Re: Generics - Is this possible?

From:
Lew <lew@lewscanon.com>
Newsgroups:
comp.lang.java.programmer
Date:
Tue, 15 Apr 2008 09:49:48 -0400
Message-ID:
<yr2dnRv_IIHhL5nVnZ2dnUVZ_ruqnZ2d@comcast.com>
lstephen wrote:

Hi,

I'm looking at the signature for something like a 'map' function.


'map' is a somewhat unfortunate variable name, since you aren't actually
associating it with the wildly popular 'Map' type.

For List it may be something like:

List<B> map(List<A> a, UnaryFunction<A, B> f)

But, I want I'd rather it not be List specific, so I was after
something like:

T<B> map(T<A> a UnaryFunction<A, B> f)

But, the compiler doesn't like this ;)

Any ideas on how or whether this is possible?


What you want to accomplish is possible, what you're trying to say isn't.
You're trying to indicate a type parameter that takes a type parameter; that
doesn't exist. Type parameters exist to restrict the range of acceptable
types; what you wrote, 'T<A>', is equivalent to merely 'T'.

What is the restriction that you actually want to place on T? Do you want it
to be a Collection?

  public static <A, B>
  Collection <B> xform( Collection <A> a, UnaryFunction <A, B> f )

(untried, untested, uncompiled, one of several solutions that differ subtly in
semantics)

If you have a class called 'UnaryFunction', shouldn't it overload its "unary
function" method with one that takes a Collection? Having a "friend" method
like that shown breaks encapsulation.

--
Lew

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