Japan has been doubling down on it's rhetoric against China and the new PM has even started a campaign to get rid of the article 9 provisions in the constitution. This has inflamed tensions and China would have a reasonable case for war if Japan landed troops onto sovereign Chinese soil.
But...China wouldn't nuke them?
I mean I'm not saying they wouldn't fight back, obviously, but China has a no first strike policy. Unless Japan launches nuclear weapons first, China wouldn't launch them in the first place. This has been the policy ever since China first developed nuclear weapons. [I mean, the only other use case is that if China actually feels like they need to use it to prevent occupation/invasion of China. That's what the nuclear arsenal has been developed for, to make occupation too costly for imperialist powers. I guess technically you could argue a deployment onto Taiwan would count, but...its kinda just dumb? It's not like Russia nuked Ukraine after it invaded Kursk. Xi isn't sitting at his computer like "hehe yes I love nukes let's blow up people."]
Imo the most likely case would depend on escalation. I think if it's limited to just a conflict over Taiwan island China would probably just blockade Japan and force a deal. [I mean...they could try a ground invasion bit I don't think most Chinese people would actually want to do that given the immense resources required].
Sorry, I just keep seeing it said that "Japan will learn when they realize china has nukes this time," even by non-chinese communists. And honestly it's just...not funny? Like if it is a joke, I don't see the punchline. If it's not, then it's foolish mental behavior at best and outright callous at worse. I'd expect those kinda things from Han nationalists with the username "Yonglesgreatestsoldier" or something.
Nukes are kinda weird. As far as I can tell, there really isn't any reason to actually use them unless you're going scorched earth (literally), as in pretty much either genocidal or tit for tat against an enemy that's trying to be genocidal toward you. Using them without dire backlash so far worked only once in history because at the time, the US was the only one who had them and I'm sure because of how well positioned the US was at the end of the war to get away with it. Since then, the proliferation has created the policy of MAD, Mutually-Assured Destruction, effectively creating a stalemate mindset toward nukes: what benefit is there in going scorched earth (even for the most barbaric colonial/imperial forces) if the enemy can also go scorched earth?
So instead we see decades of sieges conducted through economic policy, color revolutions, conventional bombings, and so on. Which nevertheless can be extremely murderous, including toward civilians, it just doesn't happen all at once the way a nuke does. The more insidious nukes are the violent campaigns of imperialism and colonialism that have genocided peoples over territorial control and this framed in terms of "civilizing", "peacekeeping", "freedom."
As for China, it shows no interest in, nor benefit to be gained from, such violent policy, nuke or otherwise. Its people will undoubtedly defend its interests, but not with interest toward annihilating others. Neither the material conditions, nor worldview, are there to support such barbaric action. The more salient point is the economic power that its building and the ways it can use this to hit military interests with precise restrictions (such as in the rare earth minerals stuff that was going on with the US - they may have made a deal since? I'm not sure offhand), focusing on damaging warmongers without hurting civilians in the way that US sanction-wielding does. This isn't to say they can't defend themselves if attacked, but there is no reason to believe they would view it in some gleeful way as an opportunity to go scorched earth.
That said, I guess the less literal view of "China has nukes this time" is that China is far from helpless and if Japan tries to fuck with them the way imperial/colonial Japan did in the past, it's not going to go well for Japan at all. Another way to put it is, China is the economic and military powerhouse this time, so Japan should be glad it's not run by imperialists of a Chinese kind who like the idea of revenge and take the opportunity to work on relations instead. They are probably too dependent on US controls right now though, warping them toward self-defeating interests.