[dekz]

Are there zip codes that are 2 digits? I'd fall fallacy to this post and once again argue to the contrary. If this function is about formatting zip codes with leading zeros, and zip codes are either 4 or 3 digits. Why would an invalid zip code be passed to this method, a formatter?

Normally I'd agree with your argument about failing to meet a contract, but why would this formatter method get anything but a valid zip code.

[nonchalance]

The first 3 digits of a zip code represent the sectional center facility code, the smallest of which is 005. One would assume that 00501 is the smallest zip code (which apparently maps to HOLTSVILLE NY )

http://pe.usps.com/text/LabelingLists/L002.htm#xd_L002

https://tools.usps.com/go/ZipLookupAction!input.action

[pnathan]

> why would this formatter method get anything but a valid zip code.

Ask not, lest ye receive an answer.

Seriously speaking, you can't trust that callers will be sane unless you establish a priori with a higher level contract that all possible callers will be sane.