It’s not easy to find public trash cans in Tokyo (other than
bottle-and-can recycling bins next to drink vending machines). Even so,
there’s hardly any litter on the ground. When I travel to Japan, I
always keep a plastic bag in my knapsack to store trash until I come
across a rare public gomibako (Tip: There’s one in front of the Kiddy Land toy store in Harajuku).
Better tip: right inside the doors of most combini (convenience stores) there are trash cans. Still, it’s a good idea to keep a plastic bag with you.