What is difference between an Abstract Class and an Interface?

In design, you want the base class to present only an interface for its derived classes. This means, you don’t want
anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which
gives you polymorphic behavior), so that its interface can be used. This is accomplished by making that class
abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents
it.
The interface keyword takes this concept of an abstract class a step further by preventing any method or function
implementation at all. You can only declare a method or function but not provide the implementation. The class,
which is implementing the interface, should provide the actual implementation. The interface is a very useful and
commonly used aspect in OO design, as it provides the separation of interface and implementation and
enables you to:
1. Capture similarities among unrelated classes without artificially forcing a class relationship.
2. Declare methods that one or more classes are expected to implement.
3. Reveal an object's programming interface without revealing its actual implementation.
4. Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple
inheritances, a feature that some object-oriented languages support that allow a class to have more than one
superclass.

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