Bad interaction between wildcard and functional interface conversion

Remi Forax forax at
Mon Jun 29 13:49:33 UTC 2015

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<>();
     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)


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

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