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

[Timo Kinnunen]

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.

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