Method reference and Generics in Java-8

Question

I am facing issue with method reference combined with generic types.

We have code where we need to call a overloaded method, but it is failing with error:

Cannot resolve value m1()

I have simplified my problem to make it clear where the problem lies.

The following code fails:

public class Test {
    void test() {
        // Getting error here
        setValue(C1::m1, Integer.ONE);
    }

    <E extends I1, T> void setValue(BiConsumer<E, T> cons, T value) {
    }
}

interface I1 {
}

class C1 implements I1 {
    void m1(Integer value) {
    }

    void m1(int value) {
    }
}

Can someone please why this is behaving like this ?

Kindly note this is not related to the question Java 8 Method reference with generic types

Answer 1

It seems that the type inference rules aren't "smart" enough to resolve the conflict between picking the right m1 variant and generating the correct inferred type parameters to the setValue call ( B and BigDecimal respectively).

I can't quite explain why this fails, but type inference has traditionally been an area with arcane, well-reasoned and un-intuitive rules, so I'm not very suprised.

You can work around this issue by adding a type witness (specifying which type parameters you want to call setValue with) at which point the compiler will pick the correct m1 method:

this.<B,BigDecimal>setValue(B::m1, BigDecimal.ONE);