Japan is a country with high per-capita GDP, very high HDI, high population density (home to the world largest megalopolis ), cutting edge technology, low natural resources.
It's economy can't grow by implementing already existing technologies, or increasing the population, or using more land, or exploiting more domestic natural resources. It can only grow by technological development, which, in many core areas, is quite slow: a modern car may have all kinds of digital gizmos, but it's only marginally more efficient than a car made 10 or 20 years ago, both in terms of fuel consumption and material costs.

China is far from that point, and was ever further in 1999. It's population is more or less artificially capped, but it has lots of usable land, natural resources, lot of room for technological improvement using already existing technologies, room for increase in internal consumption.
Maybe it will never reach Japan-level per capita wealth, maybe in the long run Japan and other first world countries' wealth will fall and they will equalize with China at some intermediate point, maybe they will keep oscillating, maybe world economy will collapse.

I can't make long-term predictions (other than world economy will probably not keep growing for more than 50-100 years), but I expect that China will keep growing through the next decade. Fourteen years ago, that prediction would have been even stronger.