In response to (now President-Elect) Trump's oft repeated questioning of the sanctity of alliances Japan has been quietly hedging, anticipating a gradual withdrawal of US extended nuclear deterrence.
Japan has definitly developed two of the three essential components of a nuclear weapons capability:
1. a viable nuclear weapon delivery system (see Japan's dual-use delivery system, the Epsilon rocket.
3? Over the last 65 years such an advanced nuclear energy power as Japan will have at least design
plans under lock-and-key for the third element, a viable nuclear device. "During the Sato cabinet
in the 1960's, it is reported that Japan secretly studied the development of nuclear weapons."
With today's computer modeling Japan would not even need to test a nuclear device. Certainly Japan would have little trouble developing a fission device - given plans for such devices were distributed by the A Q Khan network decades ago.
A Japanese Epsilon rocket, with dual-use potential as a future nuclear armed ICBM, launched from Uchinoura Space Center, southern Japan, September 2013, carrying satellite.
JAPAN'S DUAL-USE DELIVERY SYSTEM
These Epsilon specs are very similar to the developed but cancelled US MX ICBM. MXs specs are Height 22m, Diameter 2.3m, Weight 97 tons, 3 stages, blast yield 3 Mt total (using up to 10 MIRVs).
Modern ballistic missiles generally have solid fuel stages (for quicker preparation and more rugged handling) rather than liquid fuel typically used in civilian rockets. So it is notable that the first, second and third stages of the Epsilon are solid fuel.
The 2013 Epsilon launch, carrying satellite.
The extent to which the US assisted JAXA's Epsilon Project is unclear. Japan Aerospace eXploration Agency (JAXA) is Japan’s NASA equivalent. Space agencies have dual military-civilian use technology and dual-use career personnel.