Differences between HashMap and Hashtable

最新推荐文章于 2025-08-20 17:55:57 发布

skyer_lei

最新推荐文章于 2025-08-20 17:55:57 发布

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分类专栏： java 文章标签： hashmap collections iterator object null java

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本文详细对比了HashMap与Hashtable这两种Java集合框架中常用的数据结构。主要从同步性、空值支持、迭代方式等方面进行了阐述，并深入探讨了它们在实现上的不同之处。

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HashMap is used in the new framework instead of HashTable, Hashtable is not part of the Java Collections Framework - it just provides a similar API.

There are several differences between HashMap and Hashtable:

Hashtable is synchronized, whereas HashMap is not.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values
Enumeration is used in HashTable, but Iterator used in HashMap.

The upper are differences in appearance, there are also several differences in implement :

1. The default size of hash array is 11 in Hashtable , the size to add is old*2+1. And the default size is 16 in HashMap, the size to add is 2^x
2. The hashCode is different. HashTable directly uses the hashCOde of the key object:

int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;

And HashMap recomputs the hashCode:

int hash = hash(k);
int i = indexFor(hash, table.length);

static int hash(Object x) {
int h = x.hashCode();

h += ~(h << 9);
h ^= (h >>> 14);
h += (h << 4);
h ^= (h >>> 10);
return h;
}
static int indexFor(int h, int length) {
return h & (length-1);
}

原文：http://gaolei.me/archives/58