I think it's imperative for China not go all-in to produce too many J-20, but leave some for improved version of J-20 down the road as I am sure Japan has some counter strategy on current version of J-20 .
I can't rational the reason why USA selling so many F-35 to Japan instead of pushing for Aegis defense weapon, WWII massacres still fresh in memory of so many Asian countries especially China , Japan is now ditching it's defensive Aegis and opted for to purchased hundred of offensive F-35 stealth fighters , say what you like- F-35 is design to penetrate a country defense system and bomb the hell out of a country - it's an offensive weapon. How can USA ignore the feeling of it's Asian neighbors when mass genocides was done by Japan -until today refused to acknowledge ; needless to say a formal apology to its Asian neighbors.
It's ironic that China is seen as a power crazy nation when the history said otherwise while Japan with its too- many-to-count and too-cruel-for-words killings are given the benefit of doubt. I have little doubt that Japan still harbor to be world power and their desire to reverse humiliations on WWII and their once a mighty economic stagnated by USA. History shows us that Japan has no boundaries when it's comes WMD creation through audacious weapons experiment on human beings, they are allowed now to experiment again with weapons on top of acquiring them from USA. Beware USA !! I'm sure if they allow to continue with their weapon experiments, in few decades they will even leapfrog USA. By then you will see clearly who is the wolf in sheep clothing and who is the sheep you have put in wolf clothing.