Java :: Convert Pixel Location To Latitude/longitude & Vise Versa?

I need to convert latitude longitude values into pixel positions as well as do the opposite. I've found many solutions to go from lat/lng->pixel, but can't find anything on the reverse.

A couple of notes:

The map is a fixed size, no zooming, no tiles. I don't need anything super accurate, its not important. Preferably mercator projection, but not required. I'm not actually display the result. (Any 2D projection) I can't rely on any web based API's, ie: no Google Maps

A solution in almost any programming language would be fine, as long as it doesn't rely on any platform specific APIs.

This is an example of going from lat/lng->pixel:

var y = Math.round(((-1 * lat) + 90) * (this.MAP_HEIGHT / 180));
var x = Math.round((lng + 180) * (this.MAP_WIDTH / 360));