On May 31 2012, at 01:40 , Ulf Zibis wrote:
> Hi Mike,
>
> some more questions:
>
> public class Hashmap<K,V> {
> + int hash(Object k) {
> + int h = hashSeed;
> + if (k instanceof String) {
> + return ((String) k).hash32();
> + } else {
> + h ^= k.hashCode();
> + }
> +
> + // 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);
> }
>
> Why you declare and assign variable h before the if statement?
It used to be referenced in the String case. It could be moved. I will move it
in the followon patch.
> And why you set h ^= k.hashCode(); in the else branch, but not the remaining
> instructions to complete the calculation?
> Similar (but why little different?) for:
> Hashtable
I will move the read of hashSeed as per HashMap in the followon patch.
Compatibility with legacy behaviour.
> WeakHashMap
Will be the same as HashMap.
> ConcurrentHashMap
Improved behaviour.
> So couldn't method hash(Object) be moved to AbstractMap?
The differences in the avalanche (XOR scrambling) preclude this. It could be
decided for Java 8 to use a consistent scrambling implementation. I would want
to hear from Doug Lea whether he thinks this is reasonable/worthwhile.
Presumably there is a reason that Hashtable, HashMap and ConcurrentHashMap use
different scrambling solutions.
> Why in WeakHashMap method hash(Object) is not final?
Oversight. I will fix this in the followon patch.
Thank you for the observant feedback!
Mike
> (In this case I could exceptionally override it for a custom implementation
> as former desired for all hash maps.)
>
> -Ulf
>
>
> Am 31.05.2012 07:19, schrieb [email protected]:
>> Changeset: 43bd5ee0205e
>> Author: mduigou
>> Date: 2012-05-30 22:18 -0700
>> URL: http://hg.openjdk.java.net/jdk8/tl/jdk/rev/43bd5ee0205e
>>
>> 7126277: Alternative String hashing implementation
>> Summary: All of the hashing based Map implementations: HashMap, Hashtable,
>> LinkedHashMap, WeakHashMap and ConcurrentHashMap are modified to use an
>> enhanced hashing algorithm for string keys when the capacity of the hash
>> table has ever grown beyond 512 entries. The enhanced hashing implementation
>> uses the murmur3 hashing algorithm along with random hash seeds and index
>> masks. These enhancements mitigate cases where colliding String hash values
>> could result in a performance bottleneck.
>> Reviewed-by: alanb, forax, dl
>>
>> ! make/java/java/FILES_java.gmk
>> ! src/share/classes/java/lang/String.java
>> ! src/share/classes/java/util/HashMap.java
>> ! src/share/classes/java/util/Hashtable.java
>> ! src/share/classes/java/util/LinkedHashMap.java
>> ! src/share/classes/java/util/WeakHashMap.java
>> ! src/share/classes/java/util/concurrent/ConcurrentHashMap.java
>> + src/share/classes/sun/misc/Hashing.java
>> ! test/java/util/Collection/BiggernYours.java
>> ! test/java/util/Hashtable/HashCode.java
>> ! test/java/util/Hashtable/SimpleSerialization.java
>> + test/java/util/Map/Collisions.java
>> ! test/java/util/Map/Get.java
>> + test/sun/misc/Hashing.java
>>
>>
