public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
API
//返回指定键映射到的值，或
//如果此映射不包含密钥的映射，则代码默认值值。
default V getOrDefault(Object key, V defaultValue) {
V v;
return (((v = get(key)) != null) || containsKey(key))
? v
: defaultValue;
}
内部类
1.Node
//成员变量
final int hash;
final K key;
V value;
Node<K,V> next;
构造函数，get/set不多说
//获取hash值
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
//重写的equals方法
public final boolean equals(Object o) {
if (o == this)
return true;
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;
}
}
2.KeySet：Map的key值集合set
final class KeySet extends AbstractSet<K>
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks == null) {
ks = new KeySet();
keySet = ks;
}
return ks;
}
transient Set<K> keySet;//AbstractMap中
可用方法
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<K> iterator() { return new KeyIterator(); }
public final boolean contains(Object o) { return containsKey(o); }
public final boolean remove(Object key) { return removeNode(hash(key), key, null, false, true) != null; }
public final Spliterator<K> spliterator(){ ... }
public final void forEach(Consumer<? super K> action){ ... }
3.Values：map中value集合
public Collection<V> values() {
Collection<V> vs = values;
if (vs == null) {
vs = new Values();
values = vs;
}
return vs;
}
final class Values extends AbstractCollection<V>
可用方法
{
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<V> iterator() { return new ValueIterator(); }
public final boolean contains(Object o) { return containsValue(o); }
public final Spliterator<V> spliterator() { ... }
public final void forEach(Consumer<? super V> action) { ... }
}
4.EntrySet: Set<Map.Entry<K,V>>类型
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es;
return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
}
final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { HashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() { return new EntryIterator(); }
public final boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Node<K,V> candidate = getNode(hash(key), key);
return candidate != null && candidate.equals(e);
}
public final boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Object value = e.getValue();
return removeNode(hash(key), key, value, true, true) != null;
}
return false;
}
public final Spliterator<Map.Entry<K,V>> spliterator() { return new EntrySpliterator<>(HashMap.this, 0, -1, 0, 0); }
public final void forEach(Consumer<? super Map.Entry<K,V>> action) { ... }
5.iterators
// iterators
abstract class HashIterator {
Node<K,V> next; // next entry to return
Node<K,V> current; // current entry
int expectedModCount; // for fast-fail
int index; // current slot
HashIterator() {
expectedModCount = modCount;
Node<K,V>[] t = table;
current = next = null;
index = 0;
if (t != null && size > 0) { // advance to first entry
do {} while (index < t.length && (next = t[index++]) == null);
}
}
public final boolean hasNext() {
return next != null;
}
final Node<K,V> nextNode() {
Node<K,V>[] t;
Node<K,V> e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
if ((next = (current = e).next) == null && (t = table) != null) {
do {} while (index < t.length && (next = t[index++]) == null);
}
return e;
}
public final void remove() {
Node<K,V> p = current;
if (p == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
current = null;
K key = p.key;
removeNode(hash(key), key, null, false, false);
expectedModCount = modCount;
}
}
final class KeyIterator extends HashIterator
implements Iterator<K> {
public final K next() { return nextNode().key; }
}
final class ValueIterator extends HashIterator
implements Iterator<V> {
public final V next() { return nextNode().value; }
}
final class EntryIterator extends HashIterator
implements Iterator<Map.Entry<K,V>> {
public final Map.Entry<K,V> next() { return nextNode(); }
}
6.spliterators
static class HashMapSpliterator<K,V> {...}
static final class KeySpliterator<K,V> extends HashMapSpliterator<K,V> implements Spliterator<K> {...}
static final class ValueSpliterator<K,V> extends HashMapSpliterator<K,V> implements Spliterator<V>{...}
static final class EntrySpliterator<K,V>extends HashMapSpliterator<K,V> implements Spliterator<Map.Entry<K,V>>{...}
7.TreeNode
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V>
另见文章分析
字段Fields
/**
*Node数组，首次使用时初始化；长度是2的幂次（允许为0）
*/
transient Node<K,V>[] table;
/**
* 保存entrySet的缓存.
* Note that AbstractMap fields are used for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
/**
* 保存在map中的key-value键值对个数
*/
transient int size;
/**
*记录修改次数
*/
transient int modCount;
/**
* 阈值
* threshold表示当HashMap的size大于threshold时会执行resize操作
* threshold=capacity*loadFactor
*/
int threshold;
/**
* hash table的负载系数，计算HashMap的实时装载因子的方法为：size/capacity
*/
final float loadFactor;
private static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量
private static final int DEFAULT_CAPACITY = 16;//初始容量
构造方法
// 初始容量 负载系数
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);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // DEFAULT_LOAD_FACTOR=0.75f
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
//实现Map.putAll和构造方法
//evict=false，用在构造方法；evict=true，其他情况
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size，table是否初始化？
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);//static final int MAXIMUM_CAPACITY = 1 << 30; 约等于1 073 741 824
if (t > threshold)
threshold = tableSizeFor(t);
}
// 扩容
else if (s > threshold)
resize();
// 将m中的所有元素添加至HashMap中
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);
}
}
}
hash
//计算key.hashCode()
static final int hash(Object key) {
int h;
//key.hashCode是一个32位带符号的int
//最后得到的结果是要和key.hashCode的高16位异或
//使得高16位参与运算
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
//数组下标计算方法
//有公式 hash % n = （n-1）& hash，n为数组长度且为2的倍数
//作为“低位掩码”
tab[index = (n - 1) & hash]
image.png
put
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//table没有初始化或者长度为0，扩容
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;//resize() TODO
//(n - 1) & hash通过key的hash值计算数组存放位置
if ((p = tab[i = (n - 1) & hash]) == null)
//为空，放入新结点
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
//数组该位置第一个节点hash相等，key匹配
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//此节点红黑树结构
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
//此节点链表结构
else {
for (int binCount = 0; ; ++binCount) {
//到链表尾插入新节点
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//达到阈值，转化为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//hash,key 匹配跳出循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//和e=p.next组合遍历链表
p = e;
}
}
//key匹配，更新value
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
get
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) {
//检查n - 1) & hash的第一个节点
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//下一个
if ((e = first.next) != null) {
//红黑树
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
//链表
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
resize
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//原来的容量/长度
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;
}
//newCap = oldCap << 1变为2倍
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;
//移动元素
@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 链表保留顺序 TODO
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;
}
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