HashMap主要方法源码分析（JDK1.8）
就像空中月 人气:1本篇从HashMap的put、get、remove方法入手，分析源码流程
（不涉及红黑树的具体算法）
jkd1.8中HashMap的结构为数组、链表、红黑树的形式
（未转化红黑树时）
（转化为红黑树时的情况）
一、关于HashMap需要了解的静态常量
DEFAULT_INITIAL_CAPACITY 数组默认初始容量 16
DEFAULT_LOAD_FACTOR 默认负载因子 0.75
MIN_TREEIFY_CAPACITY 最小树容量 64
在下面的方法探究中将会提到这些静态常量的用处
二、方法探究
1、put
HashMap中的数组是第一次调用put方法时才创建对象的
下面是从进入put到创建完数组的全过程
将key、value作为参数传入后，计算key的hash，再传入putVal方法
/**
* 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;
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;
}
putVal（）的整个流程如下
1、先判断table是否为null，如果是，调用resize()
/**
* Initializes or doubles table size. 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;
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)
newThr = oldThr << 1; // double threshold
}
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;
}
resize()：
判断当前table为null后，初始化负载因子DEFAULT_LOAD_FACTOR
计算当前数组边界DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY
（第一次数组边界为16*0.75=12，当数组超过数组边界时会扩大为两倍。
也就是说数组中的元素达到或大于12时，将第一次扩大数组，大小变为16*2=32）
创建并返回大小为DEFAULT_INITIAL_CAPACITY的table对象。
（总的来说resize负责扩大数组容量和初始化数组）
以上就是调用put方法时HashMap对象内数组的创建过程
2、如果进入putVal（）判断table不为null
利用hash值与（&）计算出数组下标，并判断是否为空
如果是，创建node对象并存入数组
如果不是，从当前下标的链表第一位开始一个个往下对比
①若hash和key值都相同，则break退出循环，之后进行value的替换，并返回oldValue
②若过程中遇到null，则创建node对象
判断是否达到红黑树转换条件：如果当前链表长度达到8，进入treeifyBin方法
判断表长度（如果小于64，则调用resize()，判断要不要增大数组
反之replacementTreeNode，用红黑树代替当前链表
（向下遍历数组当前下标链表的操作）
（key相同时value的替换操作）
3、增加size（map中元素的数量）和modCount（对map的操作次数）
2、get
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>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.)
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* 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;
}
将key和key的hash传入getNode方法，并获取返回的Node对象的value
先判断数组不为null，且长度大于0
利用hash值先找到node可能在的列的第一个元素（当前传入key可能不存在）
再竖向遍历链表对比hash和key值
（上面的first即为node可能存在的链表的第一个元素）
查找时如果第一个即匹配则直接返回
否则往下遍历直到匹配或node为null
3、removeNode
/**
* Implements Map.remove and related methods.
*
* @param hash hash for key
* @param key the key
* @param value the value to match if matchValue, else ignored
* @param matchValue if true only remove if value is equal
* @param movable if false do not move other nodes while removing
* @return the node, or null if none
*/
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K,V>[] tab; Node<K,V> p; int n, index;
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node<K,V> node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
else if ((e = p.next) != null) {
if (p instanceof TreeNode)
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
与getNode的逻辑类似，利用hash值查找可能存在的位置
如果链表第一位就匹配：
对链表的遍历：
判断node的类型，如果是链表则用node.nest来代替当前位置
最后操作数+1，size-1，返回被remove的node对象
加载全部内容