Both countries have a fair claim to this section of the ocean. It lies dead centre between them.
This is just a full scale Asian pissing match and I would bet on China winning. They have made the first move. And more to the point in the relationship between the two, Japan needs China more as a market for its goods than China needs Japans investment in China's economy.
"The purpose of fighting is to win. There is no possible victory in defense. The sword is more important than the shield and skill is more important than either. The final weapon is the brain. All else is supplemental." - John Steinbeck