I’m assuming Japan has access to weapons grade plutonium and yellow cake. Am I incorrect? They had a reactor at Fukushima.
It's been said that they could build and assemble nukes within a week if necessary. They have the components and know-how to do so. They have plenty of weapons-grade plutonium. I think they have shipped much to the USA for disposal but only a fool would think they didn't keep plenty at home for themselves.
Regarding some articles that state Japan could build nukes within 6 months, that reflects the judgement that it takes about that long to create the weapons grade plutonium from by-products at their reactors. However, Japan has 44 tons of plutonium and well over a ton of enriched uranium that they’ve been creating over the last 30 years. They have enough material to create many thousands of nukes. (All you need is 5 to 10 kg of plutonium for a bomb.) So forget about that 6 month timeframe, they can assemble them much faster if required. This is something the Chinese are quite aware of and afraid of.