Well, since J005 is not a number then why do you think you can parse it to an int?
If you want to be able to increment a job_id then why not simply have an auto-increment (or equivalent depending on what database you are using) field with the id? Then you wouldn't have to worry about doing this yourself, or about a clash with any other system trying to do the same thing.
Review String.replace() and String.replaceFirst() to toggle J
If you are feeling confident, use StringBuilder to do the toggling.