Hi,
What's the difference between the following type definitions
<E extends Number>
and
<? extends Number>
Cheers, Don
views:
541answers:
3
+14
A:
This version:
<? extends Number>
can appear in a non-generic method/type, and it basically means "I don't care what the type is, so long as it derives from Number. I'm not going to really use the type, I just need it to be appropriate."
This version:
<E extends Number>
requires E to be a type parameter. It allows you to do more (for instance, creating an ArrayList<E> later on) but the extra type parameter can make things more complicated when you don't really need them to be.
Jon Skeet
2008-12-07 22:05:12
More formally, would it be true to say that the second one is a formal type parameter definition, whereas the first is simply a parameter definiton?
Don
2008-12-07 22:21:59
Would it be true to say that you should use the second definition only when you need to refer to the type (via the 'alias' E)?
Don
2008-12-07 22:25:41
I wouldn't like to say on the first comment without consulting the spec (and I'm off to bed in a minute). The second comment seems reasonable though. Check what Josh Bloch has to say in Effective Java - he probably covers it :)
Jon Skeet
2008-12-07 22:57:22
Actually, the question occurred to me while reading Effective Java!
Don
2008-12-07 23:34:19
A:
As an expansion on @Jon Skeet's answer, see
Which types are permitted as type parameter bounds?
in the excellent resource Java Generics FAQ from Angelika Langer
Ken Gentle
2008-12-08 15:24:27
A:
For all Java Generics related questions, I highly recommend "Java Generics and Collections" by Maurice Naftalin, Philip Wadler.
Angelika Langer's FAQ is OK, but it is also huge and somewhat rote.
Julien Chastang
2008-12-08 16:39:29