HashMap源码解析（简单易懂）
2024-08-25 20:51:43
/*
每一个key-value存储在Node<K,V>中，HashMap由Node<K,V>[]数
组组成。
*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next; //hash:每个节点插入时先计算hash
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) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
HashMap常量与属性：
常量：
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认初始化容量是16
static final int MAXIMUM_CAPACITY = 1 << 30;//数组的最大容量 2的30次方
static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认加载因子 ，阈值=capacity*DEFAULT_LOAD_FACTOR
static final int TREEIFY_THRESHOLD = 8; //默认链表长度大于8时，链表转化成红黑数
static final int UNTREEIFY_THRESHOLD = 6; //默认元素个数小于6时，红黑数退化成链表
static final int MIN_TREEIFY_CAPACITY = 64;//在转变成树之前还会有一次判断，只有键值对数量大于64才会发生转换，
//这是为了避免在哈希表建立初期，多个键值对恰好被放入同一个链表中导致不必要的转化 属性:
transient Node<K,V>[] table;
transient Set<Map.Entry<K,V>> entrySet;
transient int size;
transient int modCount;//扩容次数
int threshold;//阈值，超过这个值就扩容
final float loadFactor; //加载因子
方法：
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
public V put(K key, V value) {
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) {//onlyIfAbsent==true:如果key值已存在，则不改变其value值
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;//null或者长度为零，需要初始化大小，或者扩容
if ((p = tab[i = (n - 1) & hash]) == null) //(n - 1) & hash:确定index,因为n是2的次幂，n-1各进制位都为1，
// &：按位与 任何数与n-1做 &运算结果小于n-1,所以保证任何hash的下标都在n-1中
tab[i] = newNode(hash, key, value, null);//p==null表明：该下标的数组节点未被使用，直接赋值
else {//hash在 Node<K,V>[] 数组产生冲突，则在该节点的链表或红黑树查找
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))//要插入的元素和冲突节点的第一个元素相等
e = p;//p:保存根据hash值找到的节点，e保存实时查找的最后一次找到的元素
else if (p instanceof TreeNode)//桶中的元素按红黑树排列
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {//桶中的元素按链表排列
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {//将元素添加到p的后面
p.next = newNode(hash, key, value, null);
//元素个数大于8，链表转红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//遍历到了key相等的元素，退出遍历
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;//e=p.next;p=e;往后遍历
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)//判断是否赋值
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;//添加元素，modCount++
if (++size > threshold)//判断是否需要扩容
resize();
afterNodeInsertion(evict);
return null;//默认值
}
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)//容量2倍，阈值2倍，
newThr = oldThr << 1; // double threshold
}
//oldCap>=0&& oldThr>0
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;//使用原来的阈值作为新的容量
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//终于用到阈值的计算方法
}
//以上判断，确定了新的容量
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;
}
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