in abstract class the methods may or may not be implemented, when we implement the methods in the abstract class, then why cant we instantiate objects in for the abstract class. is it mandatory that the objects are not instantiated in abstract class. can anyone tell me the internal working of the abstract class.

If even one method is declared as abstract then the class must be abstract and cannot be directly instantiated. There may be a lot of other functionality in the class, but declaring a method abstract means "this behavior must be specified by a subclass". To instantiate, you must at the very least implement the methods that are declared as abstract in a subclass and thus provide that "missing" behavior of the method. Abstract classes let you code a lot of common functionality into a base class and leave only the specific differences in implementation to be supplied.