Re: Value types - compatibility with existing “value objects”

[Vitaly Davidovich]

My question would be why not compose value types using ... well,
composition? Is there an example where inheritance is workable but
composition isn't?

On Thu, Jan 8, 2015 at 1:35 PM, Brian Goetz <brian.goetz at oracle.com> wrote:

> Regarding “Value types” seem to assume that you will use it just for
>> small types and you will implement bigger types as objects (with
>> different syntax)."...
>>
>
> There seem to be a lot of mis-assumptions on this topic.  There is no
> arbitrary limit on the size of a value type (other than existing classfile
> limitations such as constant pool size).  However, many of the performance
> benefits of value types drop off when you get beyond a handful or two of
> components.  This is why we say the "sweet spot" is things like numerics,
> small-ish tuples, or algebraic data types.  But there's no "it stops
> working beyond" point that you are forced to switch to, (and where there
> are visible performance differences will be a function of hardware and
> time, just like cache effects are today.)
>
>  Value types are "final" which means it won't be possible to subclass.
>> However, I am disappointed with this restriction because I would like to
>> use inheritance as an easy way of composing value types. Brian, what are
>> your thoughts about this?
>>
>
> Nothing is free.
>
> Here's one example of where the flexibility you want (and I understand why
> you want it, restrictions usually feel restrictive) interferes with the
> performance you want.  Suppose I could do this:
>
> value class A {
>    int x;
>    int y;
> }
>
> value class B extends A {
>    int z;
> }
>
> A[] array;
>
> OK, how do I lay this out now?  Part of the point was that I want a packed
> layout without object headers.  But if I don't know how big a B is, I can't
> have this.  (I don't even know whether there are other subclasses of A out
> there that haven't even been loaded yet, and might be even bigger than B.
> So I can't do the simplistic thing of "take the biggest subclass layout
> I've seen at time of array instantiation".)
>
>
>