HashMap源码理解
jdk1.8 主要的几个方法理解
属性
/**
* 默认初始容量。
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* 最大支持容量
* MUST be a power of two <= 1<<30.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* 默认加载因子
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 链表长度大于8转为红黑树.
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* 链表长度等于6的时候从红黑树转为链表
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* 红黑树的多大的时候开始扩容
*/
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* 存储元素的数组
*/
transient Node<K,V>[] table;
/**
* 存储具体元素的集
*/
transient Set<Entry<K,V>> entrySet;
/**
* key-value的数量
*/
transient int size;
/**
* HashMap扩容和结构改变的次数
*/
transient int modCount;
/**
* 扩容临界值 (capacity * load factor).
*
* @serial
*/
int threshold;
/**
* 填充因子
*/
final float loadFactor;
Node节点定义
/**
* 节点的Node,继承Entry
*/
static class Node<K,V> implements Entry<K,V> {
final int hash; //hash用来定位数组索引位置
final K key;
V value;
Node<K,V> next; //下一个节点对应的Node
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);
}
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) {
Entry<?,?> e = (Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
构造器
/**
* 构造器,设置默认大小和负载因子
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) {
/***
* 如果initialCapacity赋值大小小于0抛出异常
*/
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
/***
* 如果initialCapacity赋值大小大于最大支持容量,就设置容量为最大支持容量。
*/
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
/***
* 如果负载因子小于0或者不是float,抛出异常
*/
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
/***
* 设置负载因子
*/
this.loadFactor = loadFactor;
/***
* 扩容临界值的大小
*/
this.threshold = tableSizeFor(initialCapacity);
}
/**
* 构造器,设置默认大小和负载为默认的DEFAULT_LOAD_FACTOR
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* 无参构造器,只设置赋值因子DEFAULT_LOAD_FACTOR
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* 对于给定的目标容量,返回两倍大小的幂.
*/
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;
}
添加方法及添加涉及到的方法
两个算法说明
hash = (h = key.hashCode()) ^ (h >>> 16)
”东东“ .hashCode() : 10011100001110000000
”东东“ .hashCode() >>> 16: 11111111111111111001
”东东“ .hashCode()^”东东“ .hashCode() >>> 16: 10011100001110001001
(lenth - 1) & hash :存储的位置
如果默认lenth为最初始化的16:16二进制: 00000000000000010000
16 - 1 = 15 的二进制: 00000000000000001111
hash 10011100001110001001
(lenth - 1) & hash: 00000000000000001001
这样就能完美的把值放到16个桶中了。。。
/***
* 根据key获得hash值
* @param key
* @return
*/
static final int hash(Object key) {
int h;
//返回的值如果是null,就返回0,否者
//h = key.hashCode()获取key的hashCode;
//h >>> 16 无符号右移16位
//h = key.hashCode()) ^ (h >>> 16) hash值和无符号右移16位的值进行异或运算
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
/**
* HashMap底层是用数组+链表(红黑树)组成的
* @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; //存储数据的数组
Node<K,V> p;
int n, i; //记录数组的长度
//如果数组还是空
if ((tab = table) == null || (n = tab.length) == 0)
//进行扩容(HashMap不是开始赋值的初始值的)
n = (tab = resize()).length;
//(n - 1) & hash 位置上没有值
// n - 1 长度都为2的n次方获的值所有的位置都是1
// 与hash进行进行&运算,巧妙的获取到了具体位置
if ((p = tab[i = (n - 1) & hash]) == null)
//put到 (n - 1) & hash上
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//如果当前位置的值与已有的值一毛一样,就覆盖以前的值
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果当前位置的值是TreeNode,就进行挂到红黑树
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//如果当前位置的值是链表,就进行挂到链表上
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
//说明挂在链表的下一个位置
p.next = newNode(hash, key, value, null);
// 如果达到了转树的条件,进行链表转树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//挂在列表的位置的值与已有的值一毛一样,就覆盖以前的值
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;
}
}
//扩容和结构改变的次数+1
++modCount;
//如果长度大于扩容临界值进行扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
/**
* 扩容操作
* @return the table
*/
final Node<K,V>[] resize() {
//数组先复制给oldTab
Node<K,V>[] oldTab = table;
//获得数组的长度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//获取当前数组的扩容临界值
int oldThr = threshold;
//初始化新的数组和扩容临界值
int newCap, newThr = 0;
/***
* 设置扩容临界值
*/
//如果以前数组有值
if (oldCap > 0) {
//如果超过了最大支持容量,返回旧的数组
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//否者设置新的临界值
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 {
// 第一次put的时候,没有值,就设置默认初始大小16,且设置其的扩容临界值
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注解去掉代码警告
*/
@SuppressWarnings({
"rawtypes","unchecked"})
/***
* 正式开始put操作及扩容操作
*/
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
/***
* 如果以前数组有值
*/
if (oldTab != null) {
//遍历以前的数组
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
//如果当前数组的Node下一个值为空,把值重写hash赋值到新的数组上
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
//如果当前数组是红黑树,把当树上的值重新赋值到新的Table上
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else {
// 数组上是链表。对链表的只进行挂到新的table上
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;
}