give me example of exercise to do with java
I think it's better to do some simple projects with math and numbers if you want to improve your coding. for example Odd or Even, Prime Numbers, Average Of Numbers and... . And if you know about SQL you can make a lot of exercises with connecting databases into java in android.
Start with small project like resolve equation of 1st degree then 2nd degree, just real solution. When you finish, you can create class complex, and resolve the equation in C.