Woe is me for using ? super T in a method call (part 1)

[Zhong Yu]

Wildcard capture works on the 1st level wildcard:

    a List<? extends String> object is a List<x>, for some x that x<:String

It does not work on deeper levels:

    a List<List<? extends String>> object is NOT a List<List<x>> for some x.

In your example, we'll need

    Consumer<? super T> <: Consumer<E>

no choice of E can make that happen.

The solution is to almost always use wildcard on co/contra-variant types:

    internalTake(Consumer<? super E>[] c)

The problem of the solution is that our code is litterred with
wildcards (in a very ugly notation no less)

Zhong Yu

On Sun, Mar 2, 2014 at 8:08 AM, Timo Kinnunen <timo.kinnunen at gmail.com> wrote:
> Hi,
>
>
>
>
> These examples used to compile in ECJ until recently and I didn't bat an eyelid at them, but not anymore. Looks like they haven't compiled in javac for maybe lot longer.
>
>
>
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> The first one is just precious in its simplicity:
>
>
>
>
> private static <E> void internalTake(Consumer<E>[] c) {}
>
> public static <T> void take(Consumer<? super T>[] c) {
>
>   internalTake(c);//error?
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> }
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>
>
>
> Wow, WTF? Just pick one super-type of T, any one!
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>
>
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> Here's a more fully-fledged motivating example:
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>
>
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> private static void callerOfTake2() {
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>   take2(
>
>     "A test string",
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>     asArray(
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>       (CharSequence s) -> System.out.println(s),
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>       (String s) -> System.out.println("S" + s),
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>       (Object s) -> System.out.println("O" + s)));
>
> }
>
> @SafeVarargs
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> private static Consumer<? super String>[] asArray(Consumer<? super String>... cees) {
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>   return cees;
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> }
>
> static <T> void take2(T t, Consumer<? super T>[] c) {
>
>   internalTake2(t, c);//error?
>
> }
>
> private static <SUB_E extends E, E> void internalTake2(SUB_E e, Consumer<E>[] c) {
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>   for(Consumer<E> consumer : c) {
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>     consumer.accept(e);
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>   }
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> }
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>
>
>
>
> So what's going on here? Javac's explanation doesn't really explain where the disconnect is between my understanding of how these wildcards should work and how they actually work.
>
>
> I'm splitting the other example to another email because this is getting long enough already.
>
>
> --
> Have a nice day,
> Timo.
>
> Sent from Windows Mail
>