Java implementation of String#next() successor

I've found the Ruby's String#next() or #succ very useful and productive, specially when generating data for testing. Here is what the Ruby doc says:

succ -> new_str

next -> new_str

Returns the successor to str. The successor is calculated by incrementing characters starting from the rightmost alphanumeric (or the rightmost character > if there are no alphanumerics) in the string. Incrementing a digit always results in another digit, and incrementing a letter results in another letter > of the same case. Incrementing nonalphanumerics uses the underlying character set’s collating sequence.

If the increment generates a “carry,” the character to the left of it is incremented. This process repeats until there is no carry, adding an additional > character if necessary.

"abcd".succ        #=> "abce"
"THX1138".succ     #=> "THX1139"
"<<koala>>".succ   #=> "<<koalb>>"
"1999zzz".succ     #=> "2000aaa"
"ZZZ9999".succ     #=> "AAAA0000"
"***".succ         #=> "**+"

So when I saw Groovy actually has provided a String extension #next() method, I was happy to try it out. But then I was quickly disappointed when the behavior is very different. The Groovy version is very simple and actually not very productive since it simply loop through Character set range in incrementally (including non-printable characters blindly!). The Ruby's version, however, is much more productive since it produce visible characters. For examples:

    bash> ruby -e 'puts "Z".next()'
    AA
    bash> groovy -e 'println("Z".next())'
    [

I wish Groovy version would improve in future as it's not very useful at the moment. Just for fun, I wrote a Java implementation version that mimics the Ruby's behavior:

And here is my unit test for sanity check: