HashMap允许key为null,也允许value为null
HashMap跟HashTable两个类是差不多的,除了HashTable是线程安全且不允许null值这一点外.
基本概念:HashMap底层是数组+链表(数组的每个值都是一条链表的头结点),1.8后加入了红黑树(当链表长度达到8就自动将该链表替换为红黑树),通过计算key的哈希码,在经过高位参与位运算计算得出键值对(将key和value包装起来的对象)所在的数组的下标,采用头插入法插入该位置的链表(若该位置是空的就直接插入)
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
static final int TREEIFY_THRESHOLD = 8;
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
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DEFAULT_INITIAL_CAPACITY: 默认底层数组的初始大小(2^4),可通过构造参数指定
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MAXIMUM_CAPACITY: 数组的最大长度(2^31),超过将替换为此值
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DEFAULT_LOAD_FACTOR: 默认负载因子,为0.75,可通过构造参数指定
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TREEIFY_THRESHOLD: 链表转换为红黑树的阙值,当链表长度达到8自动转为红黑树进行存储(前提是数组长度大于等于64)
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UNTREEIFY_THRESHOLD: 红黑树转为链表的阙值,当红黑树结点个数减小到6时,自动转为链表存储
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MIN_TREEIFY_CAPACITY: 链表进行树化的前提条件,数组长度要达到64或一上,在这之前只能通过数组扩容来减少链表长度
transient Node<K,V>[] table; transient Set<Map.Entry<K,V>> entrySet; transient int size; transient int modCount; int threshold; final float loadFactor; -
table: 底层数组,可动态扩容,数组长度为2的整数次方
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size: 这个map中存放的键值对数目
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modCount: 记录这个map数据结构发生改变的次数(发送插入删除或者链表与树相互转换的操作),由于fail-fast机制
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threshold: 数组进行扩容的下个阙值(当前键值对数量达到这个值后进行
resize()(扩容)操作)(threshold = capacity * load factor) -
loadFactor: 实际的负载因子
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
- 局部变量h存放hashCode()放回的初始哈希码,通过h右移16位与h异或(右移后,前16位为0,异或不改变h的前16位值)得到最终的哈希吗.
通过高位参与位运算可以减少数组长度较低时的哈希码冲突问题(取模时,高位变低位不变,冲突几率会变高)
static class Node<K,V> implements Map.Entry<K,V> {
final int hash; // 计算得到的hash码
final K key; // 键对象
V value; // 值对象
Node<K,V> next; // 下一个结点
方法略...
}
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
TreeNode<K,V> parent; // 父亲节点
TreeNode<K,V> left; // 左子树
TreeNode<K,V> right; //右子树
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red; // 红色还是黑色
TreeNode(int hash, K key, V val, Node<K,V> next) {
super(hash, key, val, next);
方法略...
}
- 通过继承
LinkedHashMap.Entry<K,V>,实际上间接继承了链表的Node<K,V>
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value; //1
}
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) { //2
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k)))) //3
return first;
if ((e = first.next) != null) { //4
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
- 计算key的哈希码,传入getNode方法,放回Node对象或者null
- 如果table为null,table是空的或者数组( (length-1)&hash )处的值为null,就返回null,否则进入3
- 检查第一个结点,若是指定的key,直接返回该结点,否则进入4
- 如果这个树/链表不止一个结点,先判断是树还是链表,再进行对应的结点查找,找到就返回,否则返回null.
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) {
Node<K,V>[] tab;
Node<K,V> p;
int n, i;
if ((tab = table) == null || (n = tab.length) == 0) //1
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null) //2
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)))) //3
e = p;
else if (p instanceof TreeNode) //4
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else { //5
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 //6
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
} //7
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
- 如果数组为null或是空的,则
resize()扩充容量 - 通过hash计算并位运算取摸获得数组下标,若该位置是空的,新建链表结点直接填坑然后跳到7,否则进入3
- 判断头结点的key跟要put进去的key是否同一个,是则将其引用赋给e,进入6,否则进入4
- 判断头结点是不是树结点,是则执行
putTreeVal,若树中已存在该key,则直接返回该键值对(赋给e),否则新建并插入结点并返回null,然后进入6.如果不是树节点则进入5 - 在链表中遍历,如果不存在,就新建一个结点,然后是否达到树化的阙值,是就转化为树结构,之后跳到7.如果存在就把它的引用赋给e跳到6
- 在搜索到当前map中存在相同key时候将该键值对赋给e,在这里进行值的覆盖,并返回旧值
- 对改动进行计数,判断是否需要进行数组扩容,返回null
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;
if ((tab = table) != null && (n = tab.length) > 0 && //1
(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)))) //2
node = p;
else if ((e = p.next) != null) { //3
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 || //4
(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;
}
- 判断底层数组是否为null或者是空的,是就直接返回null,否则2
- 判断头结点是否就是要移除的键值对,是就赋给e,进入4,否则进入3
- 判断是树还是链表并进行相应遍历,找到符合的键值对,并赋给e,进入4,若查无,返回null
- 针对不同的存储结构进行相应的移除操作,并更新相关的计数值
/**
* Replaces all linked nodes in bin at index for given hash unless
* table is too small, in which case resizes instead.
*/
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
- 先将链表结点转化成树结点,构造成双向链表,在
treeify进行红黑树的构造
/**
* 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) { // 1
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && // 2
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold // 3
newCap = oldThr;
else { // zero initial threshold signifies using defaults // 4
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) { // 5
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr; // 6
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) { // 7
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null) // 7-1
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode) // 7-2
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order // 7-3
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;
}
- 若底层数组长度大于等于允许的最大值,将扩容阙值设为MAX_INT,直接不做任何操作,直接返回原数组
- 如果底层数组长度是大于默认初始长度且当前长度*2小于允许的最大值,则将新的数组长度,扩容阙值都设为原来的两倍
- 当前数组未初始化,且扩容阙值已经初始化(不为0),将新的数组长度设定为扩容阙值,跳到5
- 当前数组与扩容阙值都未初始化,将新的数组长度和扩容阙值设为默认初始值
- 根据新的数组长度值计算新的扩容阙值,如果新的数组长度值或者新的阙值大于数组长度的允许最大值,则将其替换为MAX_INT,反之保留
- 将经过上述计算得到的新值进行更新(设置threshold为新值, 实例化一个新长度的底层数组)
- 遍历数组的每个坑位,将老数组的值搬运到新的数组中
7-1. 若该坑位只有一个结点,直接搬运到新数组对应坑位,需要重新计算下标,因为新数组的长度已经改变
7-2. 若该坑位放的是树,则调用对应方法进行换坑
7-3. 若该坑位是是链表,遍历这条链表,根据其hash&旧数组长度是0还是1分为两组,一组在新数组下标不变,另一组是原来下标+旧数组长度
注: 因为每次扩容都是2扩容两倍,位运算时只增加一个高位(右数第oldCap个),按位与时,若键值对的右数第oldCap位是0则下标不会受扩容影响,若不是,则下标是原下标加上oldCap.
以上分析为个人理解,欢迎指正!
关于红黑树的实现与操作并没有深入代码层次解析，有兴趣可阅读 红黑树深入析及Java实现