funny the n, ++n part :)

Is not tail rec (can go on stackoverflow) and moreover Java does not support tail rec optimization.
If you really want to go with rec mode you should use an accumulator.

Most efficient, cannot say, but the style is elegant and very functional way.

I tend to disagree, this way of doing thing is similar to functional programming.
I woulad have added a map step for the replace call.
ALready said but of course a joining is better than this reduce combined with the get call.
If you absolutely want to a use a reduce, maybe you can use the other version with a initializer
".reduce("", ...)

A switch is more appropriate :)

Indeed, but in this kind of pb, I like person using overwhelming magic ! :D