Ordering of hashtable keys
in [Java Help]
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From: Boris Punk on 17 Jul 2010 02:55 Hi Could someone tell me what determines the order of the keys in a hashtable?
From: Patricia Shanahan on 17 Jul 2010 03:18 On 7/16/2010 11:55 PM, Boris Punk wrote: > Hi > > Could someone tell me what determines the order of the keys in a hashtable? > > Nothing you can rely on, unless you use a LinkedHashMap. The order can even change due to rehashing when a new item is added. A hash table divides items up into a number of buckets, using the key's hash code to select a bucket. A search only looks at the items in the bucket corresponding to the hash code of the key. The iteration order depends on the current assignment of keys to buckets. Patricia
From: Kevin McMurtrie on 17 Jul 2010 03:27 In article <1uc0o.87059$hS4.7243(a)newsfe26.ams2>, "Boris Punk" <khgfhf(a)hmjggg.com> wrote: > Hi > > Could someone tell me what determines the order of the keys in a hashtable? If the JavaDoc doesn't state an order then it's a mix of JVM version, initial table size, insertion order, and _lots_ of bit hashing. After all, a hashing algorithm works best when it eliminates bucket mapping patterns. Use something like LinkedHashMap or TreeMap if order matters. -- I won't see Google Groups replies because I must filter them as spam
From: Lew on 17 Jul 2010 11:02 Boris Punk wrote: >> Could someone tell me what determines the order of the keys in a hashtable? Kevin McMurtrie wrote: > If the JavaDoc doesn't state an order then it's a mix of JVM version, > initial table size, insertion order, and _lots_ of bit hashing. After > all, a hashing algorithm works best when it eliminates bucket mapping > patterns. > > Use something like LinkedHashMap or TreeMap if order matters. And don't use 'java.util.Hashtable' even when order doesn't matter. It's been out of date since '98. -- Lew
From: markspace on 17 Jul 2010 12:26
Boris Punk wrote: > Hi > > Could someone tell me what determines the order of the keys in a hashtable? Hash tables work by distributing their keys randomly, and therefore presumably evenly, in an array. The hash code and the hashing algorithm that uses the hash code are basically random number generators that produce indexes into this array. So the order of the keys is random, and as Patricia pointed out will change at runtime if the array needs to be re-hashed. |