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.
According to the Asia Maritime Transparency Initiative, "More worrying over the long-term is that, while the total number of Chinese ships patrolling the Senkakus has been relatively constant, their size is steadily increasing. According to data released by the CCG, the ships it sent to the Senkakus in 2014 displaced an average of about 2,200 tons. In 2015, that average had grown to over 3,200 tons."
This is important because China has modified several naval warships and redeployed them as coast guard ships to help patrol the Senkakus.
"They are much better armed than most coast guard vessels, though in the law enforcement field, where ramming or shouldering is more likely than an exchange of fire, the extra mass alone could tip the balance in any contest."
Meanwhile, Japan is scrambling its fighter jets at Cold War levels because of incursions by Russian bomber and Chinese fighter jets.
According to James Manicom of the Online Asia Times, "The East China Sea dispute stems from overlapping jurisdictional claims under UNCLOS. Japan claims an exclusive economic zone (EEZ) as far as its median line that bisects the East China Sea. China, meanwhile, has always claimed a continental shelf as far as the Okinawa Trough based on the principle of natural prolongation, the basis upon which continental shelf claims are made."