Cooking together is ok, but black rice and mung beans are relatively difficult to cook. It is best to cook black rice and mung beans first (or soak them for half a day in advance) until they are five or six ripe, and then add rice, millet and walnuts. Millet is the easiest to cook, and walnuts can be eaten directly. This taste is the best and the nutritional loss is small. If you don't mind the taste and nutrition, you need to cook together for a long time.