API for looking up Japanese postal code
I'm skeptical of accepting postal codes as Integers.  Japanese postal codes are of the format NNN-NNNN.  Accepting a String, which may contain an optional hyphen and leading zeros, would be more appropriate.

Furthermore, an API that is designed to take an integer could never be extended to countries like the UK and Canada, where postal codes are alphanumeric.  It would also be problematic for US ZIP+4 codes, which, despite being mostly numeric like Japanese codes, may overflow an Integer.  (It's odd that you take an Integer instead of an Int, but that's a moot point.)