How to create an array of custom java object class in Clojure?

Question

This is my code. I would like to make an array of Fruit (an custom java object class).

(defn build-fruit
  [part]
  (let [name (:name part)
        color (:color part)
        price (:price part)
        is_available (:is_available part)]
  (-> (Fruit/newBuilder)
      (.setName name)
      (.setColor color)
      (.setPrice price)
      (.setIsAvailable is_available)
      (.build))))

(defn build-fruits
  [result length]
  (loop [remaining-fruits result
         final-fruits (make-array Fruit length)]
    (if (empty? remaining-fruits)
      final-fruits
      (let [[part & remaining] remaining-fruits]
        (recur remaining
               (into-array final-fruits (build-fruit part)))))) ;got an error here
  )

(defn -getAllFruits [this ^Empty req ^StreamObserver res]
  (let [result (clj-grpc.database/findAllFruit)
        length (count result)
        fruits (build-fruits result length)]
    (doto res
      (.onNext (-> (ListOfFruits/newBuilder)
                   (.addAllFruit fruits)
                   (.build)))
      (.onCompleted))))

But got an error

java.lang.ClassCastException: [Lorg.example.fruitproto.Fruit; cannot be cast to java.lang.Class
    at clojure.core$into_array.invokeStatic(core.clj:3431)
    at clojure.core$into_array.invoke(core.clj:3431)
    at clj_grpc.fruitservice$build_fruits.invokeStatic(fruitservice.clj:118)
    at clj_grpc.fruitservice$build_fruits.invoke(fruitservice.clj:110)

My guess is I cann't use make-array. I also can't use simple array like this [] because it also got another error. What is the proper way to achieve this?

It is often more helpful to provide a repro that is not dependent on any Java libraries, so people can more easily help you. — Michiel Borkent
– Michiel Borkent, Commented Mar 2, 2022 at 10:54
into-array has a 2-arity version, where the first argument is a component type. Note the second argument is a sequence, so there's no need for using a loop in build-fruits. I would provide a code sample, but as Michiel points out, the repro code in the question is not standalone, so it's hard to provide an example. — Steffan Westcott
– Steffan Westcott, Commented Mar 2, 2022 at 11:21
@SteffanWestcott without loop? How do i build Fruit object for every item in result variable? — Shalahuddin Al-Ayyubbi
– Shalahuddin Al-Ayyubbi, Commented Mar 2, 2022 at 12:09
See my answer below, building an array of LocalDate, where map is used to transform the input maps to LocalDate instances. — Steffan Westcott
– Steffan Westcott, Commented Mar 2, 2022 at 12:44

Steffan Westcott · Accepted Answer · 2022-03-02 13:04:51Z

2

Here is a standalone example of using into-array, building an array of LocalDate rather than Fruit:

(ns overflow.example
  (:import (java.time LocalDate)))

(defn build-local-date [{:keys [year month day]}]
  (LocalDate/of year month day))

(defn build-local-dates [xs]
  (into-array LocalDate (map build-local-date xs)))

edited Mar 2, 2022 at 13:04

answered Mar 2, 2022 at 12:42

Steffan Westcott

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