valhalla-dev Digest, Vol 17, Issue 3

Vitaly Davidovich vitalyd at
Mon Dec 28 16:14:25 UTC 2015

On Monday, December 28, 2015, Peter Levart <peter.levart at> wrote:

> Hi,
> It seems this thread has not mentioned an existing tool used to
> abstract-away the selection of a particular implementation class - the good
> old static factory method. See EnumSet for an example.

I briefly mentioned factories earlier in this thread.  They can work but
have limitations:

1) you need to define them upfront if you want to avoid users changing
their code later on.  If you don't know of a specialization a priori,
should you define a factory method just in case? Are constructors 2nd class
citizens now?

2) your example below won't even compile as-is? You'd need to add a cast to
the BooleanBitPackedList to return it as the generic type.  And this is
ugly as we now have generic code mixed with specialized code.

> Valhalla has hooks to make the selection possible at runtime given the
> instantiated type parameters:
> public abstract class PackedList<any T> implements List<T> {
>     public static <any E> PackedList<E> newInstance() {
>         if (E.class == boolean.class) {
>             return new BooleanBitPackedList();
>         } else {
>             return new DefaultPackedList<E>();
>         }
>     }
>     private static class BooleanBitPackedList extends PackedList<boolean>
> { ... }
>     private static class DefaultPackedList<any T> extends PackedList<T> {
> ... }
> }
> Regards, Peter
> On 12/25/2015 01:31 AM, Thomas W wrote:
>> Hi Vitaly,
>> I don't see why types/wrappers over one other type are special.  Optional
>> is special in that when it abstracts over a ref type the
>>> semantic of the type can be fulfilled naturally by virtue of ref types
>> already having an absence value: null.  The main point is you
>>> can exploit type info when it's provided.  That's kind of the point of
>> specialization.
>> Well, when you're talking about specializing ArrayList<something> I keep
>> feeling that ArrayList<Optional<T>> is much more useful -- in terms of
>> anybody using it -- than ArrayList<boolean>. Am I wrong in this?
>> Monads/ wrappers such as Optional are probably an "obvious" case to
>> optimize, since they're at the same cardinality & logical level as the
>> underlying data;  but currently add significant instance, indirection,
>> wrapping & storage cost.
>> If we want to go to lengths & consider tackling storage size & efficiency,
>> why not tackle it generically at the storage level -- with a system for
>> "unpacking" value types into arrays -- rather than by specializing at the
>> Collection implementation (eg. ArrayList) level?
>> If we want to address storage, array[] is where storage of pluralities
>> mostly happens. Why not target that directly?
>> I could propose a "packedarray" type, which would implement (either at
>> specialization or at VM level) as an array for each component field. Store
>> & retrieve would be automatically packed & unpacked into it.
>> We would then have:
>> public class ArrayList<any T> {
>>      packedarray T[] elements;
>>      int size;
>> }
>> Store & retrieve from 'elements' would inline the packing & unpacking of
>> elements. Functionality (at least some) needed for operations within the
>> collection would also be inlined -- eg. inlined versions of equals() and
>> hashCode() method would be available, to operate directly onto the packed
>> layout.
>> There'd be some heuristic that the value-type was small & simple enough to
>> benefit from this -- I'd suggest this would apply for value-types with no
>> more than 4 fields for now. Larger value-types would just get stored flat
>> in a single array.
>> If we are trying to target storage, I think general approaches are worth
>> considering vs. code-level specialization.
>> I'd far prefer to consider boolean -> single-bit storage in a generic
>> "packed array" that's implemented once, which everyone -- including all
>> user types implemented subsequently to the library -- get to use
>> automatically, than need to code that stuff M*N times (M per-Collection, N
>> per Element Type) in the library.
>> How is this different from creating named types for all those
>> specializations? It's always preferable to put more burden on lib
>>> author than its users, when given the choice.
>> Nice intent, but placing support in the library means it's not extensible
>> &
>> it's far less useful. The type instantiation site is outside the library
>> --
>> if somebody needs a special collection for their type, that can be
>> instantiated out there too.
>> If we require specialized sections of code in the library, it means only
>> Java core types can get that special support -- and probably not many of
>> them. Essentially other people's small types & wrappers become
>> second-class
>> citizens -- ArrayList will offer bets support only for JDK value types,
>> etc.
>> Many many great libraries such as Guava, Apache, Spring, Hibernate etc are
>> leaders in innovation. They implemented features & types the Java platform
>> needed, long before Java were aware they needed it :) They have also led
>> with great OO design and innovation, often when the core platform sorely
>> lacked it. Numerous times Java has followed the third-party & gained from
>> this innovation.
>> The Java platform & ecosystem need to support third-party libraries --
>> including their types --  as first-class citizens. M*N implementation
>> costs
>> and having efficient support only for "core Java" stuff makes it all seem
>> a
>> bit limited, in my view.
>> Regards,
>> Thomas

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