HashMap原理-1.7
阿新 • • 發佈:2017-07-14
ria 裝載 get方法 非線程安全 factor sun off tom dentry
之所以分兩篇文章記錄,因為之前一直看的1.7的源碼,而且網上很多的都是關於1.7的,今天在1.8上打開源碼一看,居然懵了。 沒想到1.8的實現變化這麽大。所有特地拿一篇文章來記錄下。
本章只介紹1.7的情況
1.HashMap存儲結構
哈希表是由數組+鏈表組成的,一個長度為16的數組中,每個元素存儲的是一個鏈表的頭結點。那麽這些元素是按照什麽樣的規則存儲到數組中呢。一般情況是通過hash(key)%len獲得,也就是元素的key的哈希值對數組長度取模得到。比如上述哈希表中,12%16=12,28%16=12,108%16=12,140%16=12。所以12、28、108以及140都存儲在數組下標為12的位置。當下標有沖突的時候,就需要解決沖突,目前解決沖突的方法有:
- 開放定址法(線性探測再散列,二次探測再散列,偽隨機探測再散列)
- 再哈希法
- 鏈地址法
- 建立一個公共溢出區
Java中的hashmap的解決辦法就是采用的鏈地址法。
源碼分析
數據結構
/**
* The table, resized as necessary. Length MUST Always be a power of two.,長度必須為2的指數次,為什麽?其中非常重要的原因就是為了hash的平均分布
*/
transient Entry<K,V>[] table; //定義table數組,類型為Entry
static class Entry<K,V> implements Map.Entry<K,V> { //entry,即鏈表結構,存放key,value,next節點
final K key;
V value;
Entry<K,V> next;
int hash;
2.HashMap put方法
打個比方, 第一個鍵值對A進來,通過計算其key的hash得到的index=0,記做:Entry[0] = A。一會後又進來一個鍵值對B,通過計算其index也等於0,現在怎麽辦?HashMap會這樣做:B.next = A,Entry[0] = B,如果又進來C,index也等於0,那麽C.next = B,Entry[0] = C;這樣我們發現index=0的地方其實存取了A,B,C三個鍵值對,他們通過next這個屬性鏈接在一起。所以疑問不用擔心。也就是說數組中存儲的是最後插入的元素。
public V put(K key, V value) { if (key == null) return putForNullKey(value); //空值的put,放到index0的位置 int hash = hash(key); int i = indexFor(hash, table.length); //獲取Hash值對應的數組index for (Entry<K,V> e = table[i]; e != null; e = e.next) { //獲取數組index的entry,如果不為空則尋找下一個節點,直到到entry為null Object k; if (e.hash == hash && ((k = e.key) == key || key.equals(k))) { //如果key相同,則替換key的value,或者 V oldValue = e.value; e.value = value; e.recordAccess(this); return oldValue; } } modCount++; addEntry(hash, key, value, i); //添加元素 return null; } void addEntry(int hash, K key, V value, int bucketIndex) { if ((size >= threshold) && (null != table[bucketIndex])) { //如果超過裝載因子的最大值,則需要對數組進行擴容 resize(2 * table.length); hash = (null != key) ? hash(key) : 0; bucketIndex = indexFor(hash, table.length); //獲取hash對應的新的index } createEntry(hash, key, value, bucketIndex); //創建entry } /** * Like addEntry except that this version is used when creating entries * as part of Map construction or "pseudo-construction" (cloning, * deserialization). This version needn‘t worry about resizing the table. * * Subclass overrides this to alter the behavior of HashMap(Map), * clone, and readObject. */ void createEntry(int hash, K key, V value, int bucketIndex) { Entry<K,V> e = table[bucketIndex]; //將當前數組index的節點保存 table[bucketIndex] = new Entry<>(hash, key, value, e); //數組index的值更新為新節點,之前的節點變為新節點的next節點; size++; }
3.HashMap get方法
public V get(Object key) { if (key == null) return getForNullKey(); //null值獲取 Entry<K,V> entry = getEntry(key); return null == entry ? null : entry.getValue(); }
final Entry<K,V> getEntry(Object key) { int hash = (key == null) ? 0 : hash(key); //求key的hash值 for (Entry<K,V> e = table[indexFor(hash, table.length)]; //在數組中查找hash值對應的下標 e != null; e = e.next) {//如果entry不為null的情況下,繼續遍歷鏈表的下一個節點,直到查找到key相同,且hash值相同的,返回搜索結果 Object k; if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) return e; } return null; }
/** * Offloaded version of get() to look up null keys. Null keys map * to index 0. This null case is split out into separate methods * for the sake of performance in the two most commonly used * operations (get and put), but incorporated with conditionals in * others. */ private V getForNullKey() { for (Entry<K,V> e = table[0]; e != null; e = e.next) { if (e.key == null) return e.value; } return null; }
4.HashMap擴容
/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
**/
void resize(int newCapacity) { Entry[] oldTable = table; int oldCapacity = oldTable.length; if (oldCapacity == MAXIMUM_CAPACITY) { //判斷是否達到最大容量 threshold = Integer.MAX_VALUE; return; } Entry[] newTable = new Entry[newCapacity]; boolean oldAltHashing = useAltHashing; useAltHashing |= sun.misc.VM.isBooted() && (newCapacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD); boolean rehash = oldAltHashing ^ useAltHashing; transfer(newTable, rehash); //擴容,將原來的數據添加到新的數組中 table = newTable; threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1); } /** * Transfers all entries from current table to newTable. */ void transfer(Entry[] newTable, boolean rehash) { int newCapacity = newTable.length; for (Entry<K,V> e : table) { while(null != e) { Entry<K,V> next = e.next; if (rehash) { e.hash = null == e.key ? 0 : hash(e.key); } int i = indexFor(e.hash, newCapacity); e.next = newTable[i]; newTable[i] = e; e = next; } } }
5.HashMap缺點
a.不支持多線程,即非線程安全;
HashMap原理-1.7