Coffee, anywhere and everywhere, to get you through another day in Tokyo. It's very rare however to see someone walking around with a coffee cup with a lid, such as one you may get at a coffee shop.
There is no shortage however of coffee in small aluminum cans that can be bought at basically any vending machine you come across, which are on every block, literally.
With a can of coffee, you can buy it and then put it in your bag for later consumption, when you don't have the time or opportunity to stop by a cafe. With a cup, you have to walk with it open, and getting on the train with an open cup of coffee is not exactly the best idea, especially in the crowded morning hours.