What is the definition of interface in object oriented programming

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Ok  a friend of mine go back and forth on what  interface  means in programming   What is the best description of an  interface    To me an interface is a blueprint of a class  is this the best definition

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In my opinion  interface has a broader meaning than the one commonly associated with it in Java  I would define  interface  as a set of available operations with some common functionality  that allow controlling monitoring a module    In this definition I try to cover both programatic interfaces  where the client is some module  and human interfaces  GUI for example    As others already said  an interface always has some contract behind it  in terms of inputs and outputs  The interface does not promise anything about the  how  of the operations  it only guarantees some properties of the outcome  given the current state  the selected operation and its parameters

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An interface defines what a class that inherits from it must implement   In this way  multiple classes can inherit from an interface  and because of that inherticance  you can   be sure that all members of the interface are implemented in the derived class  even if its just to throw an exception  Abstract away the class itself from the caller  cast an instance of a class to the interface  and interact with it without needing to know what the actual derived class IS    for more info  see this http   msdn microsoft com en-us library ms173156 aspx

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In short  The basic problem an interface is trying to solve is to separate how we use something from how it is implemented  But you should consider interface is not a contract  Read more here

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Technically  I would describe an interface as a set of ways  methods  properties  accessors    the vocabulary depends on the language you are using  to interact with an object  If an object supports implements an interface  then you can use all of the ways specified in the interface to interact with this object   Semantically  an interface could also contain conventions about what you may or may not do  e g   the order in which you may call the methods  and about what  in return  you may assume about the state of the object given how you interacted so far

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I don t think  blueprint  is a good word to use   A blueprint tells you how to build something   An interface specifically avoids telling you how to build something   An interface defines how you can interact with a class  i e  what methods it supports

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Conventional Definition - An interface is a contract that specifies the methods which needs to be implemented by the class implementing it   The Definition of Interface has changed over time  Do you think Interface just have method declarations only   What about static final variables and what about default definitions after Java 5   Interfaces were introduced to Java because of the Diamond problem with multiple Inheritance and that s what they actually intend to do   Interfaces are the constructs that were created to get away with the multiple inheritance problem and can have abstract methods   default definitions and static final variables    http   www quora com Why-does-Java-allow-static-final-variables-in-interfaces-when-they-are-only-intended-to-be-contracts

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Personally I see an interface like a template  If a interface contains the definition for the methods foo   and bar    then you know every class which uses this interface has the methods foo   and bar

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Let us consider a Man User or an Object  wants some work to be done  He will contact a middle man Interface  who will be having a contract with the companies real world objects created using implemented classes   Few types of works will be defined by him which companies will implement and give him results  Each and every company will implement the work in its own way but the result will be same  Like this User will get its work done using an single interface  I think Interface will act as visible part of the systems with few commands which will be defined internally by the implementing inner sub systems

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A boundary across which two systems communicate   Interfaces are how some OO languages achieve ad hoc polymorphism  Ad hoc polymorphism is simply functions with the same names operating on different types

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An interface is one of the more overloaded and confusing terms in development   It is actually a concept of abstraction and encapsulation  For a given  box   it declares the  inputs  and  outputs  of that box  In the world of software  that usually means the operations that can be invoked on the box  along with arguments  and in some cases the return types of these operations   What it does not do is define what the semantics of these operations are  although it is commonplace  and very good practice  to document them in proximity to the declaration  e g   via comments   or to pick good naming conventions  Nevertheless  there are no guarantees that these intentions would be followed    Here is an analogy  Take a look at your television when it is off  Its interface are the buttons it has  the various plugs  and the screen  Its semantics and behavior are that it takes inputs  e g   cable programming  and has outputs  display on the screen  sound  etc    However  when you look at a TV that is not plugged in  you are projecting your expected semantics into an interface  For all you know  the TV could just explode when you plug it in  However  based on its  interface  you can assume that it won t make any coffee since it doesn t have a water intake    In object oriented programming  an interface generally defines the set of methods  or messages  that an instance of a class that has that interface could respond to    What adds to the confusion is that in some languages  like Java  there is an actual interface with its language specific semantics  In Java  for example  it is a set of method declarations  with no implementation  but an interface also corresponds to a type and obeys various typing rules    In other languages  like C    you do not have interfaces  A class itself defines methods  but you could think of the interface of the class as the declarations of the non-private methods  Because of how C   compiles  you get header files where you could have the  interface  of the class without actual implementation  You could also mimic Java interfaces with abstract classes with pure virtual functions  etc   An interface is most certainly not a blueprint for a class  A blueprint  by one definition is a  detailed plan of action   An interface promises nothing about an action  The source of the confusion is that in most languages  if you have an interface type that defines a set of methods  the class that implements it  repeats  the same methods  but provides definition   so the interface looks like a skeleton or an outline of the class

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As above  synonyms of  contract  and  protocol  are appropriate    The interface comprises the methods and properties you can expect to be exposed by a class   So if a class Cheetos Bag implements the Chip Bag interface  you should expect a Cheetos Bag to behave exactly like any other Chip Bag   That is  expose the  attemptToOpenWithoutSpillingEverywhere   method  etc

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To me an interface is a blueprint of a class  is this the best definition    No   A blueprint typically includes the internals   But a interface is purely about what is visible on the outside of a class     or more accurately  a family of classes that implement the interface   The interface consists of the signatures of methods and values of constants  and also a  typically informal   behavioral contract  between classes that implement the interface and others that use it

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In Programming  an interface defines what the behavior a an object will have  but it will not actually specify the behavior  It is a contract  that will guarantee  that a certain class can do something    Consider this piece of C  code here   using System   public interface IGenerate       int Generate          Dependencies public class KnownNumber   IGenerate       public int Generate                  return 5              public class SecretNumber   IGenerate       public int Generate                 return new Random   Next 0  10               What you care about class Game       public Game IGenerate generator                 Console WriteLine generator Generate             new Game new SecretNumber     new Game new KnownNumber       The Game class requires a secret number  For the sake of testing it  you would like to inject what will be used as a secret number  this principle is called Inversion of Control    The game class wants to be  open minded  about what will actually create the random number  therefore it will ask in its constructor for  anything  that has a Generate method     First  the interface specifies  what operations an object will provide  It just contains what it looks like  but no actual implementation is given  This is just the signature of the method  Conventionally  in C  interfaces are prefixed with an I  The classes now implement the IGenerate Interface  This means that the compiler will make sure  that they both have a method  that returns an int and is called Generate   The game now is being called two different object  each of which implementant the correct interface  Other classes would produce an error upon building the code   Here I noticed the blueprint analogy you used   A class is commonly seen as a blueprint for an object  An Interface specifies something that a class will need to do  so one could argue that it indeed is just a blueprint for a class  but since a class does not necessarily need an interface  I would argue that this metaphor is breaking  Think of an interface as a contract  The class that  signs it   will be legally required  enforced by the compiler police   to comply to the terms and conditions in the contract  This means that it will have to do  what is specified in the interface    This is all due to the statically typed nature of some OO languages  as it is the case with Java or C   In Python on the other hand  another mechanism is used    import random    Dependencies class KnownNumber object       def generate self           return 5  class SecretNumber object       def generate self           return random randint 0 10     What you care about class SecretGame object       def   init   self  number generator           number   number generator generate           print number   Here  none of the classes implement an interface  Python does not care about that  because the SecretGame class will just try to call whatever object is passed in  If the object HAS a generate   method  everything is fine  If it doesn t  KAPUTT   This mistake will not be seen at compile time  but at runtime  so possibly when your program is already deployed and running  C  would notify you way before you came close to that    The reason this mechanism is used  naively stated  because in OO languages naturally functions aren t first class citizens  As you can see  KnownNumber and SecretNumber contain JUST the functions to generate a number  One does not really need the classes at all  In Python  therefore  one could just throw them away and pick the functions on their own     OO Approach SecretGame SecretNumber    SecretGame KnownNumber       Functional Approach    Dependencies class SecretGame object       def   init   self  generate           number    generate           print number  SecretGame lambda  random randint 0 10   SecretGame lambda  5    A lambda is just a function  that was declared  in line  as you go   A delegate is just the same in C    class Game       public Game Func lt int gt  generate                 Console WriteLine generate                 new Game      gt  5   new Game      gt  new Random   Next 0  10      Side note  The latter examples were not possible like this up to Java 7  There  Interfaces were your only way of specifying this behavior  However  Java 8 introduced lambda expressions so the C  example can be converted to Java very easily  Func lt int gt  becomes java util function IntSupplier and   gt  becomes - gt

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Interface is a contract you should comply to or given to  depending if you are implementer or a user

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An interface separates out operations on a class from the implementation within  Thus  some implementations may provide for many interfaces   People would usually describe it as a  contract  for what must be available in the methods of the class   It is absolutely not a blueprint  since that would also determine implementation  A full class definition could be said to be a blueprint

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Consider the following situation      You are in the middle of a large  empty room  when a zombie suddenly attacks you       You have no weapon       Luckily  a fellow living human is standing in the doorway of the room        Quick   you shout at him   Throw me something I can hit the zombie with     Now consider  You didn t specify  nor do you care  exactly what your friend will choose to toss     But it doesn t matter  as long as    It s something that can be tossed  He can t toss you the sofa  It s something that you can grab hold of  Let s hope he didn t toss a shuriken  It s something you can use to bash the zombie s brains out  That rules out pillows and such    It doesn t matter whether you get a baseball bat or a hammer - as long as it implements your three conditions  you re good   To sum it up   When you write an interface  you re basically saying   I need something that

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