title: “理解HashMap-基於java1.8”
url: “https://wsk1103.github.io/”
tags:
- Java
java -version ：jdk 1.8.0_191
HashMap
構造
類內參數，方法
實現
基於數組 + 鏈表實現。
當單條鏈表的個數達到8個時候，鏈表就會被轉換成紅黑樹。
鏈表時間複雜度O(n)，紅黑樹時間複雜度O(log n)
靜態參數
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
初始化時，默認數組的大小
/**
* 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;
數組的最大值，2^30。
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
默認負載因子，當新增的key在Map中超過 (Map中key的數量 * 負載因子) 的時候，就會擴容。
擴容是比較耗內存的，所以設計的時候，很多時候都要考慮一下Map的大小，防止頻繁擴容或者佔用無畏的空間。
/**
* 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;
當單個鏈表裏面的key個數超過8個時候，轉換成紅黑樹。
/**
* 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;
/**
* 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;
//超過這個值則以翻倍形式擴容(resize)
//threshold = capacity * load factor
int threshold;
map的閾值
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);
}
在存入到HashMap的時候，需要先計算一下hash值，然後根據hash值存放到對應的數組和鏈表中。
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
函數作用：高16bit不變，低16bit和高16bit做了一個異或
假設 h = hashCode() = 1111 1111 1111 1111 1111 0101 1010 0011
| 步數 | 操作 | 值 |
|---|---|---|
| 1 | h = hashCode() | 1111 1111 1111 1111 1111 0101 1010 0011 |
| 2 | h | 1111 1111 1111 1111 1111 0101 1010 0011 |
| 3 | h >>> 16 | 0000 0000 0000 0000 1111 1111 1111 1111 |
| 4 | h ^ (h >>> 16) | 1111 1111 1111 1111 0000 1010 0101 1100 |
| 5 | (n - 1) & hash | 0000 0000 0000 0000 0000 0000 0000 1111(15) 1111 1111 1111 1111 0000 1010 0101 1100 |
| 6 | 結果 | 1100(12) |
則結果 (n - 1) & hash 值爲12。
可以看出，HashMap是允許key爲null的，當key爲null的時候，hash值爲0。
注意：當key存放是一個對象的時候，一般情況下我們需要重寫這個對象的hashCode()方法，防止嚴重碰撞，或者鏈表/紅黑樹過長。
/**
* 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)
//重置HashMap
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//當數組tab中對應的位置爲空，則直接存放到該tab[]中。
//i = (n - 1) & hash 取模定位數組位置
tab[i] = newNode(hash, key, value, null);
else {
//已經存在該key的hash值或者碰撞衝突。
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
// 已經存在，保存起來，用於返回oldValue
e = p;
else if (p instanceof TreeNode)
//放入樹節點中，也就是已經是紅黑樹的情況。
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//循環列表
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
//放到列表尾部
p.next = newNode(hash, key, value, null);
//當列表個數超過8個的時候，轉換成紅黑樹。
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//已經存在該key
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//如果已經存在，替換，然後返回被替換前的值。
if (e != null) { // existing mapping for key
V oldValue = e.value;
//當參數onlyIfAbsent設置爲false的時候，即使相同的key，也不插入。
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);//這個沒有什麼用，空方法
return oldValue;
}
}
//修改的次數，該值用transient修飾，無法被序列號.
++modCount;
//當key的數量超過閾值時，翻倍並重新將key插入一個新的map中。
if (++size > threshold)
resize();
afterNodeInsertion(evict);//這個沒有什麼用，空方法
return null;
}
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;
}
get方法也是用hash值，快速定位到具體的數組位置。
/**
* 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))))
// 找到hash值相等 和 key也相等
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
//已經轉換成紅黑樹，紅黑樹搜索
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
//循環遍歷鏈表查找
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
//找不到
return null;
}
resize方法
擴容
每次擴容都需要重新分配數組，並把老的值再分配到新的數組，耗能比較高，所以應該儘量避免擴容。例如在初始化的時候，判斷該Map的key的數量可以會達到最大的數值，Map map = new HashMap(keyNum);
/**
* 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) {
//當數組tab的個數超過2^32的時候，已經不需要做什麼了。
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
//位運算，翻倍。
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"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//開始將舊的值存到新的map中
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;
}
初始化構造器
HashMap提供4中初始化的構造器
1. 參數 int initialCapacity, float loadFactor
initialCapacity：初始map的數組大小
loadFactor：負載因子
/**
* 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);
}
2. 參數int initialCapacity
initialCapacity：初始map的數組大小
/**
* 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);
}
3. 無參
/**
* 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
}
4. 參數 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);
}
循環遍歷Map
map.forEach((key,value)->{
System.out.println("key=" + key + " value=" + value);
});
其他說明
HashMap 和 HashTable 區別
- HashMap是unsynchronized，使用的時候要注意線程安全。
- HashMap允許key爲null，而HashTable不允許key爲null。
- HashTable線程安全的原理是在操作Map的方法上都加入synchronized。
HashMap 和 ConcurrentHashMap 區別
- HashMap是unsynchronized，使用的時候要注意線程安全。
- ConcurrentHashMap是synchronized，多線程的情況下，直接使用這個。
- ConcurrentHashMap 的原理是在操作每個數組對應的位置加鎖，而不是整個Map加鎖，所以性能比HashTabe好。
Fail-Fast機制
在多個線程同時操作同一個Map的時候，每次修改map，則modCount++，當檢測到modCount發生改變的時候，則拋出異常 throw new ConcurrentModificationException();
取模
如果數組tab的長度爲n，如果要對hash進行取模，一般的做法是
hash % n
但是HashMap的取模是
hash & (n - 1)
因爲n在初始化的時候，我們會把n定義爲2的次冪，所以n-1 的二進制是01111111…
hash & n-1 實際上就是取保留低位值，結果是在n的範圍內，類似取模運算。
假設 n = 16，hash = 1111 1111 1111 1111 1111 0101 1010 0011
| 步數 | 計算 | 值 |
|---|---|---|
| 1 | n -1 | 0000 0000 0000 0000 0000 0000 0000 0111(15) |
| 2 | hash | 1111 1111 1111 1111 1111 0101 1010 0011 |
| 3 | hash & n-1 | 0000 0000 0000 0000 0000 0000 0000 0011(3) |
當時當在初始化數組的時候，沒有設置爲2的次冪，那麼就和一般的取模運算一樣，沒有什麼性能改進。
使HashMap線程安全
如果不使用ConcurrentHashMap，那麼可以使用java.util包。
Map m = Collections.synchronizedMap(new HashMap(...));
同時Set,List也一樣。
HashTable
實現的接口和繼承的類和 HashMap 一致，裏面的方法，變量的定義也是基本一致的。
只是在操作數組和鏈表的時候，在所有的方法上都添加 synchronized 關鍵字。
例如 add方法
/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> method
* with a key that is equal to the original key.
*
* @param key the hashtable key
* @param value the value
* @return the previous value of the specified key in this hashtable,
* or <code>null</code> if it did not have one
* @exception NullPointerException if the key or value is
* <code>null</code>
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
從這裏就可以看出， HashTable 是不允許存儲的對象爲 null
並且 HashTable 中的鏈表是不會轉爲紅黑樹。
有趣的是， HashTable 的初始容量是 11，而 擴容操作 是 int newCapacity = (oldCapacity << 1) + 1 ，即*2+1 ，
計算hash也比較簡單
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;