which language should I learn to python or java ?.
I have knowledge of both the languages but I want to learn one fully.

I guess it depends on what sort of project you want to write. Kinda' like asking should I learn to use a hammer or a screwdriver - depends on what you want to do with it.
Both are excellent, search around for a little project and try to write it in each of the languages and see which you prefer maybe?
https://projecteuler.net/ has a nice little list of challenges to take on.

If you are learning for curiosity sake, I would suggest start with Python. I have found Python being "useful" in more situations.

I want to write a best-selling novel. Should I learn French or German, to write the novel in?

@Oversmart you go to the top mate.

python requires less than 2Gb of RAM to run

python... there is so much you can do with python...