HashMap的源码解析
特点：数据存储方式为散链表(数组+链表或数组+红黑树)
初始化
HashMap有四种初始化方式：
方式一：
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
}
方式二：
public HashMap(int initialCapacity) { //initialCapacity是长度
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
方式三：
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0) //判断initialCapacity的值是否正常
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY) //判断initialCapacity的值是否超过设置的最大容量
initialCapacity = MAXIMUM_CAPACITY; //如果超过则为最大容量
if (loadFactor <= 0 || Float.isNaN(loadFactor)) //判断loadFactor的值是否正常
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity); //初始化容量
}
/**
* 返回最接近cap的2的n次幂，目的：为了减少hash冲突
* 例：8
* 8 换成二进制 0000 1000 运算后 0000 1111 =15 ，返回15+1=16
* 总结：先将cap的值转换成二进制，将从最高位的1开始一直到最低位都化成1(如：0011 0101=>0011 1111,0100 *1000=>0111 1111)，在将此数值+1不超过最大容量则返回
*/
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;
}
方式四：
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);//同上
}
//以上判断map是否正常
else if (s > threshold) //判断是否超过当前容量，超过则扩容
resize(); //见扩容标题
// 遍历输入的map，每一个都put进去
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict); //见put标题
}
}
}
get
/**
* 计算hash值
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);//防止hash值冲突过高
}
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) { //判断这个hash在散链表的数组中是否存在
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k)))) //判断找到的key是否是理想值,不是理想值说明hash冲突
return first;
if ((e = first.next) != null) { //判断是否存在下一个节点，如果为空证明查找的key不存在
if (first instanceof TreeNode) //判断冲突的hash是不是按树结构来存储
return ((TreeNode<K,V>)first).getTreeNode(hash, key); //根据树结构来查找
//不是树根据链表结构来查找
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
remove
public V remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
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;
//找
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) { //拿到key的节点
Node<K,V> node = null, e; K k; V v;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //判断是不是要找的值
node = p;
//不是按照树或链表的结构来查找
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);
}
}
//删，按链表或树结构删除，不能使其断链
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; //返回删除的节点
}
}
return null; //没有要删的节点，返回null
}
put
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
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)//如果是第一次put进行扩容
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null) //判断插入的数组位置是否为空
tab[i] = newNode(hash, key, value, null); //为空创建新节点
else {
Node<K,V> e; K k;
//判断插入的key是否等于当前的key
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//如果是树结构按树的方式插
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//不是树结构按链表的方式插
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//如果大于链表转树的预值转换成树
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;
//如果value不相等才进行赋值
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//如果当前长度大于预值进行扩容(默认预值=0.75*当前容量)
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
resize
/**
* 扩容目的：减少hash冲突
* 如果节点的hash小于老容量，在数组中的位置不变
* 节点的hash与新容量进行与运算，如果最高位是1则在数组新位置上，如果最高位是0则还在数组原位置上
* 过程：遍历老数组，找到冲突的链表或树，遍历节点，判断位置是否在新位置，如果在，原索引+原数组长度=新索引
*/
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;
}
浙公网安备 33010602011771号