java中HashCode的作用和Map的实现结构

核心提示：Map 是一种数据结构，用来实现key和value 的映射。通过Key可以找到Value。实现类包括 HashMap，LinkedHashMap，TreeMap /** * Applies a supplemental hash function to a given hashCode, which * defends against poor quality hash functions. This is cr

 

 

Map 是一种数据结构，用来实现key和value 的映射。通过Key可以找到Value。实现类包括

HashMap，LinkedHashMap，TreeMap

 /**
  * Applies a supplemental hash function to a given hashCode, which
  * defends against poor quality hash functions.  This is critical
  * because HashMap uses power-of-two length hash tables, that
  * otherwise encounter collisions for hashCodes that do not differ
  * in lower bits. Note: Null keys always map to hash 0, thus index 0.
  */
 static int hash(int h) {
     // This function ensures that hashCodes that differ only by
     // constant multiples at each bit position have a bounded
     // number of collisions (approximately 8 at default load factor).
     h ^= (h >>> 20) ^ (h >>> 12);
     return h ^ (h >>> 7) ^ (h >>> 4);
 }

 /**
  * Returns index for hash code h.
  */
 static int indexFor(int h, int length) {
     return h & (length-1);
 }

 /**
  * 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) {
     if (key == null)
         return putForNullKey(value);
     int hash = hash(key.hashCode());
     int i = indexFor(hash, table.length);
     for (Entry<K,V> e = table[i]; e != null; e = e.next) {
         Object k;
         if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
             V oldValue = e.value;
             e.value = value;
             e.recordAccess(this);
             return oldValue;
         }
     }

     modCount++;
     addEntry(hash, key, value, i);
     return null;
 }

HashMap 是以数组的结构，用哈希函数值跟数组的长度做位与运算，获取对应数组的索引。浏览key值时，不保证顺序。
LinkedHashMap 是以双向列表的结构做实现的，浏览key值时候，可以保证顺序。LinkedHashMap继承HashMap ，不同的是数据存储结构。

TreeMap 是以二叉树实现的Map接口。Map中的key值按照从小到大的顺序排列。key要实现comparable。