If the world is silent on Japan's action, then China could do the same. It's called what's good for the goose is good for the gander. If the West say China can't do the same just because it is China, then it's might makes right.
I don't know about might makes right but definitely double standards and favoritism. In this particular case it is also Japan's geographic advantage to be located next to open waters with no other immediately neighboring state to contend its claims. It is much easier to claim parts of a physical public commons for oneself when no one else is near.