If you don't do anything special, and create several Java array of bytes
(for example) and use them as keys in a Clojure map, the default hashCode
and equals implementation will cause two arrays with identical contents to
be treated as different keys in the map, unless they are also == in Java.
I'd like to use Java byte arrays as keys in Clojure maps, but haven't been
able to figure out how yet. I know that java.util.Arrays/hashCode and
java.util.Arrays/equals have the behavior I want, so one way would be to get
Clojure to somehow use those methods when given the byte arrays as keys.
That led me to think of trying deftype to make my own type Key with the
desired implementation of hashCode and equals. I'm using Clojure 1.2, and I
don't see how to give the appropriate type hints to declare that something
is a Java byte array. Here is one attempt for Java int arrays instead of
Java byte arrays:
(definterface IKey
(^ints key [])
(equals [other-key] "Compare two Keys for value equality, i.e. same length
and same bytes in array value")
(hashCode [] "Return a hash code that is the same for equal keys, and
usually different for different keys"))
(deftype Key [^ints key]
IKey
(key [this] key)
(equals [this other-key]
(let [^Key other other-key]
(java.util.Arrays/equals key (.key other))))
(hashCode [this]
(java.util.Arrays/hashCode key))
Object
(toString [this]
(apply str (map char key))))
When I try to evaluate the deftype statement, I get an error like this:
java.lang.NoClassDefFoundError: java/lang/ints
Same error if I replace ^ints with #^ints. If I leave out the type hints
for the field key completely, I get the following error when trying to
evaluate the deftype:
java.lang.VerifyError: (class: user/Key, method: hashCode signature:
()Ljava/lang/Object;) Expecting to find object/array on stack
I'm not wedded to deftype as being part of the solution. My real goal is as
high performance way of using Java byte/int/some-native-type arrays as map
keys, i.e. with no reflection warnings.
Thanks,
Andy
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