Concept of Interface

What is Interface

• An Interface is a group of constants and method declaration.
• .Net supports multiple inheritance through Interface.
• Interface states “what” to do, rather than “how” to do.
• An interface defines only the members that will be made available by an implementing object. The definition of the interface states nothing about the implementation of the members, only the parameters they take and the types of values they will return. Implementation of an interface is left entirely to the implementing class. It is possible, therefore, for different objects to provide dramatically different implementations of the same members.
• Example1, the Car object might implement the IDrivable interface (by convention, interfaces usually begin with I), which specifies the GoForward, GoBackward, and Halt methods. Other classes, such as Truck, Aircraft, Train or Boat might implement this interface and thus are able to interact with the Driver object. The Driver object is unaware of which interface implementation it is interacting with; it is only aware of the interface itself.
• Example2, an interface named IShape, which defines a single method CalculateArea. A Circle class implementing this interface will calculate its area differently than a Square class implementing the same interface. However, an object that needs to interact with an IShape can call the CalculateArea method in either a Circle or a Square and obtain a valid result.
• Practical Example

public interface IDrivable
{
void GoForward(int Speed);
}

public class Truck : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}

public class Aircraft : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}

public class Train : IDrivable
{
public void GoForward(int Speed)
{
// Implementation omitted
}
}


Extra

• Each variable declared in interface must be assigned a constant value.
• Every interface variable is implicitly public, static and final.
• Every interface method is implicitly public and abstract.
• Interfaces are allowed to extends other interfaces, but sub interface cannot define the methods declared in the super interface, as sub interface is still interface and not class.
• If a class that implements an interface does not implements all the methods of the interface, then the class becomes an abstract class and cannot be instantiated.
• Both classes and structures can implement interfaces, including multiple interfaces.

Making choice between Interface and Abstract Class

In which Scenario you will go for Interface or Abstract Class?

Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces
do not provide implementation. They are implemented by classes, and defined as separate entities from
classes. Even though class inheritance allows your classes to inherit implementation from a base class, it
also forces you to make most of your design decisions when the class is first published.

Abstract classes are useful when creating components because they allow you specify an invariant level
of functionality in some methods, but leave the implementation of other methods until a specific
implementation of that class is needed. They also version well, because if additional functionality is
needed in derived classes, it can be added to the base class without breaking code.

Difference between Interface and Abstract Class

Interfaces vs. Abstract Classes
Feature	Interface	Abstract class
Multiple inheritance	A class may implement several interfaces.	A class may extend only one abstract class.
Default implementation	An interface cannot provide any code at all, much less default code.	An abstract class can provide complete code, default code, and/or just stubs that have to be overridden.
Constants	Static final constants only, can use them without qualification in classes that implement the interface. On the other paw, these unqualified names pollute the namespace. You can use them and it is not obvious where they are coming from since the qualification is optional.	Both instance and static constants are possible. Both static and instance intialiser code are also possible to compute the constants.
Third party convenience	An interface implementation may be added to any existing third party class.	A third party class must be rewritten to extend only from the abstract class.
Is-a vs -able or can-do	Interfaces are often used to describe the peripheral abilities of a class, not its central identity, e.g. an Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.	An abstract class defines the core identity of its descendants. If you defined a Dog abstract class then Damamation descendants are Dogs, they are not merely dogable. Implemented interfaces enumerate the general things a class can do, not the things a class is.
Plug-in	You can write a new replacement module for an interface that contains not one stick of code in common with the existing implementations. When you implement the interface, you start from scratch without any default implementation. You have to obtain your tools from other classes; nothing comes with the interface other than a few constants. This gives you freedom to implement a radically different internal design.	You must use the abstract class as-is for the code base, with all its attendant baggage, good or bad. The abstract class author has imposed structure on you. Depending on the cleverness of the author of the abstract class, this may be good or bad. Another issue that's important is what I call "heterogeneous vs. homogeneous." If implementors/subclasses are homogeneous, tend towards an abstract base class. If they are heterogeneous, use an interface. (Now all I have to do is come up with a good definition of hetero/homogeneous in this context.) If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all they share is a set of method signatures, then tend towards an interface.
Homogeneity	If all the various implementations share is the method signatures, then an interface works best.	If the various implementations are all of a kind and share a common status and behavior, usually an abstract class works best.
Maintenance	If your client code talks only in terms of an interface, you can easily change the concrete implementation behind it, using a factory method.	Just like an interface, if your client code talks only in terms of an abstract class, you can easily change the concrete implementation behind it, using a factory method.
Speed	Slow, requires extra indirection to find the corresponding method in the actual class. Modern JVMs are discovering ways to reduce this speed penalty.	Fast
Terseness	The constant declarations in an interface are all presumed public static final, so you may leave that part out. You can't call any methods to compute the initial values of your constants. You need not declare individual methods of an interface abstract. They are all presumed so.	You can put shared code into an abstract class, where you cannot into an interface. If interfaces want to share code, you will have to write other bubblegum to arrange that. You may use methods to compute the initial values of your constants and variables, both instance and static. You must declare all the individual methods of an abstract class abstract.
Adding functionality	If you add a new method to an interface, you must track down all implementations of that interface in the universe and provide them with a concrete implementation of that method.	If you add a new method to an abstract class, you have the option of providing a default implementation of it. Then all existing code will continue to work without change.