Generally iterators should be thrown away. Indeed, for anything that
isn't a collection, return an Iterable (or subtype) rather than an
Iterator. Look after the design first, before starting on premature
optimisation.

The performance cost of creating a new iterator over reusing one?
Perhaps a dozen cycles.

The cost of keeping an iterator? Very much more difficult. Naive
microbenchmarks will show big improvements. Real code will not. Having
more (small) objects will increase GC times, which microbenchmarks wont
show. Even retrieving a cached object may take significant time if it
has dropped out of the CPU cache. And of course you have more code,
which generally means slower.

Almost certainly network latency will be vastly more important overall
to your application than a few tiny, tiny objects.

Tom Hawtin