Re: abstract static methods (again)

Tomas Mikula <>
Mon, 19 Oct 2009 06:06:28 -0700 (PDT)
On Oct 19, 2:19 pm, Marcin Rze=BCnicki <>

On 19 Pa=BC, 13:08, Tomas Mikula <> wrote:

On Sun, 18 Oct 2009 19:58:43 -0700, Marcin Rze=BCnicki wrote:

On 19 Pa=BC, 04:06, Tomas Mikula <> wrote:

I have searched this group for "abstract static methods" and found a
couple of threads, but I think none of them was discussing the kind =


semantics I am going to describe. As you might have guessed, I belie=


it would be useful :). I further believe it is fully complatible wit=


the current language, but there might be caveats I have overlooked. =


wonder if you would find it as useful as I do and if you see any
problems with it. I know it is a long post and some parts may be
difficult to understand. Therefore I will be thankful if you can rea=


it all and think about it.

Interesting but I doubt it is going to be useful. First of all, becau=


statics should remain non-inheritable, static abstract actually force=


each subclass to implement its own definition - extreme nuisance in m=



Yes (unless the subclass is abstract). I think in some cases it is
reasonable (as with the enforced no-arg constructor in Serializable, or
some other serialization static methods. For example, if I want to
deserialize an immutable object, I need to do it by a static method or =


special constructor, because the non-static readObject(...) method in
Java's serialization API is a mutator method. In my opinion it is
reasonable to enforce own implementation of a (de)serialization method)=


Hi, possibly it is reasonable, but what is wrong with how it is done
today (readObject/writeObject) which you are not required to implement
if default behavior suffices?

I'm saying it is wrong, but just don't like that the implementation
requires a lot of reflection. (I don't mind that implementation of
statndard Java API requires reflection, because someone has already
implemented it for me. But if I want to create my own serialization
framework (e.g. for xml serialization), I need to do a lot of
reflection which could be automated.) Probably one thing I find wrong
with readObject - as I already mentioned, it prevents the object to be
immutable. Though this could also be solved by declaring it static and
use reflection.

Example with generics can easily be substituted by some kind of
"trait" parameter

Sorry, I don't know what you mean by "trait" parameter? Do you mean tha=


I would call the zero() method on some instance?

I borrowed the terminology from C++. More or less, you add type
parameter (let's say <Zero extends ZeroVector>) which has a method
like getZeroVector() (strictly speaking ZeroVector has this method).
Actual type parameter provides concrete implementation.

I don't see how this would help. Would I call Zero.getZeroVector()?
Probably you meant something else because this leads to the same
problem with calling static method getZeroVector() on a type
parameter. Could you provide an example?

or suitable simple design pattern (for example
Factory), or even with classic sub-typing (zero vector needs not know
its dimension, it can simply 'answer' with neutral element of the rin=


on which it is constructed for each and every component query),

Allowing operations between a concrete vector and this general zero
vector would require to also allow operations between 2D and 3D vetors =


the original type safety would disappear.

I don't get it, could you provide an example?

If I understood well, you meant something like this:

Class Vector {
    public static Vector getZeroVector(){
        return someSpecialZeroVectorInstance;
    public abstract Vector add(Vector v);

Class Vector2D {
    public Vector add(Vector v){...}

class MyClass<V extends Vector> {
    V v; // V is some concrete class, such as Vector2D
    Vector zero = Vector.getZeroVector();
    v.add(zero); // adding a general Vector zero to concrete v
    // if this is allowed, then also the following is
    v.add(new Vector3D(1,2,3)); // summing 2D and 3D vector

 no big
win here either (eliminating type erasure is extremely welcome but fo=


other reasons). One big advantage of inheritance is, in my opinion, t=


it enables you to compose more specialized classes from generic ones,=


is easy to imagine algebraic ordering relation between types based on
inheritance. Your version of statics breaks this assumption without
promise of any reward in exchange.

I don't see how it breaks this relation between classes. Also now it is
possible to hide supertype's static methods by own implementation. I
would only add that in some cases this hiding would be required.

I was not very clear, it was late when I was writing :-) I guess what
I was trying to say was that you can impose ordering based on
specialization (as opposed to parent-child relationship). Each class
in an inheritance chain either extends or redefines partially its
ancestor (I am using 'or' as logical or). Therefore each class is
either more specialized (if redefinition occurs and it accepts
stronger contract, as in Rectangle->Square) or equally specialized (if
extension occurs and all redefinitions do not change contract - I
treat extension as an interface extension so that class can be used
_additionally_ in different context). Your proposal forces implementor
to provide implementation for non-inheritable method, so it really
can't take any benefit from redefinitions up the chain. Therefore all
concrete classes are at most equally specialized as their context of
usage is determined by a static method. So it does not play well with
most "inheritance patterns". That's how I see it.

Now I don't get it. Can you provide an example where you have a class
and its specialized subclass and adding an abstract static method to
their interface removes/prohibits this specialization?

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