[noelwelsh]

If you changed reasonably quickly to extremely quickly I would agree. There is some overhead involved but it is very small. In a copying collector, like the 1st generation of Hotspot's generational GC, the GC cost is proportional to the amount of non-garbage you allocate. Creating lots of very short-lived objects is almost free (allocation is very fast in all GCs). The cost is in memory consumption -- a classic tradeoff of time for space.

Of course if this is an issue you can always fall back to Java's collections or a collection class specialised to Longs. I think the Scala implementors made the right tradeoff for a general purpose library.