Blah.java

import java.util.function.Function;

public class Blah {

    private Blah() {
    }

    static class A {
    }

    static class B extends A {
        String foo(final String x) {
            return x + "!";
        }
    }

    static void f1(final Function<A, Void> k) {
        k.apply(new A());
    }

    static void f2(final Function<B, Void> k) {
        f1(k); // compile error - quite rightly: "b => Void" is not an "a => Void"
    }

    static {
        f2(x -> {
               System.out.println(x.foo("ABC"));
               return null;
           }
        );
    }
}