If you mean the lunches bought in Japanese convenience stores, they are generally divided into hot and cold ones. If it is hot food, such as all kinds of lunch boxes with rice, most cashiers in convenience stores will ask if they need heating when checking out, and a few stores will use the microwave oven next to them to heat it after checking out.

There are many kinds of cold lunches, including salads (some with cold noodles and salads), diced fruits, rice balls (such as hand-held rice balls and triangular rice balls, packaged separately), sandwiches, comprehensive sushi lunch boxes, various side dishes, snacks, etc. (generally packaged separately for sale). Generally, you can eat directly without heating. If you want to eat the rice balls while they are hot, you can also ask the cashier for a microwave oven when you check out.