Japan has been one of China's largest sources of imports, along with South Korea and the United States. Because there is still political enmity between China and Japan due to the Japanese occupation of parts of China during World War II, the best explanation for the present Japanese trade relationship with China is

The Japanese and the Chinese have not been on the best of terms for centuries and this became worse in the 20th century with Japanese attacks on the Chinese and then after those ended with the second world war, Japan was and still is firmly in the U.S. camp which China does not appreciate.

This hasn't stopped Japan and China from trading however because they are so close to each other and so logistical benefits ensure that they are some of each others' biggest trading partners.