Sized

[Brian Goetz]

> Of course, some collections, like the key set of a ConcurrentHashMap,
> don't know their size any more than a garden-variety Iterable does.

That's not exactly true.  In the absence of interference from concurrent 
modification, the key set of a CHM absolutely knows its size.  It's just 
that in the presence of interference, the reported size may be stale by 
the time you receive it.