Java 8 - how do I declare a method reference to an unbound non-static method that returns void

Question

Here's a simple class that illustrates my problem:

package com.example;

import java.util.function.*;

public class App {

    public static void main(String[] args) {
        App a1 = new App();

        BiFunction<App, Long, Long> f1 = App::m1;
        BiFunction<App, Long, Void> f2 = App::m2;

        f1.apply(a1, 6L);
        f2.apply(a1, 6L);
    }

    private long m1(long x) {
        return x;
    }

    private void m2(long x) {
    }
}

f1 , referring to App::m1 , and being bound to a1 in f1 's call to apply , works perfectly fine - the compiler is happy and the call can be made through f1.apply just fine.

f2 , referring to App::m2 , doesn't work.

I'd like to be able to define a method reference to an unbound non-static method with no return type, but I can't seem to make it work.

Answer 1

BiFunction represents a function that accepts two arguments a nd produces a result .

I'd like to be able to define a method reference to an unbound non-static method with no return type

use a BiConsumer instead which represents an operation that accepts two input arguments and returns no result .

BiConsumer<App, Long> f2 = App::m2;

then change this:

f2.apply(a1, 6L);

to this:

f2.accept(a1, 6L);

Answer 2

The method reference is App::m2, just like you have, but it's not assignable to a BiFunction, because it doesn't return a value, even a Void value (which has to be null ). You'd have to do:

f2 = (a,b) -> { m2(a,b); return null; }

if you want a BiFunction. Alternatively, you could use a BiConsumer as mentioned in other answers.