Probably. According to an unnamed official cited by the NBC some years back:
But according to a senior Japanese government official deeply involved in the country’s nuclear energy program, Japan has been able to build nuclear weapons ever since it launched a plutonium breeder reactor and a uranium enrichment plant 30 years ago.
“Japan already has the technical capability, and has had it since the 1980s,” said the official. He said that once Japan had more than five to 10 kilograms of plutonium, the amount needed for a single weapon, it had “already gone over the threshold,” and had a nuclear deterrent.
Japan now has 9 tons of plutonium stockpiled at several locations in Japan and another 35 tons stored in France and the U.K. The material is enough to create 5,000 nuclear bombs. The country also has 1.2 tons of enriched uranium.
Germany's HEU stockpile, albeit officially owned by EU (apparently) was pretty similar. The Japanese (on-shore) plutonium stockpile seems sans pareil though in countries that are not nuclear weapons holders, but seemingly similar to what India has.
From the same NBC article:
Technical ability doesn’t equate to a bomb, but experts suggest getting from raw plutonium to a nuclear weapon could take as little as six months after the political decision to go forward. A senior U.S. official familiar with Japanese nuclear strategy said the six-month figure for a country with Japan’s advanced nuclear engineering infrastructure was not out of the ballpark, and no expert gave an estimate of more than two years.
One also has to think in terms of delivery capabilities. Japan has a space program, but still I think developing SLBMs, which (if I recall correctly) is how France or the UK have much of their deterrent placed, can take years. Building silos in Japan (like the US, China and Russia have, and France once had) is probably a non-starter in Japan due to geography, population density etc.
There's a bit more interesting background story how Japan ended up with such a large stock of plutonium. The TLDR version could easily make the plot for movie:
- Japan started a ($17 billion) fast breeder reactor program, but its Monju prototype suffered a sodium-leak fire and was decommissioned.
- Fuel sent for MOX reprocessing in the UK (at BNFL) came back with falsified (copy-pasted) data, which caused a scandal and loss of confidence, with environmental groups protesting that all MOX was thus not safe. The MOX program was set back some 12 years compared to its original schedule; the first operational/commercial MOX fuel was used in 2009.
- Then Fukushima happened in 2011, shutting down much of the country's nuclear power plants in the aftermath. Of the 16-18 LRWs that were planned to use MOX, only 3 were operational in 2018.