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

Sun Mar 2 21:53:06 PST 2014

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.
>>>>> 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?
>> }
>>>>> Wow, WTF? Just pick one super-type of T, any one!
>>>>> Here's a more fully-fledged motivating example:
>>>>> private static void callerOfTake2() {
>> take2(
>> "A test string",
>> asArray(
>> (CharSequence s) -> System.out.println(s),
>> (String s) -> System.out.println("S" + s),
>> (Object s) -> System.out.println("O" + s)));
>> }
>> @SafeVarargs
>> private static Consumer<? super String>[] asArray(Consumer<? super String>... cees) {
>> return cees;
>> }
>> 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) {
>> for(Consumer<E> consumer : c) {
>> consumer.accept(e);
>> }
>> }
>>>>>> 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
>