本文深入解析了HashMap的内部结构，包括其基于数组和链表的数据结构、关键属性如初始容量、最大容量、加载因子等，以及核心方法如put、get、resize的实现原理。详细解释了HashMap如何处理hash冲突，以及在不同情况下如何调整容量和结构。
HashMap简介
HashMap基于哈希表的 Map 接口的实现。此实现提供所有可选的映射操作，并允许使用 null 值和 null 键。（除了不同步和允许使用 null 之外，HashMap 类与 Hashtable 大致相同。）此类不保证映射的顺序，特别是它不保证该顺序恒久不变。
值得注意的是HashMap不是线程安全的，如果想要线程安全的HashMap，可以通过Collections类的静态方法synchronizedMap获得线程安全的HashMap。
Map map = Collections.synchronizedMap(new HashMap());
HashMap的数据结构
HashMap的底层主要是基于数组和链表来实现的，它之所以有相当快的查询速度主要是因为它是通过计算散列码来决定存储的位置。HashMap中主要是通过key的hashCode来计算hash值的，只要hashCode相同，计算出来的hash值就一样。如果存储的对象对多了，就有可能不同的对象所算出来的hash值是相同的，这就出现了所谓的hash冲突。学过数据结构的同学都知道，解决hash冲突的方法有很多，HashMap底层是通过链表来解决hash冲突的。
下面一幅图，形象的反映出HashMap的数据结构：数组加链表实现
HashMap属性
/**
* 初始容量，必须是2的倍数，默认是16
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
* 最大所能容纳的key-value 个数
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 默认的加载因子
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
* 树化链表节点的阈值，当某个链表的长度大于或者等于这个长度，则扩大数组容量，或者数化链表
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
* 由树转换成链表的阈值UNTREEIFY_THRESHOLD
* 当执行resize操作时，当桶中bin的数量少于UNTREEIFY_THRESHOLD时使用链表来代替树。默认值是6
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
* 当桶中的bin被树化时最小的hash表容量。（如果没有达到这个阈值，即hash表容量小于MIN_TREEIFY_CAPACITY，当桶中bin的数量太多时会执行resize扩容操作）这个MIN_TREEIFY_CAPACITY的值至少是TREEIFY_THRESHOLD的4倍。
*/
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* The table, initialized on first use, and resized as
* necessary. When allocated, length is always a power of two.
* (We also tolerate length zero in some operations to allow
* bootstrapping mechanics that are currently not needed.)
* 存储数据的Node数组，长度是2的幂
*/
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
* keyset 方法要返回的结果
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* The number of key-value mappings contained in this map.
* map中保存的键值对的数量
*/
transient int size;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
* hashmap 对象被修改的次数
*/
transient int modCount;
/**
* The next size value at which to resize (capacity * load factor).
*
* @serial
*/
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
// 容量乘以装在因子所得结果，如果key-value的 数量等于该值，则调用resize方法，扩大容量，同时修改threshold的值。
int threshold;
/**
* The load factor for the hash table.
* 装载因子
* @serial
*/
final float loadFactor;
构造方法
默认构造方法
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
默认构造方法将使用默认的加载因子（0.75）初始化。
HashMap(int initialCapacity, float loadFactor)
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @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;
this.threshold = tableSizeFor(initialCapacity);
}
使用指定的初始容量和默认的加载因子初始化HashMap，这里需要注意的是，并不是你指定的初始容量是多少那么初始化之后的HashMap的容量就是多大，例如new HashMap(20,0.8); 那么实际的初始化容量是32，因为tableSizeFor（）方法会严格要求把初始化的容量是以2的次方数成长只能是16、32、64、128…
tableSizeFor方法
/**
* 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;
}
根据入参 返回2的指数 容量值
HashMap(int initialCapacity)
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
其实这个方法也是调用HashMap(int initialCapacity, float loadFactor) 方法实现的
HashMap(Map<? extends K, ? extends V> m)
/**
* 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);
}
该方法是按照之前的hashMap的对象，重新深拷贝一份HashMap对象，使用的加载因子是默认的加载因子：0.75
put方法
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @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) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param key的hash值
* @param key值
* @param value值
* @param onlyIfAbsent如果是true，则不修改已存在的value值
* @param evict if false, the table is in creation mode.
* @return 返回被修改的value，或者返回null。
*/
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)
//如果是第一次调用，则会调用resize 初始化table 以及threshold
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//如果对应的索引没有Node，则新建Node放到table里面。
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))))
//如果hash值与已存在的hash相等，并且key相等，则准备更新对应Node的value
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//如果hash值一致，但是key不一致，那么将新的key-value添加到已有的Node的最后面
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // 当某个节点的链表长度大于8，则扩大table 数组的长度或者将当前节点链表变成树节点链表
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)
//hash值和key值相等的情况下，更新value值
e.value = value;
//留给LinkedHashMap实现
afterNodeAccess(e);
//返回旧的value
return oldValue;
}
}
//修改次数加1
++modCount;
//判断table的容量是否需要扩展
if (++size > threshold)
resize();
//留给LinkedHashMap扩展
afterNodeInsertion(evict);
return null;
}
执行逻辑：
1）根据key计算当前Node的hash值，用于定位对象在HashMap数组的哪个节点。
2）判断table有没有初始化，如果没有初始化，则调用resize（）方法为table初始化容量，以及threshold的值。
3）根据hash值定位该key 对应的数组索引，如果对应的数组索引位置无值，则调用newNode（）方法，为该索引创建Node节点
4）如果根据hash值定位的数组索引有Node，并且Node中的key和需要新增的key相等，则将对应的value值更新。
5）如果在已有的table中根据hash找到Node，其中Node中的hash值和新增的hash相等，但是key值不相等的，那么创建新的Node，放到当前已存在的Node的链表尾部。
如果当前Node的长度大于8,则调用treeifyBin（）方法扩大table数组的容量，或者将当前索引的所有Node节点变成TreeNode节点，变成TreeNode节点的原因是由于TreeNode节点组成的链表索引元素会快很多。
5）将当前的key-value 数量标识size自增，然后和threshold对比，如果大于threshold的值，则调用resize（）方法，扩大当前HashMap对象的存储容量。
6）返回oldValue或者null。
put 方法比较经常使用的方法，主要功能是为HashMap对象添加一个Node 节点，如果Node存在则更新Node里面的内容。
resize方法
上面调用到了一个resize方法， 我们来看看这个方法里面做了什么，
/**
* 初始化，或者是扩展table 的容量。
* table的容量是按照2的指数增长的。
* 当扩大table 的容量的时候，元素的hash值以及位置可能发生变化。
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//当前table 数组的长度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//当前的阈值
int oldThr = threshold;
int newCap, newThr = 0;
//如果table数组已有值，则将其容量（size）和阈值（threshold）扩大两倍
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 { // 当第一次调用resize的时候会执行这个代码，初始化table容量以及阈值
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);
}
//将新的阈值存储起来
threshold = newThr;
//重新分配table 的容量
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//将以前table中的值copy到新的table中去
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;
}
实现逻辑如下：
-
如果当前数组为空，则初始化当前数组
-
如果当前table数组不为空，则将当前的table数组扩大两倍，同时将阈值（threshold）扩大两倍
数组长度和阈值扩大成两倍之后，将之前table数组中的值全部放到新的table中去数组长度和阈值扩大成两倍之后，将之前table数组中的值全部放到新的table中去
treeifyBin方法
/**
* 如果table长度太小，则扩大table 的数组长度
* 否则，将所有链表节点变成TreeNode，提高索引效率
*/
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);
}
}
get方法
/**
* 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;
}
/**
* Implements Map.get and related methods
*
* @param hash hash for key
* @param key the key
* @return the node, or null if none
*/
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) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
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;
}
根据key的hash值和key，可以唯一确定一个value，下面我们来看看get方法执行的逻辑
-
根据key计算hash值
-
根据hash值和key 确定所需要返回的结果，如果不存在，则返回空
containsKey方法
/**
* Returns <tt>true</tt> if this map contains a mapping for the
* specified key.
*
* @param key The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return getNode(hash(key), key) != null;
}
containsKey方法实际也是调用getNode方法实现的，如果key对应的value不存在则返回false
containsValue方法
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
*/
public boolean containsValue(Object value) {
Node<K,V>[] tab; V v;
if ((tab = table) != null && size > 0) {
for (int i = 0; i < tab.length; ++i) {
for (Node<K,V> e = tab[i]; e != null; e = e.next) {
if ((v = e.value) == value ||
(value != null && value.equals(v)))
return true;
}
}
}
return false;
}
containsValue方法的话需要遍历对象所有的value，遇到value相等的，则返回true
remove方法
/**
* Implements Map.remove and related methods
*
* @param key的hash值
* @param key值
* @param 需要remove 的value，
* @param 为true时候，当value相等的时候才remove
* @param 如果为false 的时候，不会移动其他节点。
* @return 返回被移除的Node，或者返回null
*/
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 && //如果定位到的第一个元素符合条件，则跳出if else
((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 {//定位到的第一个Node元素不符合条件，则遍历其链表
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
//移除符合要求的节点，将链表重新连接起来
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;
//修改次数加1
++modCount;
//当前的key-value 对数减一
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
执行逻辑：
- 根据key得到key的hash值
- 根据key 和hash值定位需要remove的Node
- 将Node从对应的链表移除，然后再将Node 前后的节点对接起来
- 返回被移除 的Node
- key-value的数量减一，修改次数加一
replace方法
@Override
public boolean replace(K key, V oldValue, V newValue) {
Node<K,V> e; V v;
if ((e = getNode(hash(key), key)) != null &&
((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) {
e.value = newValue;
afterNodeAccess(e);
return true;
}
return false;
}
@Override
public V replace(K key, V value) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) != null) {
V oldValue = e.value;
e.value = value;
afterNodeAccess(e);
return oldValue;
}
return null;
}
replace(K key, V oldValue, V newValue)
根据key和value定位到Node，然后将Node中的value用新value 替换，返回旧的value，否则返回空。
replace(K key, V value)
根据key定位到Node，然后将Node中的value 替换，返回旧的value，否则返回空
clear方法
/**
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
*/
public void clear() {
Node<K,V>[] tab;
modCount++;
if ((tab = table) != null && size > 0) {
size = 0;
for (int i = 0; i < tab.length; ++i)
tab[i] = null;
}
}
clear 方法将每个数组元素置空
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