OSM / JAVA : latitude & longitude for each point of the tile

Hi everyone

It's my first time using osm, which is a great project. I am using osm tiles in a java program. I get each tile using url like : http://a.tile.openstreetmap.org/zoom/x/y.png

I choose latitude & longitude and get x & y, thanks to the wiki : https://wiki.openstreetmap.org/wiki/Slippy_map_tilenames But i only know the latitude & longitude in the middle of my tile. How could i know latitude & longitude for each point of the tile ?

Thanks Gabriel