HashMap源码解析_jdk1.8（二）
构造函数
HashMap提供了如下几个构造函数：
/**
* 构造一个具有指定初始容量和负载因子的空HashMap.
*
* @param initialCapacity 初始容量
* @param loadFactor 负载因子
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
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;
// 如果用户通过构造函数指定了一个数字作为容量，那么Hash会选择大于该数字的第一个2的幂作为容量。(1->1、7->8、9->16)
this.threshold = tableSizeFor(initialCapacity);
}
/**
* 构造一个具有指定初始容量和默认负载因子(0.75)的空HashMap.
*
* @param initialCapacity 初始容量
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* 构造一个具有默认初始容量(16)和默认负载因子(0.75)的空HashMap.
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
如果用户通过构造函数指定了一个数字作为容量，那么Hash会选择大于该数字的第一个2的幂作为容量。(1->1、7->8、9->16)
/**
* Returns a power of two size for the given target capacity.
*/
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;
}
从构造函数我们可以看出，在常规构造器中，没有为数组table分配内存空间（有一个入参为指定Map的构造器例外），而是在执行put操作的时候才真正构建table数组。
put方法
/**
* 将指定的value与该map中的指定key相关联.
* 如果map中已经包含指定key，key对应的value将会被新值覆盖
*
* @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>.)
*/
public V put(K key, V value) {
// 根据key计算hash值
return putVal(hash(key), key, value, false, true);
}
/**
* 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
* @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;
// 如果table为null或长度为0，为数组table分配内存空间
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 判断插入的位置是否是冲突的，如果不冲突就直接newNode，插入到数组中即可
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
// 如果插入的hash值冲突了
Node<K,V> e; K k;
// 判断table[i]中的元素是否与插入的key一样，若相同那就直接使用插入的值p替换掉旧的值e。
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 判断插入的数据结构是红黑树还是链表，在这里表示如果是红黑树，那就直接putTreeVal到红黑树中
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
// 数据结构是链表
else {
// 遍历table数组是否存在
for (int binCount = 0; ; ++binCount) {
// 不存在直接newNode(hash, key, value, null)
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 判断当前链表的长度是否大于阈值8，如果大于那就会把当前链表转变成红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 如果存在了跳出循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 如果存在了，直接使用新的value替换掉旧的value
if (e != null) {
// existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
// 插入成功，判断是否需要扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize扩容方法
/**
* table数组初始化或长度加倍. 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;
// 扩容前table数组长度
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)
// 将阈值扩大为2倍
newThr = oldThr << 1; // double threshold
}
// 阈值已经初始化了，就直接使用
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else {
// zero initial threshold signifies using defaults
// 没有初始化阈值那就初始化一个默认的容量和阈值
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({
"rawtypes","unchecked"})
// 新建一个数组长度为原来2倍的数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
// 旧数据保存在新数组里面
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
// 只有一个节点，通过索引位置直接映射
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
// 如果是红黑树，需要进行树拆分然后映射
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else {
// preserve order
// 如果是多个节点的链表，将原链表拆分为两个链表
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
// 扩容后，若hash值新增参与运算的位=0，那么元素在扩容后的位置=原始位置
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
// 扩容后，若hash值新增参与运算的位=1，那么元素在扩容后的位置=原始位置+oldCap
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
// 链表1存于原索引
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
// 链表2存于原索引加上原hash桶长度的偏移量
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
参考：
https://zhuanlan.zhihu.com/p/79219960
https://www.cnblogs.com/theRhyme/p/9404082.html#_lab2_1_1