Japan is not a nuclear power only because they choose not to be. I believe they have the information and the technical capability to create nuclear weapons. If the NKs launch a nuke figure Japan will probably successfully intercept it, a couple of months later several NK military installations disappear from aircraft delivered weapons.
I would bet good money Japan already has nuclear weapons.
They’ve had everything they need for many decades.
And to date, they’ve had no reason to show their hand.