Alone, the Japanese Navy could defeat the Chinese Navy.
Fairly quickly.
Why? Well, for one the frenetic development that the Chinese military has gone through. However, there is another metric - what I call the 'FR statements of certainty.' Examples include a FReeper saying Avatar would be a commercial fop when it was first released. As pertains to China, 15 years ago several FReepers would claim that China would never have the capability to cross the 180 km wide Formosa Strait to attack Taiwan. They were absolutely adamant, based on the fact that at the time the Chinese did not have a proper naval mass transport capability. That was 15 years ago.
In ten years the only navy that will be a challenge to China will be the USN, and the gap will be closer than any of us dare believe today.