Question regarding interface java.util.Sized

Andrew Thompson lordpixel at
Fri Oct 26 05:25:38 PDT 2012

I have found myself wanting to write

<C extends Iterable<?> & Sized> doSomething(S input)

Or variations on that theme in real code from time to time. I've always ended up using Collection but didn't like that. Of course it would be a lot more useful if it also worked with arrays…

On Oct 26, 2012, at 7:54 AM, Gernot Neppert <mcnepp02 at> wrote:

> Hello all,
> while browsing the latest lambda-related sourcecode, I came across the
> new mixin-style interface "java.util.Sized".
> The JavaDoc says:
> "Implementing this interface allows an object to indicate that its
> elements may be efficiently counted."
> OK, so what do we do with this interface? In what contexts do we want
> to test for "instanceof Sized" and then query its size()?
> As far as I can see, "Sized" is implemented by:
> java.util.Collection
> java.util.Map
> java.util.streams.ops.Node
> I think that interfaces should have both of the following properties:
> 1. Provide uniform access to an aspect of a class.
> 2. Provide a common handle to a polymorphic set of instances that can
> reasonably put to use
> IMHO, "Sized" fails to have property 2.)
> It is hard to see what usecase would want to handle a polymporphic set
> of Collections, Maps and Node, let alone handle them as "Sized"
> objects, where the only operation you could invoke would be size().
> This, java.util.Collection and java.util.Map gain nothing from
> implementing "Sized"
> If there are cases where it is useful to discriminate between sized an
> non-sized flavours of a specific class or interface, it makes more
> sense to provide an interface that extends that baseclass or
> interface.
> e.g.
> interface SizedStream extends Stream {
>  public int size();
> }
> Just my 2 cents...

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