Can I use file reading system in java if I use an online ide ???

Answer 1

mapleGUARD

Why not? see here:

https://code.sololearn.com/cA14a144a4A1/?ref=app

Answer 2

some IDEs allows it,
by sololearn if you create it programmatically

https://code.sololearn.com/cZ1Xc38SjXKK/?ref=app