Here's a capsule explanation of what's going on in the East China Sea. This territorial dispute is one of the flashpoints in the region, since the Japanese insist on making ports of call.
Here's a link to an image that focuses on the China/Japan territorial dispute. There are other conflicting claims in the region.
In 2012, the Japanese government bought three of five islands from a private family. Japanese vessels regularly patrol the sea, off the coast of Okinawa. The Japanese and the Chinese government have disputed claims to the islands.
The Chinese call them the Diaoyu Islands. Dispute over them has been going on since the 1970s.