Invoke non-static Java method in Clojure

Question

Invoking a static Java method in Clojure is smooth; no problem. But when I invoke a non-static method, it throws an error, although I have tried several variations for dot (.) notation, as explained in Clojure Interop documentation .

The Java class:

public class CljHelper {

  public void test(String text) {
    System.out.println(text);
  }

}

The Clojure code:

(ns com.acme.clojure.Play
  (:import [com.acme.helpers CljHelper]))

(. CljHelper test "This is a test")  

The error:

java.lang.IllegalArgumentException: No matching method: test

This is another attempt, which makes the Java method to be executed but throws an error right after:

(defn add-Id
  [x]
  (let [c (CljHelper.)]
    ((.test c x))))        ;;; java.lang.NullPointerException: null in this line

(add-Id "id42")

The error:

java.lang.NullPointerException: null

Answer 1

You have two different issues here. In the first example you're trying to invoke a method on the class CljHelper . You should be calling it on the instance, not the class.

(.test (CljHelper.) "This is a test")

For the second example, you have an extra set of parentheses. So, you're correctly calling the method test , but then you are taking the result, which is null, and attempting to call that as a function. So just remove the parentheses.

(defn add-id
  [x]
  (let [c (CljHelper.)]
    (.test c x)))

Answer 2

Here is the easiest way to do it. Make a java class Calc:

package demo;

public class Calc {
  public int answer() {
    return 42;
  } 
}

and call it from Clojure:

(ns tst.demo.core
  (:use tupelo.core tupelo.test)
  (:import [demo Calc]))

(let [c (Calc.)]                   ; construct an instance of Calc
  (println :answer (.answer c)))   ; call the instance method

with result:

:answer 42

You can clone a template Clojure project to get started.