Another generics conundrum

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Twisted - 11 Jun 2006 18:51 GMT

public abstract class Vector<S extends Scalar> {

public abstract <T extends Scalar> Vector<T> add (Vector<? extends T>
y);

}

What I additionally want to specify, somehow, is that S extends T. In
fact T should extend Scalar and be the last common ancestor of S and
whatever the right-hand ? turns out to be.

How??

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Danno - 12 Jun 2006 06:51 GMT

> public abstract class Vector<S extends Scalar> {
>
[quoted text clipped - 8 lines]
>
> How??

Wow, dude, I think you need to rethink your plan and focus more on the
inheritance.
Can't you do (forgive my old rusty mathematics ignorance)?

public class Scalar {
private double value;

//removed setters and getters for brevity
}

public class Vector extends Scalar {
private double angle;
private double magnitude;

//removed setters and getters for brevity
}

If you need a collection of Scalars or Vectors you can do:

ArrayList<Scalar> scalarsAndVectorList = new ArrayList<Scalar>();

scalarsAndVectorList.add(new Scalar(234.00));
.
.
.
.

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Dimitri Maziuk - 12 Jun 2006 17:24 GMT

Twisted sez:

> public abstract class Vector<S extends Scalar> {
>
[quoted text clipped - 8 lines]
>
> How??

All generics do is add (fairly limited) compile-time type checking
to collections (which cast everything to Object under the hood).
Among the things they _don't_ do is specialization, nesting,
metaprogramming, and about any other feature of c++ templates
you can think of.

HTH
Dima

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Twisted - 12 Jun 2006 18:44 GMT

> All generics do is add (fairly limited) compile-time type checking.
> Among the things they _don't_ do is specialization, nesting,
> metaprogramming, and about any other feature of c++ templates
> you can think of.

Considering all I'm trying to add is some compile-time type checking...

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Dimitri Maziuk - 14 Jun 2006 16:34 GMT

Twisted sez:

>> All generics do is add (fairly limited) compile-time type checking.
>> Among the things they _don't_ do is specialization, nesting,
>> metaprogramming, and about any other feature of c++ templates
>> you can think of.
>
> Considering all I'm trying to add is some compile-time type checking...

That's where "fairly limited" bit comes in. Basically, if
you want to limit the element to a specific type, you use
regular types (as in
... MyVector add( MyVector y )
). If you want the element to be "anything, but all of the
same type", you use
... T add( T y )

In other words, there's usually little point in writing generic
containers for a particular application.

Dima

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Sufficiently advanced incompetence is indistinguishable from malice.

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Another generics conundrum

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