the functions are specific for scala... but this is general enough to work for anything

toggle-test.el

(defvar scala-test:main-file-format
  "%s/src/main/scala/%s%s.scala"
  "scala test format")

(defvar scala-test:test-file-format
  "%s/src/test/scala/%s%sSpec.scala"
  "scala test format")

;; (setq scala-test:main-file-format "%s/src/main/scala/%s%s.scala")
;; (setq scala-test:test-file-format "%s/src/test/scala/%sTest%s.scala")

(defun scala-test:format-to-regex (fmt)
  (format
   (replace-regexp-in-string "\\." "\\\\." fmt)
   "\\(.+\\)" "\\(.+/\\)?" "\\([^/]+\\)"))

(defun scala-test:toggle-spec-test ()
  "Toggle between test and spec"
  (interactive)
  (let* ((path buffer-file-name)

         (main-regex (scala-test:format-to-regex scala-test:main-file-format))
         (spec-regex (scala-test:format-to-regex scala-test:test-file-format))

         (other-path 
          (cond ((string-match spec-regex path)
                 (format scala-test:main-file-format
                         (match-string 1 path)
                         (match-string 2 path)
                         (match-string 3 path)))
                ((string-match main-regex path)
                 (format scala-test:test-file-format
                         (match-string 1 path)
                         (match-string 2 path)
                         (match-string 3 path))))))
    (message "%s" other-path)
    (find-file other-path)))