Not to ruin your party, Princeps, but...
As of early 2007, in terms of major surface combatants :
China : 29 destroyers, 40 frigates.
Japan : 54 destroyers, 0 frigates.
While this appears to give China a slight number advantage, consider that destroyers are generally heavier and more powerful warships than frigates, that the vast majority of Chinesse ships are of 70s design (and based off an even earlier Soviet design), whereas only a handful of Japanesse ships are even that old, even those were based off contemporary american design, and virtually every cold-war-era ship still in service in the JMSDF has been extensively refitted and modernized.
Submarine wise is more or less the same story. Yes, China has a LOT of subs, but again - their latest subs are at best roughly comparable with late 70s and early 80s Soviet subs, whereas Japan's submarines (keeping in mind that Japan build their ships generally up to date with American tech) are all state-of-the-art ships.
If you bring in airpower, it's more or less the same. Sure, China has as a huge number advantage there, but Japan has the nasty piece of work that are the F-15Js and E-767 AWACS that go a long way toward evening out the odds.
Neither side has carriers ; Japan lacks the political fortitude and Chinesse the tech know-how to build them. Given current trends, I expect that to change - for both sides - within the next decade or two. Certainly, if China actually starts building a carrier or three, you can expect Japan to look hard at whether to call quit on the "no carrier" business.
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By the way, For comparison's purpose :
Japan : 0 Carrier, 54 destroyers, 0 Frigates.
France : 1 Carrier, 12 Destroyers, 11 Frigates.
UK : 2 Carrier, 8 Destroyers, 17 Frigates.
Frankly, until the Queen Elizabeth class enter service, the Royal Navy would probably lose a naval fight against Japan - 40-odd Sea Harriers just won't make up for that gross number advantage, especially given Japan's AEGIS ships. France isn't much better, even if their Charles de Gaulle is a BIT better than an Invincible ; they only have one.