Abstract class can contain both abstact methods and concrete methods. In a class, if one method is declared abstract, the class must be declared abstract. However, reverse of this is not necessarily true. I mean, if class is declared abstract class, it may not have abstract method in it.
What is abstract method?
If a method does not provide actual implementation but just provides method signature, it is called abstract method. Actual implementation is left for the sub classes who extends abstract class.
What is concrete method?
If a method provide actual implementation, it is called concrete method.
Abstract method can not be instantiated. It can only be extended by another class.