Is it possible to override the main method?

NO, because main is a static method. A static method can't be overridden in Java.

What methods java providing for Thread communications ?

Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll. 

can i Call Abstract method from non abstract method?

Yes, We can call a abstract method from a Non abstract method in a Java abstract class