The meaning of "capture of ? e" in Java

Asked Oct 07 '16 at 07:43

Active Oct 07 '16 at 07:50

Viewed 2,877 times

In IntelliJ IDEA, especially when I create the instance of Collection class like:

Collection<?> collection = ...;
collection.

And try typing the method add, IntelliJ helps me telling that add's first argument is capture of ? e.

Anyway, I can't use any value as the argument like:

collection.add(new Integer(5));

and the message is

add(capture<?>) in Collection cannot be applied to (java.lang.Integer)

What does it mean?

edited Oct 07 '16 at 07:50

asked Oct 07 '16 at 07:43

Beraliv

It means that your collection is _declared_ to contain elements of _some_ type and _not_ `java.lang.Object`, so the compiler cannot an integer. – Lyubomyr Shaydariv Oct 07 '16 at 07:45
1

Another one worth reading: http://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super – Andy Turner Oct 07 '16 at 07:48
@LyubomyrShaydariv thank you, helping me a great deal! – Beraliv Oct 07 '16 at 07:52
To understand the concept of wildcard capture: https://docs.oracle.com/javase/tutorial/java/generics/capture.html – Marko Topolnik Oct 07 '16 at 07:53
@AndyTurner thank you as well, nice answer from `@anoopelias` – Beraliv Oct 07 '16 at 07:57

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