String calculator kata in Ruby
The changes I will propose are from ruby-style-guide since that is accepted by the majority of Ruby developers.

What you can improve:


Remove initialize method since there is no need for it in your case.
Use white space to separate groups of logic, something along these lines:

raise 'only accepts a string' unless string_of_numbers.is_a?(String)

string_array = string_of_numbers.split(/[^0-9-]+/)   integer_array = string_array.map(&amp;:to_i)

raise "cannot accept negatives -#{check_for_negatives(integer_array)}" if  check_for_negatives(integer_array)

integer_array.inject(0){|sum,x| x &lt;= 1000? sum + x : sum }

It's not a good practice to hard code the data in the name, instead of string_of_numbers replace with numbers, and instead of integer_array you can figure something else (naming is hard).
Use public and private API methods, since int_add is the only method your class is using, you can rename it to add, and make check_for_negatives private so the reader will know to rely only on add as a stable method.
Ruby returns the last expression, so you don't need an assignment for return negatives_string = negatives_array.join(","), it can be negatives_array.join(",")
You can reduce the check_for_negatives method to this:    

def check_for_negatives(numbers)
  negatives = numbers.select{ |i| i &lt; 0 }
  # Ruby return the last statement which is the negatives string,
  # or nil, that is treated as false
  negatives.join(",") if negatives.length &gt; 0
end