source for learning: docs.oracle.com/javase
source for practicing :
http://codegym.cc/mytask
http://www3.ntu.edu.sg/home/ehchua/programming/java/j3f_oopexercises.html#zz-1.1
https://practiceit.cs.washington.edu/problem/list
https://courses.fit.hcmus.edu.vn/ NhapMonLapTrinh CQ