Problems calling a clojure function that takes a map as parameter from java

Question

I'm using maven with several modules, one in java, another in clojure. I'm calling a clojure function from java and want to pass in a HashMap as a parameter and return a HashMap. (I ran lein uberjar and lein pom on the clojure project to make it work with maven. I can get things to work for clojure function with simple types e.g. String, so the maven setup does work.)

I am getting the following error when I run some java unit tests calling the java code:

java.lang.ClassCastException: class clojure.lang.LazySeq cannot be cast to class java.util.Map (clojure.lang.LazySeq is in unnamed module o
f loader 'app'; java.util.Map is in module java.base of loader 'bootstrap')

How can I get this to work? Is this the proper way to call clojure methods from java? What about if the HashMap had a POJO object as a value rather than a String?

My java code:

import interop.Core;

public class BillingCalc {
       static Map<String, String> nonEmptyItems(Map<String, String> items) {
           return Core.non_empty_seats(new HashMap<String, String>());
       }
}

My clojure code:

(ns interop.core
     (:gen-class
      :name interop.Core
      :methods [^{:static true} [apply_vat_to_netto [java.math.BigDecimal java.math.BigDecimal] java.math.BigDecimal]
                ^{:static true} [non_empty_seats [java.util.Map] java.util.Map]]) )


(defn -filter-empty-seats
  "filter out those with empty seats"
  [seats]
  (filter (fn [[_ v]] (pos? (:booked-items v))) seats))

(defn -non_empty_seats
  [java-seats]
  (-filter-empty-seats (into {} java-seats)))

Martin Půda · Accepted Answer · 2022-07-07 16:59:36Z

1

I guess that your error is caused by this definition in :gen-class:

[non_empty_seats [java.util.Map] java.util.Map]]

From docs for :gen-class:

:methods [ [name [param-types] return-type], ...]

The expected type of returned value is java.util.Map, but filter in -filter-empty-seats returns instance of clojure.lang.LazySeq. You should rewrite -non_empty_seats like this:

(defn -non_empty_seats
  [java-seats]
  (into {} (-filter-empty-seats java-seats)))

answered Jul 7, 2022 at 16:59

Martin Půda

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2 Comments

Stephan Kennedy Over a year ago

You are perfectly right. And also I did not realize clojure maps are automatically converted to java Map. Thx!

Jeremy Over a year ago

@StephanKennedy They're not automatically converted. Instead, Clojure maps implement the java.util.Map interface.

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