java中HashCode的功用和Map的实现结构

2012-11-01

java中HashCode的作用和Map的实现结构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Valu

java中HashCode的作用和Map的实现结构

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

?TreeMap? 是用二叉树结构存储的，根据key找value的时间复杂度是o(以2为底，n的对数）

查找和插入的性能都没有hashmap好，但是可以实现key的有序存放。所以增加了hashMap的类。

二叉树的中序遍历就是从小到大的顺序排列。

1 楼 lovejavaei 2009-06-28 理解的还不够深入，再往下研究，会有更多的收获 2 楼 liu0107613 2009-06-28 非常感谢楼主的支持，刚开始研究这方面，很多方面的知识还需要学习，请楼主多多包涵。
我会继续努力的。。。。 3 楼 mycigar 2009-07-01 <div class="quote_title">liu0107613 写道</div>
<div class="quote_div">
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
?
</div>
?
原来看数据结构的时候，我觉得没啥大用，后来看Java的时候，发现数据结构的重要性了。

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java中HashCode的功用和Map的实现结构