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。