Bad interaction between wildcard and functional interface conversion (original) (raw)

Remi Forax forax at univ-mlv.fr
Wed May 27 15:29:13 UTC 2015


Hi all,

The way the conversion between a lambda (or a method reference) and a functional interface is specified doesn't take wildcard (exactly ? super) into account making the concept of contravariance of functional interface less intuitive that it should be.

The following code compiles: private static void create(Consumer<Consumer> consumer) { consumer.accept(s -> System.out.println(s)); }

This one doesn't compile because "? super Consumer<? super String>" is not a functional interface: private static void create2(Consumer<? super Consumer<? super String>> consumer) { consumer.accept(s -> System.out.println(s)); }

The workaround is to introduce a cast :( private static void create3(Consumer<? super Consumer<? super String>> consumer) { consumer.accept((Consumer)s -> System.out.println(s)); } which is stupid in this case because there is no ambiguity. This cast is just here because the JLS doesn't consider that ? super Consumer<...> is a valid target type

IMO, this bug is very similar to JDK-6964923 and i think the spec should be changed to allow ? super Foo to be a valid target type for a lambda conversion (obviously if Foo is a functional interface).

regards, Rémi



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