Can I have both Japanese nationality and China nationality?

No, dual citizenship is not allowed in China, but you can get a permanent residence permit (green card) in Japan, provided that you have worked and lived in Japan for about ten years, which depends on the immigration laws and regulations in Japan. It's relatively easy to acquire Japanese citizenship, about five years, but you have to give up your China citizenship. . . . . . I only know a rough idea. Please know more about it.