Custom equality policy / hashing strategy for keys

[batya239]

Hi, Ben!

Currently caffeine relies only for canonical equality/hashing policy via proper defined #equals() and #hashCode() methods. Do you have any plans to support something like

public interface HashingStrategy<K> {
  int hashCode(K key);

  boolean equals(K k1, K k2);
}

?

Or would you mind for pull request to support it?

[ben-manes]

There is no planned support. It would fall under two strategies, which at this time don't make sense for us.

Wrap the key

The key can be wrapped with a custom equivalence strategy, like with any other hashmap type. As this can easily be done in user code, it doesn't carry its weight to make as a feature in this library. See Guava's Equivalence and the wrap method.

Custom hash table

The wrapper can be avoided by having the hash table call the equivalence strategy's methods with the key. However, this requires forking ConcurrentHashMap to rewrite its usages, which is not desirable. It would require that we keep in sync with a complex, evolving class which uses low-level feature only available to the JVM authors. However, if the Java authors decided to support this then we would expose the plumbing (you could ask on core-libs-dev).