java.util.HashMap
本文目录：
- 一、HashMap 的特点概述和说明
- 二、HashMap 的内部实现：从内部属性和构造函数说起
- 三、HashMap 的 put 操作
- 四、HashMap 的扩容
- 五、HashMap 的 get 操作
- 六、HashMap 的 remove 操作
- 七、参考
一、HashMap的特点概述和说明
| 关注点 | HashMap的相关结论 |
| 是否允许空的 key | 是 |
| 是否允许重复的 key | 否，实际上可能会进行覆盖更新 |
| 元素有序：读取数据和存放数据的顺序一致 | 否，读取和存放都无序 |
| 是否线程安全 | 否 |
| 通过 key 进行随机访问的效率 | 较快 |
| 添加元素的效率 |
较快 涉及扩容、列表转红黑树、遍历列表时相对慢 |
| 删除元素的效率 | 较快 |
这里主要提几点：
- Java8 中 HashMap 源码的大方向就是：数组 + 单向链表(数组的元素，Node 实例，包含四个属性：key, value, hash 值和用于单向链表的 next) + 红黑树(链表超过8个元素且总元素个数超过 64 时转换为红黑树)，对于hash冲突的元素，使用链表进行存储，每次存储在链表末尾。
- capacity：当前数组容量，默认值是 16，自动扩容，但始终保持 2^n，即扩容后数组大小为当前的 2 倍。
- loadFactor：负载因子，默认为 0.75。
- threshold：扩容的阈值，等于 capacity * loadFactor，当元素实际个数 size 大于等于 threshold 时，进行扩容。
Java8的 HashMap，最大的改变，是使用了数组 + 链表 + 红黑树。当链表中的元素达到了 8 个且总元素个数超过64个时，会将链表转换为红黑树，在这些位置进行查找的时候可以由原来的耗时 O(N)，降低到时间复杂度为 O(logN)。另附上简要示意图：
二、HashMap的内部实现：从内部属性和构造函数说起
1、常用的类属性
常用的类属性如下，比如默认容量、负载因子等。
/**
* The default initial capacity - MUST be a power of two.
* 默认的初始容量 16 = 2 ^ 4
*/
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.
* 允许的最大容量
*/
static final int MAXIMUM_CAPACITY = 1 << 30; /**
* The load factor used when none specified in constructor.
* 默认的负载因子
*/
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.
* 达到需要转化为红黑树时的链表容量阈值
*/
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.
* 红黑树转回链表的下限阈值
*/
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.
* 达到需要转化为红黑树时的Map总容量最低阈值
*/
static final int MIN_TREEIFY_CAPACITY = 64;
2、实例属性
实例属性，包括内部实际存储元素的数组、Map 的实际大小、实际的负载因子、修改次数等
/**
* 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.)
* 内部实际存储元素的数组
*/
transient Node<K,V>[] table; /**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet; /**
* The number of key-value mappings contained in this map.
* map的大小，即实际的元素个数
*/
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).
* 修改次数，用于 fail-fast 机制校验
*/
transient int modCount; /**
* The next size value at which to resize (capacity * load factor).
* 容量阈值，元素个数超过阈值是会进行扩容
* @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.)
int threshold; /**
* The load factor for the hash table.
* 负载因子
* @serial
*/
final float loadFactor;
3、节点内部类
这个是实际的元素存储节点。
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
...
}
4、构造函数
几个构造函数中最主要的还是要设定初始的负载因子 loadFactor。
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
* 指定 初始容量 和 负载因子
* @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);
// 设定负载因子
this.loadFactor = loadFactor;
// 设定阈值为 大于等于指定初始容量且最小 的 2^n
this.threshold = tableSizeFor(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) {
// 使用默认的负载因子 0.75
this(initialCapacity, DEFAULT_LOAD_FACTOR);
} /**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
// 使用默认负载因子 0.75，而其他使用默认值，包括 阈值 threshold = 0
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
} /**
* 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) {
// 使用默认的负载因子 0.75
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
} /**
* Implements Map.putAll and Map constructor
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size，说明还没有初始化，进行阈值的预设定
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t); // 大于等于指定容量且最小 的 2^n
}
else if (s > threshold)
resize(); // 先进行一次扩容
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); // 内部还会检查是否需要扩容
}
}
}
三、HashMap 的 put 操作
put 操作的源码如下：
/**
* 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 hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* onlyIfAbsent 为 true 时表示不修改已存在的（key 对应的）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;
// 因为前面构造器都没有对数组 table 进行初始化，因此第一次进行put操作时需要进行扩容
// 由于 table 本身为 null，最终也只是新建大小为默认容量 16 的数组而已
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 当前 key 对应的具体数组下标，如果对应数组元素为 null，则直接初始化 Node 元素并设置即可
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else { // 说明该下标位置存在相关的元素数据
Node<K,V> e; K k;
// 数组对应位置的元素的 key 与 put 操作的 key “相等”
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
// 否则需要判断是链表还是红黑树来执行 put 操作
// 红黑树节点则按照红黑树的插值方法进行
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// 执行到这里说明数组的该位置是一个链表
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// 插入到链表的最后位置
p.next = newNode(hash, key, value, null);
// binCount 为 7 时触发红黑树转化，明显此时0-6已经有节点了，
// 再加上原来的 tab[i]（相当于 -1），新插入的是链表的第9个位置
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// put 操作的 key 与链表中的该位置的 key “相等”
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
// 说明在链表中存在并找到了与 key 一致的节点
break;
p = e;
}
}
// 存在旧的 key 与要 put 的 key 一致，考虑是否进行值覆盖，然后返回旧值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
// 记录修改次数
++modCount;
// 超过阈值，则进行扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
四、HashMap 的扩容
HashMap扩容的过程也就是内部数组的扩容，源码如下：
/**
* 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.
* 初始化 或者 倍增扩容
* @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;
}
// 将数组大小扩大一倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold 阈值扩大一倍
}
// 有进行初始化容量的设定时会设置 threshold，此时的第一次 put 操作进入这里
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
// 没有指定初始容量的 new HashMap()，第一次 put 操作进入这里
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;
// 初始化数组或者创建容量翻倍的数组
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) { // 初始化的则会跳过这里直接返回
// 遍历数组进行节点的迁移
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
// 该位置是单个节点元素，不是数组也不是红黑树，则直接迁移
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode) // 红黑树时的节点迁移
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
// 链表情况下的迁移
// 将链表拆成两个链表，放到新的数组中，并且保留原来的先后顺序
// loHead、loTail 对应一条链表，hiHead、hiTail 则对应另一条链表
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;
// 第一条链表
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
// 第二条链表的新的位置是 j + oldCap，也比较好理解
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
五、HashMap 的 get 操作
get 操作比较直接，流程大致如下：
- 计算 key 的 hash 值，根据 hash 找到对应的数组下标即 (table.length – 1) & hash；
- 判断对应下标处的节点元素是否正好是要寻找，如是即返回；否，继续下一步；
- 如果是红黑树，则用红黑树的方法获取数据；
- 如果是链表，则按照链表的方式寻找相应的节点；
- 找不到的，返回 null。
/**
* 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) {
// 第一个节点的判断
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
// 红黑树的走法
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
// 链表的遍历
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
六、HashMap 的 remove 操作
源码走起：
/**
* Removes the mapping for the specified key from this map if present.
*
* @param key key whose mapping is to be removed from the map
* @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 remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
} /**
* 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) { // 先找到对应的数组下标
Node<K,V> node = null, e; K k; V v;
// 总是先判断第一个是否是要找的
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
else if ((e = p.next) != null) {
// 红黑树的找法
if (p instanceof TreeNode)
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else { // 链表的找法
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)))) {
// 红黑树按照红黑树的方式移除
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
// 刚好是数组节点的移除
else if (node == p)
tab[index] = node.next;
// 链表的移除
else
p.next = node.next;
// 记录 修改次数 和元素节点的 实际个数
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
七、参考
Java8 中 HashMap 的相关基本操作源码介绍，这里也可以直接参考【Java7/8中的 HashMap 和 ConcurrentHashMap 全解析】，介绍得还是挺详细的。
备注：关于红黑树和ConcurrentHashMap，有待后续的进一步研究。
