What is alternative hashing for String keys in Java 8?

问题

Java 8 is providing alternative hashing for String keys to improve performance when a large number of key hash code collisions are encountered. Can anybody explain what is that and how it will work?

回答1:

From this email of core-lib-devs@openjkd:

A new interface Hashable32 is introduced.

Hashable32 provides a method hash32()

String implements Hashable32 and hash32() method

HashMap et al recognize String and invoke hash32() rather than hashCode()

The revisions of the code:

Murmur3 : https://code.google.com/p/smhasher/wiki/MurmurHash3
althashing "7" webrev : http://cr.openjdk.java.net/~mduigou/althashing7/8/webrev/
althashing "8" webrev : http://cr.openjdk.java.net/~mduigou/althashing8/8/webrev/

回答2:

To bring more relevance to this question, the alternative hashing has been removed from JDK 8. Check out :

http://docs.oracle.com/javase/8/docs/technotes/guides/collections/changes8.html

http://openjdk.java.net/jeps/180

It is interesting to note that once the number of items in a hash bucket grows beyond a certain threshold, that bucket will switch from using a linked list of entries to a balanced tree.

The hash(Object key) function in the HashMap has been revised to follows with no special treatment to String objects:

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

回答3:

It should be noted that the shift to MurmurHash3 will not prevent DoS attacks: http://emboss.github.com/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/

来源：https://stackoverflow.com/questions/11946486/what-is-alternative-hashing-for-string-keys-in-java-8

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java

collections

hashmap

hash

hashcode