As a country Japan is much more united than China.... Will for independence in Japanese province is not as strong as it can be seen with China's integrity where it can be clearly seen in Xinjiang, Tibet, Qinghai, Sichuan, Fujian, Guangdong, Yunnan, Hainan, Zhejiang .... Taiwan is already separate and do not at all consider itself a part of China....
Once these states are separate already semi-landlocked country like China will further bogged down with very small coastal line useful for only fishing...
These are all forced stated into an artificial state called China...
Add to that Chinese hans have ruined further by trying to change the caste system in China....
To me and the rest of the world Japan is much more united compared to China.... So China needs to worry about its own unification...