was thinking one should provide one's own implementation of such and then use is in an assert equals test to find, with run(), the perfect chromosome, which is used to build the fitness function???

Nothing. Regular string concatenation faster that String#format. You can find in google performance of regular concatenation and String#format.

changed

Well, people may find out automatic boxing/unboxing ;) that may be useful.

Indeed, even the definitions of alpha and digit are missing. Can identifiers use é? It's an "alpha" character...

This kata has a very specifc definition of "identifier". It doesn't matter how a specifc language defines valid identifiers.

Note that the Java version is probably a translation from one of the other languages, which have other rules for valid identifiers.

(If you think that the definition of identifier in the kata is misleading, post an issue in the kata description)

Yes, you' re right!
Actually i thought about that after i submitted my code :)