HashMap源码分析
继承和接口
- 继承
AbstractMap.java - 实现接口
Map，Cloneable，Serializable
静态变量
- 初试容量默认是16 -必须是2的次方
/** * The default initial capacity - MUST be a power of two. */
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
- 最大容量
如果具有参数的任一构造函数隐式指定较高值，则使用最大容量。必须是2的次方，且小于2^30
/** * 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. */
static final int MAXIMUM_CAPACITY = 1 << 30;
- 默认加载因子 0.75
/** * The load factor used when none specified in constructor. */
static final float DEFAULT_LOAD_FACTOR = 0.75f;
- 阈值TREEIFY_THRESHOLD
当存储的节点数目达到该值时，链表转换为红黑树数据结构。
/** * 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. */
static final int TREEIFY_THRESHOLD = 8;
- UNTREEIFY_THRESHOLD
/** * 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. */
static final int UNTREEIFY_THRESHOLD = 6;
- MIN_TREEIFY_CAPACITY
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。
/** * 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. */
static final int MIN_TREEIFY_CAPACITY = 64;
成员变量
- table 数组
在第一次使用时初始化，动态扩容，长度总是为2的次方。
/** * 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.) */
transient Node<K,V>[] table;
- entrySet
/** * Holds cached entrySet(). Note that AbstractMap fields are used * for keySet() and values(). */
transient Set<Map.Entry<K,V>> entrySet;
- size
记录Map中节点个数
/** * The number of key-value mappings contained in this map. */
transient int size;
- modCount
顾名思义就是修改次数，当HashMap有remove,add等操作时，或者其他方式修改了内部结构时，如rehash，都会有记录次数。 此字段用于在HashMap的fail-fast Collection-views上生成迭代器。
HashMap 不是线程安全的，因此如果在使用迭代器的过程中有其他线程修改了map，那么将抛出ConcurrentModificationException，这就是所谓fail-fast策略。
/** * 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). */
transient int modCount;
- threshold 临界值
threshold = capacity * load factor；
当size达到threshold时，HashMap就会自动扩容（resize）
/** * 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.)
int threshold;
- loadFactor 加载因子
在初始化对象时，不指定则默认为0.75f，
/** * The load factor for the hash table. * * @serial */
final float loadFactor;
关键方法
1. hashCode()
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
2. equals()
Objects.equals和Object.equals都是针对地址比较，即判断两个引用对象是不是同一个对象。（String中的equals是重写后的。）
public final boolean equals(Object o) {
if (o == this)
return true;
// 判断是否为实例对象
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
/*其中Objects.equals*/
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
/*其中Object.equals*/
public boolean equals(Object obj) {
return (this == obj);
}
3. hash()
从方法实现中看出：
- 键为空时，hash值为0；同时可说明，null只能存放一个。
- 键非空时，先计算出object.hashCode()的结果h，将h其右移后再与object.hashCode()值异或。
（>>> : 无符号右移，忽略符号位，空位都以0补齐）
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
4. putMapEntries()
/** * 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
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)
resize();
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);
}
}
}
5. get()
根据键（key）获取值
先计算key的hash值，索引到数组的位置，
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) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
// 此处的equals比较的是对象的hash值，而不是HashMap中计算出来的hash值
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)//判断是否转化为红黑树存储，使用getTreeNode
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do { //链表结构，顺序查找
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
6. put()
- 如果table为空，调用resize初始化；
- 如果根据hash到的table节点为空，无链表挂载，则初始化链表节点信息；
- 如果该键存在，直接替换value；不存在，则判断是否为红黑树结构，插入红黑树节点
- 如果是链表结构，插入节点，判断是否需要转成红黑树；
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
// put的具体实现
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)
// 把节点添加到树中
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
// 这时候就是链表结构了，要把待插入元素挂在链尾
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
// 由于初始即为p.next，所以当插入第9个元素才会树化
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 找到了对应元素，就可以停止了
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
// 继续向后
p = e;
}
}
// e就是被替换出来的元素，这时候就是修改元素值
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;
//是否扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
7. resize()
- HashMap中的table不为空：
容量已经达到最大值，resize扩容失败；
容量增加为原来2倍，依然未到最大值，同时原来的容量大于默认初试容量，threshold也扩大为2倍。 - table为空，初始化了threshold，则新容量赋值为threshold的值。
- 默认为空时，即第一次初始化，赋值默认的容量capcitty和threshold
- 申请扩容后的新的数组table，如果HashMap不为空，则复制原来的节点到现在的table中，同时清除Old table的引用，以便GC回收。
在复制的过程中，因为每个table位是链表，重新计算hash，是否位置发生变化，实质只将引用知道头节点即可；
如果红黑树结构存储，要做一个split操作，因为threshold也扩大了，此时数量可能小于threshold，需要转换为链表结构。
/** * 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"})
// 申请扩容后的新的数组table
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;
// 得到的是 元素的在数组中的位置是否需要移动
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;
}
KeySet
顾名思义，键key的集合
官方释义：Map中所有key的集合视图
提供的删除操作：Iterator.remove、 Set.remove、 removeAll、retainAll
不支持添加操作：add，addAll
public Set<K> keySet() {
Set<K> ks;
return (ks = keySet) == null ? (keySet = new KeySet()) : ks;
}
Values
Map中所有value值的视图，操作同KeySet
public Collection<V> values() {
Collection<V> vs;
return (vs = values) == null ? (values = new Values()) : vs;
}
EntrySet
Map中Entry的集合视图，即所有key-value的集合。
操作同KeySet。
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es;
return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}
使用keySet, valueSet, entrySet，如果对该 set 进行迭代的同时修改了映射（通过迭代器自己的 remove 操作除外），则迭代结果是不确定的。
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator’s own remove operation, or through the
* setValue operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the Iterator.remove,
* Set.remove, removeAll, retainAll and
* clear operations. It does not support the
* add or addAll operations.
*
* @return a set view of the mappings contained in this map
*/
