Re: abstract static methods (again)

From:

Tomas Mikula <tomas.mikula@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 20 Oct 2009 01:16:21 +0000 (UTC)

Message-ID:

<hbj315$4l4$1@aioe.org>

On Mon, 19 Oct 2009 17:23:33 -0700, Marcin Rze??nicki wrote:

On 20 Pa??, 01:45, Tomas Mikula <tomas.mik...@gmail.com> wrote:

On Tue, 20 Oct 2009 00:05:41 +0100, Tom Anderson wrote:

What are you planning to do with this zero if you don't have any
Vector instances? I don't really buy your scenario.

Consider you want to implement a method that takes a collection of
vectors and returns their sum. Simple, eh? If I can call static methods
on type parameters, it will look like this:

public V sum(Collection<V> coll){
?? ?? V s = V.zero();
?? ?? for(V v: coll)
?? ?? ?? ?? s = s.add(v);
?? ?? return s;

}

However, we cannot call V.zero(). What will you return if coll is an
empty collection?

But what is so wrong with passing neutral element to the method and
implement it like foldl? For me it is quite natural

Isn't so bad, but not quite natural for me (ok, this is subjective). I
want a method that takes a collection and returns its sum. Instead, I
will have a method that takes a collection and an element and returns
that element plus the sum of the collection. If I was reading this code
after someone else, I would wonder why they have written it like this,
because I would not see the problem at the first sight.

What if the method that calls sum() doesn't have an instance at hand
either? Then I have to propagate this instance-passing up the call
hierarchy.