HashMap原码阅读
JDK1.8 HashMap
文章目录
- JDK1.8 HashMap
- HashMap继承的类，和接口
- HashMap的常量
- 静态内部类Node
- hash函数
- comparableClassFor方法
- tableSizeFor
- 成员变量
- table
- entrySet
- size
- modCount
- threshold
- loadFactor
- HashMap的4个初始化函数
- putMapEntries
- getNode函数
- put函数
- putVal
- resize函数
HashMap继承的类，和接口
HashMao继承了AbstractMap类，实现了Map,Cloneable,Serializable接口。主要看探讨HashMap的实现。
HashMap的常量
默认初始化容量为1<<4。
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
最大的容量大小
static final int MAXIMUM_CAPACITY = 1 << 30;
默认的装载因子0.75，当当前容量大小size大于当前总容量*DEFAULT_LOAD_FACTOR 就会扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;
默认当链表长度大于8时转变成红黑树维护.
红黑树原理
代码实现
static final int TREEIFY_THRESHOLD = 8;
默认树转链表的阈值:
static final int UNTREEIFY_THRESHOLD = 6;
默认table中至少包含64个键值对才转化成树
static final int MIN_TREEIFY_CAPACITY = 64;
静态内部类Node
实现了Map.Entry接口
static class Node<K,V> implements Map.Entry<K,V> {
final int hash; //hash被申明为final
final K key; //key也被申明为final
V value; //value不是final
Node<K,V> next; //链表只想下一个
Node(int hash, K key, V value, Node<K,V> next) { //构造函数
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; } //被申明为final
public final V getValue() { return value; } //被申明为final
public final String toString() { return key + "=" + value; } //被申明为final
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value); //重写hashCode方法
}
public final V setValue(V newValue) { //
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) { //重写equals方法
if (o == this)
return true;
if (o instanceof Map.Entry) { //必须要实现Map.Entry接口才能比较，并且key和value值都相等才能比较
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
hash函数
hash函数是一个static final修饰的函数，静态的，不可修改。
从源代码来看hashmap允许值为空。
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
//如果key == null 那么返回0 否则返回(h = key.hashCode() )^(h >>>16 )
//充分利用了key.hashCode()的高16位，增加key.hashCode 低16位的随机性。
}
comparableClassFor方法
先来看看他的注释
/**
* Returns x's Class if it is of the form "class C implements
* Comparable", else null.
*/
如果他是这种形式class C implements Comparable 那么就返回x的Class对象。
那么这个函数的具体作用是干什么的呢？
当链表中的数量大于8时那么他会转化成一个棵红黑树，当put一个新元素时，如果该元素键的hash值小于当前节点的hash值的时候，就会作为当前节点的左节点；hash值大于当前节点hash值得时候作为当前节点的右节点。那么hash值相同的时候呢？这时还是会先尝试看是否能够通过Comparable进行比较一下两个对象（当前节点的键对象和新元素的键对象），要想看看是否能基于Comparable进行比较的话，首先要看该元素键是否实现了Comparable接口，此时就需要用到comparableClassFor方法来获取该元素键的Class，然后再通过compareComparables方法来比较两个对象的大小。
static Class<?> comparableClassFor(Object x) {
if (x instanceof Comparable) {
Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
if ((c = x.getClass()) == String.class) // bypass checks
return c;
if ((ts = c.getGenericInterfaces()) != null) {
for (int i = 0; i < ts.length; ++i) {
if (((t = ts[i]) instanceof ParameterizedType) &&
((p = (ParameterizedType)t).getRawType() ==
Comparable.class) &&
(as = p.getActualTypeArguments()) != null &&
as.length == 1 && as[0] == c) // type arg is c
return c;
}
}
}
return null;
}
/**
* 如果x所属的类是kc，返回k.compareTo(x)的比较结果
* 如果x为空，或者其所属的类不是kc，返回0
*/
@SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
static int compareComparables(Class<?> kc, Object k, Object x) {
return (x == null || x.getClass() != kc ? 0 :
((Comparable)k).compareTo(x));
}
tableSizeFor
static final int tableSizeFor(int cap) 当创建一个新的hashmap的时候带了cap参数，这个时候这个函数用来寻找大于它且离他最近的2的幂。
具体算法原理参考:算法原理
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;
//如果n = 0，结果这些操作之后应该返回1
}
成员变量
table
/**
* 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.)
*/
transient Node<K,V>[] table;
entrySet
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
size
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
modCount
/**
* 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;
threshold
/**
* 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;
loadFactor
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;
HashMap的4个初始化函数
带初始化的容量和装载因子
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity); //如果初始化容量<0则抛出异常
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor); //如果装载因子小于0或者是一个非数字则跑出异常
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity); //获取大于且离他最近的2次幂
}
这个就简单了
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
默认的这里只设置了loadFactor。估计capacity在具体初始话的时候设置
/**
* Constructs an empty HashMap with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//构造一个映射关系与指定 Map 相同的新 HashMap。
public HashMap(Map<? extends K, ? extends V> m) { //
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false); //主要通过这个函数去构造
}
putMapEntries
/**
* Implements Map.putAll and Map constructor
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size(); //获取传入map的大小,如果为null则抛出异常
if (s > 0) {
if (table == null) { // pre-size table还没初始化
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t); //计算阈值
}
else if (s > threshold) //如果已经table已经初始化了并且s>阈值那么就需要扩容
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);
}
}
}
/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map
*/
public int size() {
return size;
}
/**
* Returns true if this map contains no key-value mappings.
*
* @return true if this map contains no key-value mappings
*/
public boolean isEmpty() {
return size == 0;
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* 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.)
*
*
A return value of {@code null} does not necessarily
* 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;
//如果getNode返回的值为null就直接返回空否则返回e.value
}
getNode函数
//代码很简介，易懂,先把要用的变量全部声明好。
/**
* 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) {
//如果tab已经初始化了 并且长度大于0 并且first不为空
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first; //每次都先比较第一个
if ((e = first.next) != null) {
if (first instanceof TreeNode) //在判断是不是treeNode treeNode和Node感觉有继承关系
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;
}
put函数
/**
* 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 key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
putVal
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value //可以修改相关参数 使得不改变已经存在的值
* @param evict if false, the table is in creation mode. //默认不是这个
* @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;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
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;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize函数
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold; //threshold如果当前size大于这个就扩容
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) { //如果oldCup大于MAXIMUM_CAPACITY的最大容量了
threshold = Integer.MAX_VALUE; //那么把threshold变成max的最大值
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold、
//如果newCap = oldCap的两倍 并且小于最大的容量 并且大于初始的容量 阈值变为两倍
}
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;
}