On 07/05/2015 03:13 PM, Gernot Neppert wrote:
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?
yes I can.
The java.util.function.Consumer (or its method accept if you prefer)
acts as a consumer of T ( :) ) so using the PECS rule [1], it should be
a Consumer<? super ...>.
The thing is that if you have a functional interface which is parametrized,
if the functional interface is parametrized by a wildcard, it stops to
be a functional interface because you can not do any lambda/method
reference conversion (without adding an useless cast, that will be
removed in the generated code BTW).
So it's a stupid bug and I'm ashamed to haven't thought about it before
the release of 8.
Cheers, Gernot
regards,
Rémi
[1] https://sites.google.com/site/io/effective-java-reloaded
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
