[Aaron Zapashniy]

Hi. i'm starting to add generics to a lot of my 1.4 code. i have a utility method which takes in a container of SuperClass. i'm now getting compile errors when i pass it a container of Subclass. why?

[Starwarrior]

Because a container of a superclass is not a superclass of a container of a subclass.

[Aaron Zapashniy]

I.e. void doIt(List<Parent>){ ...} ... List<Child> list = new List<Child>(); doIt(list);
That's not legal?

[Ikopar]

No

[Poolkop]

Aaron Zapashniy, generics is http://java.sun.com/j2se/1.5.0/docs/guide/language/generics.html and the generics tutorial is http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf and http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

[Aaron Zapashniy]

Hm. odd. how useful are these things then?

[amoralis]

How about List<Parent> list = new List<Child>(); doIt(list); ?
Is that still invalid?

[Ikopar]

Yep

[amoralis]

<- doesn't get to use generics

[Ikopar]

It's unnecessary too

[amoralis]

It seems like it should be valid, please explain why it isn't valid

[Ikopar]

List<Parent> list = new ArrayList<Parent>(); list.add(instanceOfChild);
Because generics don't work that way, there's no inheritence

[amoralis]

Ikopar: perhaps in that list instance you don't want parent to be accepted

[Ikopar]

Read the generics FAQ or tutorial, they cover this topic much better than I could

[amoralis]

Perhaps you only want a specific child object there
Not that I have a use case but maybe Aaron Zapashniy does heh

[Ikopar]

You generally don't want that

[Aaron Zapashniy]

I see. hm. interesting. okay.
Should still work fine, as 99% of the time i think i'm only performing parent methods when acting on collections.

[amoralis]

Generics in java kinda suck

[Aaron Zapashniy]

Is it just me or does the term generics seem completely contrary to what they actually are?
They're not generics. they're specifics.
Treating things as Object is generic.

[Gladis]

"templates" would be a better word for them.
Except that was taken. By something GOOD.

[Aaron Zapashniy]

To me, List list = new ArrayList(); is a GENERIC list, and List<Wookie> = new ArrayList<Wookie>(); is ..well i wouldn't call it a Template either but it's not generic.

[amoralis]

If you passed in a List of Parent that was populated by all child classes that would not introduce any problems, but as soon as you try to passing a generified list parameterized to child into a method requiring a list parameterized as parent it has issues... that's stupid. It would not cause any problems.

[Gladis]

Aaron Zapashniy, std::vector<wookie>; <- I would call that templates.

[Aaron Zapashniy]

I would ask how you got an STD from a wookie

[amoralis]

Gladis, I think the reason for the name difference is that in C++ it effects code outcome, in java it doesn't