Hi Remi, I think it's fair to say that I know my share of Java Generics, so I generally understand the motivation of introducing wildcards into method signatures. Just in your particular case (and in your original example), I don't see what you gain by having the "super" wildcard for the outer Consumer's type parameter. If you leave it out, the code compiles without problems: static <K, T> Function<K, T> factory(Consumer<BiConsumer<? super K, ? super T>> consumer, Function<? super K, ? extends T> ifAbsent) { ... } Can you enlighten me? Cheers, Gernot Am 29.06.2015 15:49, schrieb Remi Forax:
Bitten again by the very same issue :(
The following code doesn't compile: static <K, T> Function<K, T> factory(Consumer<? super BiConsumer<? super K, ? super T>> consumer, Function<? super K, ? extends T> ifAbsent) { HashMap<K, T> map = new HashMap<>(); consumer.accept(map::put); return key -> map.computeIfAbsent(key, ifAbsent); }
I really think that it's a serious bug, the only workaround is to not use wildcards correctly, i.e. <K, T> Function<K, T> factory(Consumer<BiConsumer<? super K, ? super T>> consumer, Function<? super K, ? extends T> ifAbsent)
cheers, Rémi
On 05/27/2015 05:29 PM, Remi Forax wrote:
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<String>> 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<String>)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