Clojure - Reflection Warning with primitive array .length

Question

I'm having some trouble trying to use instance methods on a java float array in clojure. Specifically, the following code gives a reflection warning on the call to .length

(defn make-matrix
  ([^floats fs]
   (let [len-fs (.length fs)]
     (cond (>= len-fs 16) (Matrix4. fs)
           (>= len-fs 9)  (Matrix3. fs)
           :else (throw (IllegalArgumentException. (str "Array must have at least 9 elements (found" len-fs ")")))))))

By my understanding, the type-hint should eliminate the need for reflection to resolve the call to .length. What am I missing?

Michał Marczyk · Accepted Answer · 2013-12-12 06:24:20Z

10

Arrays on the JVM are not quite like regular objects and their lengths are not obtained through a field lookup, but rather through a specialized opcode arraylength. Java's array.length notation is just syntactic sugar that compiles down to this opcode. Therefore, (.length fs) will simply fail at runtime, because fs will be found to have neither a length method nor a length field (assuming it is in a fact an array).

Thus, you should use the clojure.core/alength function:

(alength fs)

Without a type hint, this will result in a reflective call; with a type hint, it will be a direct call to a static method returning fs.length (in Java notation).

answered Dec 12, 2013 at 6:24

Michał Marczyk

84.5k13 gold badges204 silver badges212 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

juan.facorro · Accepted Answer · 2013-12-12 06:24:37Z

3

Primitive arrays are a somewhat special case in the JVM, a proof of this is that there's a bytecode instruction for getting the length of them: arraylength. So the reason why Clojure is showing a reflection warning is because the class for arrays of floats [F does not declare a field with the name length, so when it tries to get it (here) the field is not there.

You can check it yourself doing the following:

(-> (float-array [1])
  class
  (.getField "length"))

answered Dec 12, 2013 at 6:24

juan.facorro

9,9602 gold badges36 silver badges41 bronze badges

Collectives™ on Stack Overflow

Clojure - Reflection Warning with primitive array .length

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related