Re: [Vala] Final / Sealed classes in Vala

[Mikkel Kamstrup Erlandsen <mikkel kamstrup gmail com>]

2009/9/22 Jan Hudec <bulb ucw cz>:
> On Tue, Sep 22, 2009 at 06:40:10 +0200, Marco Trevisan (Treviño) wrote:
> > Is it possible to define both in VAPIs and in a vala code a class which
> > can not be extended?
> >
> > I mean something like "final" does for Java and "sealed" for C#...
>
> I don't think so. Though some classes are not inheritable because they lack
> proper construction functions. That's (unfortunately) the case for most
> classes in the GLib namespace.
>
> And I think vala should *NOT* get 'sealed'. It does not need it. In fact, C#
> does not need it either -- they added it because Java had 'final', but in
> Java it partly replaces missing non-virtual methods. In C# it's mostly
> annoying.
>
> > I think they could be useful for grouping static methods in "virtual
> > classes" (I mean in a class which name has not a reference in a C
> > type/structure, but that has just been written for organizing similar
> > methods) without the risk of methods overriding or of extending a
> > "virtual" class that has been defined in a VAPI but that is not
> > available in the relative C code.
>
> I think you are forgetting, that vala has non-member functions. Static
> methods -- functions -- should be grouped in namespaces. There are many cases
> of that in glib-2.0.vapi and gobject-2.0.vapi.
>
> > For example:
> >
> > public final class Utility {
> >       public static void my_method();
> > }
>
> That's only useful in Java which does not have non-member methods. And even
> there the final keyword is useless -- it's static, so it cannot be virtual
> and if you call it as Utility.my_method(), you always call that method and
> nothing else. Only instance methods may be overriden.
>
> In vala, you just use namespace as:
>
> namespace Utility {
>    public void my_method();
> }
>
> Note, that in C# you can now use a namespace now too, but in earlier versions
> where non-member functions were not allowed there was special 'static class'
> declaration for this purpose.
>
> > or in VAPI:
> >
> > public abstract final MyOwnType {
>
> Abstract final makes no sense, because 'abstract' means it must be inherited
> and final means it must not.
>
> In Java, it's an idiom for a static class. In C# it would be declared 'static
> class' and in all languages that support non-member methods, including vala,
> a namespace would be used instead.
>
> And you forgot 'class' or 'struct'.
>
> >       public static void c_referenced_method1();
> >       public static void c_referenced_method2();
> > }
> >
> > I hope you got the idea... :P
>
> Yes. It is Java-specific idiom. In Vala, you just use namespace with
> non-member functions.

Jan, I agree with you on almost everything you write, you do forget
one important use case for 'final' which (to some extent) applies to
Vala as well. Namely that of object serialization.

If I am writing/using a network protocol or other serialization that
has native object mapping (like fx. Java's RMI), then I will be quite
interested in assuring that I am serializing the right class, and not
some hacked up subclass which could cause in-compatibilities on the
wire. I've learned this the hard way in Java - and while it doesn't
apply directly to Vala, it might do so in the future in some wicked
DBus Object mapping.

-- 
Cheers,
Mikkel