If the String gets longer one would better use a StringBuilder instead of String concatenation.

Easy to understand solution using OOP patterns. However I think if there would be thousands/millions of operations this code would cause significant perfomance issues because of object creation.

This is exactly what I had in mind before unlocking the solution. I was missing the .boxed() call so my List from Arrays.stream() could not be assigned to List. I've learned something new :) Thanks.

This is crap. Don't use labels.

The code is too verbose. At least you should reduce it to a single return statement.