10. What is the difference between an abstract class and an interface and when should you use them?

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 behaviour), 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:

• Capture similarities among unrelated classes without artificially forcing a class relationship.
• Declare methods that one or more classes are expected to implement.
• Reveal an object's programming interface without revealing its actual implementation.

• 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.

Abstract Class	Interface
Have executable methods and abstract methods.	Have no implementation code. All methods are abstract.
Can only subclass one abstract class.	A class can implement any number of interfaces.
Can have instance variables, constructors and any visibility: public, private, protected, none.	Cannot have instance variables, constructors and can have only public and none (aka package) visibility.

When to use an abstract class?: In case where you want to use implementation inheritance then it is usually provided by an abstract base class. Abstract classes are excellent candidates inside of application frameworks. Abstract classes let you define some default behaviour and force subclasses to provide any specific behaviour.

When to use an interface?: For polymorphic interface inheritance, where the client wants to only deal with a type and does not care about the actual implementation use interfaces. If you need to change your design frequently, you should prefer using interface to abstract. Coding to an interface reduces coupling and interface inheritance can achieve code reuse with the help of object composition. Another justification for using interfaces is that they solve the ‘diamond problem’ of traditional multiple inheritance as shown in the figure. Java does not support multiple inheritances. Java only supports multiple interface inheritance. Interface will solve all the ambiguities caused by this ‘diamond problem’.

Design pattern: Strategy design pattern lets you swap new algorithms and processes into your program without altering the objects that use them.