Compare and contrast abstract classes and interfaces. Why would you use an abstract class? Why would you use an interface?

Question

What will be an ideal response?

Answer 1

An abstract class describes the general notion of what it means to be an object of that class. Abstract classes are incomplete—they normally contain data and one or more methods and properties that are declared abstract because they cannot be implemented in a general sense. Objects of an abstract class cannot be instantiated. Concrete derived classes must declare the “missing pieces”—the implementations of the abstract methods and properties that are appropriate for each specific derived class.
Abstract class references can refer to objects of any of the classes below the abstract class in an inheritance hierarchy and therefore can be used to process any such objects polymorphically. An interface also describes abstract functionality that can be implemented by objects of any number of classes. Classes that implement an interface can be completely unrelated, whereas concrete derived classes of the same abstract base class are all related to one other by way of a shared base class. An interface is often used when disparate (i.e., unrelated) classes need to provide common functionality (i.e., methods and properties). An interface can also be used in place of an abstract class when there are no default implementation details (i.e., method and property implementations and instance variables) to inherit. When a class implements an interface,
it establishes an is-a relationship with the interface type, just as a derived class participates in an is-a relationship with its base class. Therefore, interface references can be used to evoke polymorphic behavior, just as abstract base class references can.