Currently, Japan has the second strongest navy in the world. Behind only the US. Unless of course you are including nuclear options in your naval comparisons. In that case China, India, Russia, Britain, France, and even Israel have a strategic naval superiority over Japan.
But if you presume nukes are for deterrrence only, then Japan has the second strongest navy in the world. Japan has a large surface fleet, some of the best naval technology, and extremely well trained crews. Don't let the innocuous name 'self-defence force' fool you.
China's navy is still in a massive transition phase. They have recently retired all their Whiskey class submarines, and are still phasing out large numbers of Romeo class submarines. These old noisy obsolete subs are worthless in attack, but they do provide China with patrol services and require diversion of enemy resources for tracking. As China continues to replace them with new advanced subs, China's submarine fleet has grown smaller and more lethal. There is no actual naval buildup in terms of numbers. China's fleet is actually shrinking. But once the old junk is completely decommissioned (in about ten years) there will be a buildup, and it will be of a very potent variety.