Japan has ICBM and nuclear ability, the range they need to cover all China mainland is just 3000-4000km, for China east coast, only 1500 km is enough.
We all know that Japan nuclear is for balancing to China nuclear program ... the decisive method to solve the conflict still the conventional naval battles ... which Japan still have competitive power.
I don't really care about Japan acquire Nuclear bomb or not but Japan always be the 1st nation after US to severely punish other nation for acquiring nuclear weapons. Everyone can have nukes to counter each other and no one would complain.