the current mask will
* always collide. (Among known examples are sets of Float keys
* holding consecutive whole numbers in small tables.) So we
* apply a transform that spreads the impact of higher bits
* downward. There is a tradeoff between speed, utility, and
* quality of bit-spreading. Because many common sets of hashes
* are already reasonably distributed (so don't benefit from
* spreading), and because we use trees to handle large sets of
* collisions in bins, we just XOR some shifted bits in the
* cheapest possible way to reduce systematic lossage, as well as
* to incorporate impact of the highest bits that would otherwise
* never be used in index calculations because of table bounds.
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
可以看出HashMap是支持key为null的当key的值等于null的时候hash值返回0,HashTable是直接用key来获取HashCode,所以key为空会抛异常.
首先计算出key的hashCode为h,然后与h无条件右移16位的二进制进行按位异或(^)得到最终的hash值.
下面看下putVal方法的具体实现.
/**
* Implements Map.put and related methods
* 实现了map的put和相关方法
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @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;
// 判断table是空或者长度为0,如果是就进行初始化.
// 这里resize方法只是进行初始化实际并没有分配空间,所以在这里调用进行空间分配
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 对hashcode进行取模运算,
// i = (n - 1) & hash;这里i的值就是获取到值的位置. 如果tab[i] 为null就新增一个元素.
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
// 如果tab[i] 不等于空表示这个位置已经有值了.
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
// 如果key的值已经存在,直接去替换已有值.
e = p;
// 判断是否是红黑树.
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// 否则就是链表.
else {
// 如果是链表的话需要遍历到最后节点然后插入.
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// 节点的最后一位的标志就是next值为null
p.next = newNode(hash, key, value, null);
// 判断节点的长度大于等于TREEIFY_THRESHOLD红黑树的阀值,就转换红黑树.
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))))
break;
p = e;
}
}
// 如果链表中重复就直接替换.并返回旧值.
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
// 记录修改次数.
++modCount;
// 判断当前元素数量是否超过threshhold阀值,如果超过调用resize()
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize()方法
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
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;
}
get()方法
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
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;
}
