Japan is a six-day nuclear power. They have delivery vehicles, they have nuclear tech. The only thing they haven’t done is assembled a device. According to some reports, it would take Japan six days at most to assemble a device, stick it on top of one of their space program’s rockets, and send someone the unwelcome wagon card.
A careful study by some US Naval War College authors, including information from an internal Japanese study leaked by the Japanese press, is that it would take 3-5 years for the Japanese to develop a usable nuclear warhead.
My gut feeling tells me they already assembled a device, and plans for a test detonation. The only thing not assembled is the trigger and nuclear fuel. They probably code-labeled it "popcorn machine" to keep it secret. If things get really hot, they may need it in 15 minutes, not six days. Problem is, using nukes is not a solution for either country. But publicly announcing one via a test, when China is hostile may work in Japan's favor to get China to back off.
Japan has no shortage of fissle material either.