"Conbini" (convenience store) operators are jostling for eminence in a famously cut-throat industry, and one product has come up trumps for the market's top players — coffee.

Millions and millions of cups of it.

Inspired by a popular ¥100 blend launched by McDonald's Holdings Co. (Japan) in 2008, Seven-Eleven Japan Co. touched off a boom in conbini coffee last year, lifting Japan's long-stagnant coffee market and rankling rival businesses.