Will China's J-20 stealth fighter jet dominates the airfight?

The J-20 looked like a modified SU-27 with a delta wing added to it nothing really new or advanced in it as that design is really old so they added some stealth features to it. That doesn't really do much when you have a fighter jet that has advance computer systems. Also the J-20 wouldn't be completely steathly from all angles as its engines as they are in all current pictures would give off a lot of heat which would mean a heat seeking missile would be able to find it. So it would be of little challenge to anything NATO or the US currently has in operation as most missiles have both fly by wire and heath seeking abilities our jets would be able to take it down. Also they are currently believing that it will be 5 to 10 years from being ready to roll out.

In the US, even though testing has been slow for the F-35, progress continues to be made in fighter design. With modern improvements, the F-16 remains one of the best dogfighters out there and the largely carrier-based F-18 is still a brilliant multi-role aircraft. The J-20 barely matches the F-22 and has no practical flight experience. Also, China has no real aircraft carrier program (they recently refurnished an old Soviet one but it looks to be set-up for scientific use). That makes tactical use of their aircraft extremely difficult in the modern era. Finally, the most important aspect of a plane is it's pilot. Whether we're talking about the old-school jet jockey with thousands of hours of flight experience in his (or her) bird or a new generation of pilots who will control planes, like the Predator, by remote control, American pilots are infinitely superior to the average Chinese pilot. In a World War versus China, NATO would own the seas and skies within hours of declared war. Only the ground war in SouthEast Asia and Russia coupled with short range missile strikes against Japan would give China's enemies cause for concern. This is why China will never declare war in the foreseeable future, making all these conjectures moot.

Considering they have stolen the technology of every country on earth...they ought to have the best stealth fighter. How could they miss? All they have to do is stick the pieces together like an erector set basically. They probably even get the parts from all of us. Screw them anyway.

It is hard to say, and I have no idea about those military weaponry. The U.S. also constantly improve their weapons when they got the wind of China's advanced weapons. There will be a contest between U.S. and China on those air fighter weapons, and ocean carrier too. I hope that will not continue to be a cold war.