Unable to assign method reference to generic interface method

Question

I am trying to assign a method reference to the following generic interface.

interface Interface1<R> {
  public R doIt();
}

interface Interface2<P, R> {
  public R doIt(P x);
} 

public class Test {
  public static Long foo(Long x) {
      return x;
  }

  public static Long bar() {
      Long x = 1L;
      return x;
  }

  public static void main(String[] args) {
    Interface1<? extends Number> fun1 = Test::bar; //works
    Interface2<? extends Number, ? extends Number> fun2 = Test::foo; // does not work
  }
}

The second assignment gives me

incompatible types: Number cannot be converted to Long

What am I missing here!

Note by way of comparison that in the standard library you always see `Function super T, ? extends U>`. — chrylis -cautiouslyoptimistic-, Aug 06 '21 at 05:54
But then why Interface1 extends Number, ? extends Number> f1 = (Long x) -> x; works? — anupamD, Aug 06 '21 at 06:11
Because in the second case it is defined to return `Number`, not `Long`. — chrylis -cautiouslyoptimistic-, Aug 06 '21 at 06:13
Ah.. thanks @chrylis-cautiouslyoptimistic-. Doubt cleared. And learnt something new today — anupamD, Aug 06 '21 at 06:14

score 0 · Answer 1 · answered Aug 06 '21 at 05:53

0

You cant do this because then you can do fun2.doIt((Number)0.5); The idea is that you need something like Interface2<? super Number, ? extends Number>

answered Aug 06 '21 at 05:53

Ofek

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Unable to assign method reference to generic interface method

1 Answers1