Japan has very high technology and probably had nuclear weapons for the longest time.
My guess is since before 1980.
Japan officially claim they do not have nuclear weapons.
They mean they did not connect Part A to Part B.
US is aware but in order to offset the China/Russia and North Korea nuclear threat, allows Japan to carry on.
It is a kind of {I don't ask and you don't tell} situation about nuclear weapons in Japan.
Understand the Japanese psyche.
Japanese perceive themselves as the foremost Asian.
It is intolerable for Japan not to have nuclear weapons when Russia, China, India, Pakistan and North Korea have.
(75% of Russian territory lies in Asia continent)
The Fukushima disaster had a nuclear meltdown.
US tried to help and rushed emergency crews to Japan to contain the nuclear accident.
Japan obstinately refused all help from US and disallowed Americans entering the nuclear plant.
There may be some aspects of Japan nuclear industry that it wants to keep Top Secret.