To understand recursion, you must understand recursion

Using stringbuilder is better

I smell StackOverFlow + Heap polution with huge amount of intermediate String object.
Differently will not be used in production.

I love this one.

Not a big deal. By the way, your solution concatenates strings, too.

Very very intelligent of you you caught that Xehpuk, I should have said concatenate instead. Well done indeed.

No, you cannot increment strings.

yes

Doesn't this increment string object which is immutable?

This isn't even tail-recursive so it doesn't matter if Java supports tail call elimination.
I've rewritten it using StringBuilder and tail-recursion. You may want to check it out.

Concatenation, each time a new string is created.