Most traditional Japanese confectioneries are made of stewed beans and sugar. The most popular beans for this purpose are small red beans called adzuki. They are served hot in winter, cold in summer, sometimes mashed, sometimes strained, served by themselves or with other ingredients, and occasionally shaped in a mold.
During summer, I love to eat chilled sweet adzuki beans with vanilla ice cream, or with shaved ice and green tea syrup. In winter, drop a couple of toasted mochi in hot adzuki bean sauce as oshiruko.
Once you stewed red beans, you can keep it in a refrigerator for about a week or so. If you use more sugar, the stewed beans last longer.
When you buy adzuki beans (dried red beans), try to buy new beans as much as possible. The newer beans can be cooked easily within one hour or so, but old beans are tough and take forever to be soften.
If you are busy, you can find packaged or canned stewed sweet adzuki beans at Asian grocery stores. But when it comes to adzuki beans, I prefer to cook by myself, because home cooked adzuki beans just taste better. And, if you cook beans on your own you can make a lot of stewed beans cheaply.