1 概述
採用陣列+連結串列的形式對資料進行儲存。 key可以為null。 執行緒非安全。
public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
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HashMap繼承了AbstractMap<K,V>抽象類,實現了Map<K,V>介面。
給定表M,存在函式f(key),對任意給定的關鍵字值key,代入函式後若能得到包含該關鍵字的記錄在表中的地址,則稱表M為雜湊(Hash)表,函式f(key)為雜湊(Hash) 函式。
2 雜湊函式
常的雜湊函式有:
- 直接定址法:取關鍵字或關鍵字的某個線性函式值為雜湊地址。即H(key)=key或H(key) = a?key + b,其中a和b為常數(這種雜湊函式叫做自身函式)
- 數字分析法:分析一組資料,比如一組員工的出生年月日,這時我們發現出生年月日的前幾位數字大體相同,這樣的話,出現衝突的機率就會很大,但是我們發現年月日的後幾位表示月份和具體日期的數字差別很大,如果用後面的數字來構成雜湊地址,則衝突的機率會明顯降低。因此數字分析法就是找出數字的規律,儘可能利用這些資料來構造衝突機率較低的雜湊地址。
- 平方取中法:取關鍵字平方後的中間幾位作為雜湊地址。
- 摺疊法:將關鍵字分割成位數相同的幾部分,最後一部分位數可以不同,然後取這幾部分的疊加和(去除進位)作為雜湊地址。
- 隨機數法:選擇一隨機函式,取關鍵字的隨機值作為雜湊地址,通常用於關鍵字長度不同的場合。
- 除留餘數法:取關鍵字被某個不大於雜湊表表長m的數p除後所得的餘數為雜湊地址。即 H(key) = key MOD p, p<=m。不僅可以對關鍵字直接取模,也可在摺疊、平方取中等運算之後取模。對p的選擇很重要,一般取素數或m,若p選的不好,容易產生同義詞。
如果兩個不同的元素,通過雜湊函式得出的實際儲存地址相同怎麼辦?也就是說,當我們對某個元素進行雜湊運算,得到一個儲存地址,然後要進行插入的時候,發現已經被其他元素佔用了,其實這就是所謂的雜湊衝突,也叫雜湊碰撞。前面我們提到過,雜湊函式的設計至關重要,好的雜湊函式會盡可能地保證 計算簡單和雜湊地址分佈均勻,但是,我們需要清楚的是,陣列是一塊連續的固定長度的記憶體空間,再好的雜湊函式也不能保證得到的儲存地址絕對不發生衝突。那麼雜湊衝突如何解決呢?雜湊衝突的解決方案有多種:開放定址法(發生衝突,繼續尋找下一塊未被佔用的儲存地址),再雜湊函式法,鏈地址法,而HashMap即是採用了鏈地址法,也就是陣列+連結串列的方式。 簡單來說,HashMap由陣列+連結串列組成的,陣列是HashMap的主體,連結串列則是主要為了解決雜湊衝突而存在的,如果定位到的陣列位置不含連結串列(當前entry的next指向null),那麼對於查詢,新增等操作很快,僅需一次定址即可;如果定位到的陣列包含連結串列,對於新增操作,其時間複雜度為O(n),首先遍歷連結串列,存在即覆蓋,否則新增;對於查詢操作來講,仍需遍歷連結串列,然後通過key物件的equals方法逐一比對查詢。所以,效能考慮,HashMap中的連結串列出現越少,效能才會越好。
在JDK1.8中如果連結串列的長度大於8會將該連結串列轉換為紅黑樹。
圖中,紫色部分即代表雜湊表(雜湊陣列),陣列的每個元素都是一個單連結串列的頭節點,連結串列是用來解決衝突的,如果不同的key對映到了陣列的同一位置處,就將其放入單連結串列中。HashMap原始碼中計算hash值的方法:
/**
* Computes key.hashCode() and spreads (XORs) higher bits of hash
* to lower. Because the table uses power-of-two masking, sets of
* hashes that vary only in bits above the current mask will
* always collide. (Among known examples are sets of Float keys
* holding consecutive whole numbers in small tables.) So we
* apply a transform that spreads the impact of higher bits
* downward. There is a tradeoff between speed, utility, and
* quality of bit-spreading. Because many common sets of hashes
* are already reasonably distributed (so don't benefit from
* spreading), and because we use trees to handle large sets of
* collisions in bins, we just XOR some shifted bits in the
* cheapest possible way to reduce systematic lossage, as well as
* to incorporate impact of the highest bits that would otherwise
* never be used in index calculations because of table bounds.
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
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key.hashCode()函式呼叫的是key鍵值型別自帶的雜湊函式,返回int型雜湊值。 這個是擾動函式,為了減少雜湊碰撞。 具體可以參考知乎回答:JDK 原始碼中 HashMap 的 hash方法原理是什麼?
3 HashMap原始碼
初始容量16:
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
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最大容量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;
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載入因子係數0.75:
在容量的3/4的時候擴容。
/**
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
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HashMap底層實現了一個Node類,Node實現了Map.Entry介面:
static class Node<K,V> implements Map.Entry<K,V>
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Node陣列:
/**
* 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;
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hash方法:
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
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put方法:(如果key重複,更新value,返回老的value)
/**
* 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);
}
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putVal方法:
/**
* 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)
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)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
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)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
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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;
}
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擴容resize():
/**
* 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"})
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;
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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