HashMap源码学习笔记（一）
HashMap中的部分属性
/**
* The default initial capacity - MUST be a power of two.
* 默认的初始化容量
*/
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
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 默认的加载因子
*/
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.
* 元素节点从链表转化为树的阈值（注意：还有一个条件是容量必须大于 MIN_TREEIFY_CAPACITY）
*/
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;
/**
* 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.)
* HashMap数据存放处，该数组大小必须是2的幂次方也为0（为什么必须为2的幂次方？）
*/
transient Node<K,V>[] table;
HashMap的初始化
空构造函数和有参数的构造函数是有区别的。
###空构造函数
源码如下
Map<String, String> map = new HashMap();
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
空构造函数只是将loadFactor赋予默认值，不做其它处理。当向Map中添加数据时才调用对应的初始化
有参构造函数
Map<String, String> map = new HashMap(16, 0.75);
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);//该方法保证了容器的容量一定是2的幂次方
}
//该方法就是求不小于给定数的2的幂次方数
//eg: in : 3 4 235
// out : 4(2^2) 4(2^2) 256(2^8)
//思路：2的幂次方数的特征是只有最高位为1，其它位都为0
//将所有位全部设为1。再加1就获得2的幂次方数。
static final int tableSizeFor(int cap) {
//int类型的2进制数位32位
int n = cap - 1;//这一个步骤是为了当入参本身就是2的幂次方数的时候。
n |= n >>> 1;//保证最高位2位为1
n |= n >>> 2;//保证最高位4位为1
n |= n >>> 4;//保证最高位8位为1
n |= n >>> 8;//保证最高位16位为1
n |= n >>> 16;//保证最高位32位为1
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
HashMap 添加元素
原理：HashMap底层是一个Node数组。元素的下标index=(n - 1) & hash。其中hash是由hash算法获取，n为容量。
当键值的hash值相同时，底层的数据结构不一样。分为链表和红黑树。
###链表
当键值的hash值相同，且链表长度小于8。新加入的元素放入链表末尾。
当链表的长度大于8时，但是容器的容量小于64。会调用resize()方法进行扩容并且进行数据的重新排列
当链表的长度大于8时，但是容器的容量不小于64。则将链表转化为红黑树
//hash算法。获取hash值。思考key.hashCode()) ^ (h >>> 16)的用意
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
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);//当该下标没有数据时
else {
//当该下标已经有数据时
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;//如果新加的数据的键值和该下标的键值相同，直接覆盖该下标的元素的值
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);//当该节点是树结构时
else {
//当该节点是链表时。
for (int binCount = 0; ; ++binCount) {
//当前节点为最后一个节点时
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);//当前链表的长度大于等于8时，进行数据的重新调整
break;
}
//当前节点的的键值和新加元素的键值相同是
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();//扩容并重新调整数据
afterNodeInsertion(evict);
return null;
}
来源：CSDN
作者：sky_sigh
链接：https://blog.csdn.net/qq_38268604/article/details/103863451