What is the difference between an interface and abstract class

Question

What exactly is the difference between an interface and abstract class

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In short the differences are the following:

Syntactical Differences Between Interface and Abstract Class:

Methods and members of an abstract class can have any visibility. All methods of an interface must be public. //Does not hold true from Java 9 anymore
A concrete child class of an Abstract Class must define all the abstract methods. An Abstract child class can have abstract methods. An interface extending another interface need not provide default implementation for methods inherited from the parent interface.
A child class can only extend a single class. An interface can extend multiple interfaces. A class can implement multiple interfaces.
A child class can define abstract methods with the same or less restrictive visibility, whereas class implementing an interface must define all interface methods as public.
Abstract Classes can have constructors but not interfaces.
Interfaces from Java 9 have private static methods.

In Interfaces now:

public static - supported
public abstract - supported
public default - supported
private static - supported
private abstract - compile error
private default - compile error
private - supported

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Many junior developers make the mistake of thinking of interfaces  abstract and concrete classes as slight variations of the same thing  and choose one of them purely on technical grounds  Do I need multiple inheritance  Do I need some place to put common methods  Do I need to bother with something other than just a concrete class  This is wrong  and hidden in these questions is the main problem   I   When you write code for yourself  by yourself  you rarely think of other present or future developers working on or with your code   Interfaces and abstract classes  although apparently similar from a technical point of view  have completely different meanings and purposes   Summary   An interface defines a contract that some implementation will fulfill for you  An abstract class provides a default behavior that your implementation can reuse    Alternative summary   An interface is for defining public APIs An abstract class is for internal use  and for defining SPIs   On the importance of hiding implementation details  A concrete class does the actual work  in a very specific way  For example  an ArrayList uses a contiguous area of memory to store a list of objects in a compact manner which offers fast random access  iteration  and in-place changes  but is terrible at insertions  deletions  and occasionally even additions  meanwhile  a LinkedList uses double-linked nodes to store a list of objects  which instead offers fast iteration  in-place changes  and insertion deletion addition  but is terrible at random access  These two types of lists are optimized for different use cases  and it matters a lot how you re going to use them  When you re trying to squeeze performance out of a list that you re heavily interacting with  and when picking the type of list is up to you  you should carefully pick which one you re instantiating   On the other hand  high level users of a list don t really care how it is actually implemented  and they should be insulated from these details  Let s imagine that Java didn t expose the List interface  but only had a concrete List class that s actually what LinkedList is right now  All Java developers would have tailored their code to fit the implementation details  avoid random access  add a cache to speed up access  or just reimplement ArrayList on their own  although it would be incompatible with all the other code that actually works with List only  That would be terrible    But now imagine that the Java masters actually realize that a linked list is terrible for most actual use cases  and decided to switch over to an array list for their only List class available  This would affect the performance of every Java program in the world  and people wouldn t be happy about it  And the main culprit is that implementation details were available  and the developers assumed that those details are a permanent contract that they can rely on  This is why it s important to hide implementation details  and only define an abstract contract  This is the purpose of an interface  define what kind of input a method accepts  and what kind of output is expected  without exposing all the guts that would tempt programmers to tweak their code to fit the internal details that might change with any future update   An abstract class is in the middle between interfaces and concrete classes  It is supposed to help implementations share common or boring code  For example  AbstractCollection provides basic implementations for isEmpty based on size is 0  contains as iterate and compare  addAll as repeated add  and so on  This lets implementations focus on the crucial parts that differentiate between them  how to actually store and retrieve data   APIs versus SPIs  Interfaces are low-cohesion gateways between different parts of code  They allow libraries to exist and evolve without breaking every library user when something changes internally  It s called Application Programming Interface  not Application Programming Classes  On a smaller scale  they also allow multiple developers to collaborate successfully on large scale projects  by separating different modules through well documented interfaces   Abstract classes are high-cohesion helpers to be used when implementing an interface  assuming some level of implementation details  Alternatively  abstract classes are used for defining SPIs  Service Provider Interfaces   The difference between an API and an SPI is subtle  but important  for an API  the focus is on who uses it  and for an SPI the focus is on who implements it   Adding methods to an API is easy  all existing users of the API will still compile  Adding methods to an SPI is hard  since every service provider  concrete implementation  will have to implement the new methods  If interfaces are used to define an SPI  a provider will have to release a new version whenever the SPI contract changes  If abstract classes are used instead  new methods could either be defined in terms of existing abstract methods  or as empty throw not implemented exception stubs  which will at least allow an older version of a service implementation to still compile and run   A note on Java 8 and default methods  Although Java 8 introduced default methods for interfaces  which makes the line between interfaces and abstract classes even blurrier  this wasn t so that implementations can reuse code  but to make it easier to change interfaces that serve both as an API and as an SPI  or are wrongly used for defining SPIs instead of abstract classes    Which one to use    Is the thing supposed to be publicly used by other parts of the code  or by other external code  Add an interface to it to hide the implementation details from the public abstract contract  which is the general behavior of the thing  Is the thing something that s supposed to have multiple implementations with a lot of code in common  Make both an interface and an abstract  incomplete implementation  Is there ever going to be only one implementation  and nobody else will use it  Just make it a concrete class     ever  is long time  you could play it safe and still add an interface on top of it     A corollary  the other way around is often wrongly done  when using a thing  always try to use the most generic class interface that you actually need  In other words  don t declare your variables as ArrayList theList   new ArrayList    unless you actually have a very strong dependency on it being an array list  and no other type of list would cut it for you  Use List theList   new ArrayList instead  or even Collection theCollection   new ArrayList if the fact that it s a list  and not any other type of collection doesn t actually matter

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I don t want to highlight the differences  which have been already said in many answers   regarding public static final modifiers for variables in interface  amp  support for protected  private methods in abstract classes   In simple terms  I would like to say   interface  To implement a contract by multiple unrelated objects  abstract class  To implement the same or different behaviour among multiple related objects  From the Oracle documentation  Consider using abstract classes if     You want to share code among several closely related classes  You expect that classes that extend your abstract class have many common methods or fields  or require access modifiers other than public  such as protected and private   You want to declare non-static or non-final fields     Consider using interfaces if     You expect that unrelated classes would implement your interface  For example many unrelated objects can implement Serializable interface  You want to specify the behaviour of a particular data type  but not concerned about who implements its behaviour  You want to take advantage of multiple inheritance of type    abstract class establishes  is a  relation with concrete classes  interface provides  has a  capability for classes   If you are looking for Java as programming language  here are a few more updates   Java 8 has reduced the gap between interface and abstract classes to some extent by providing a default method feature  An interface does not have an implementation for a method is no longer valid now    Refer to this documentation page for more details    Have a look at this SE question for code examples to understand better   How should I have explained the difference between an Interface and an Abstract class

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You can find clear difference between interface and abstract class   Interface   Interface only contains abstract methods  Force users to implement all methods when implements the interface  Contains only final and static variables  Declare using interface keyword  All methods of an interface must be defined as public  An interface can extend or a class can implement multiple other interfaces    Abstract class   Abstract class contains abstract and non-abstract methods  Does not force users to implement all methods when inherited the abstract class  Contains all kinds of variables including primitive and non-primitive Declare using abstract keyword  Methods and members of an abstract class can be defined with any visibility  A child class can only extend a single class  abstract or concrete

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An interface is so called because it provides an interface of methods to a caller  or a COM client for instance  that are implemented by some class  By polymorphically casting an object pointer to the type of an interface that the object s class implements  it restricts the access of the object to functions and members of the interface that it implements  separated from other COM interfaces the coclass might implement  The client does not need to know what class implements the interface or what other methods are present in the class  the object presents as an instance of the interface it knows  where the instance of the class has been polymorphically cast to the interface subinstance  and it just uses the interface by calling the methods of the interface on the interface instance  All details of the actual implementation and extraneous functionality   details implemented by different interfaces are separated from the interface the caller expects -- the caller just uses the interface it has with the object  and the underlying object implementation is called without the caller having to know the location or the details of the implementation  Accessing the object through the interface  a pointer of the type of the interface  is a form of encapsulation that syntactically prevents unauthorised access to the object as well as hiding implementation details and other functionality that does not pertain to the interface and its defined personality  An interface is where all methods are virtual and abstract  abstract is known as pure virtual in C    all abstract methods contain the virtual specifier and therefore are virtual   An abstract class is where at least one of the methods is virtual and specified as abstract  or pure virtual in C     Other details differ across languages  All interface attributes are implicitly public static final in java but they aren t in C    Java allows non-static attributes in abstract classes but C   allows them in both  Attributes cannot be virtual   abstract in either language

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I am 10 yrs late to the party but would like to attempt any way  Wrote a post about the same on medium few days back  Thought of posting it here   tl dr  When you see    Is A    relationship use inheritance abstract class  when you see    has a    relationship create member variables  When you see    relies on external provider    implement  not inherit  an interface   Interview Question  What is the difference between an interface and an abstract class  And how do you decide when to use what  I mostly get one or all of the below answers  Answer 1  You cannot create an object of abstract class and interfaces   ZK  That   s my initials   You cannot create an object of either  So this is not a difference  This is a similarity between an interface and an abstract class  Counter  Question  Why can   t you create an object of abstract class or interface   Answer 2  Abstract classes can have a function body as partial default implementation   ZK  Counter Question  So if I change it to a pure abstract class  marking all the virtual functions as abstract and provide no default implementation for any virtual function  Would that make abstract classes and interfaces the same  And could they be used interchangeably after that   Answer 3  Interfaces allow multi-inheritance and abstract classes don   t   ZK  Counter Question  Do you really inherit from an interface  or do you just implement an interface and  inherit from an abstract class  What   s the difference between implementing and inheriting  These counter questions throw candidates off and make most scratch their heads or just pass to the next question  That makes me think people need help with these basic building blocks of Object-Oriented Programming  The answer to the original question and all the counter questions is found in the English language and the UML  You must know at least below to understand these two constructs better   Common Noun  A common noun is a name given    in common    to things of the same class or kind  For e g  fruits  animals  city  car etc   Proper Noun  A proper noun is the name of an object  place or thing  Apple  Cat  New York  Honda Accord etc   Car is a Common Noun  And Honda Accord is a Proper Noun  and probably a Composit Proper noun  a proper noun made using two nouns   Coming to the UML Part  You should be familiar with below relationships    Is A Has A Uses   Let   s consider the below two sentences  - HondaAccord Is A Car  - HondaAccord Has A Car   Which one sounds correct  Plain English and comprehension  HondaAccord and Cars share an    Is A    relationship  Honda accord doesn   t have a car in it  It    is a    car  Honda Accord    has a    music player in it   When two entities share the    Is A    relationship it   s a better candidate for inheritance  And Has a relationship is a better candidate for creating member variables  With this established our code looks like this   abstract class Car      string color     int speed    class HondaAccord   Car      MusicPlayer musicPlayer      Now Honda doesn t manufacture music players  Or at least it   s not their main business   So they reach out to other companies and sign a contract  If you receive power here and the output signal on these two wires it   ll play just fine on these speakers   This makes Music Player a perfect candidate for an interface  You don   t care who provides support for it as long as the connections work just fine   You can replace the MusicPlayer of LG with Sony or the other way  And it won   t change a thing in Honda Accord   Why can   t you create an object of abstract classes   Because you can   t walk into a showroom and say give me a car  You   ll have to provide a proper noun  What car  Probably a honda accord  And that   s when a sales agent could get you something   Why can   t you create an object of an interface  Because you can   t walk into a showroom and say give me a contract of music player  It won   t help  Interfaces sit between consumers and providers just to facilitate an agreement  What will you do with a copy of the agreement  It won   t play music   Why do interfaces allow multiple inheritance   Interfaces are not inherited  Interfaces are implemented  The interface is a candidate for interaction with the external world  Honda Accord has an interface for refueling  It has interfaces for inflating tires  And the same hose that is used to inflate a football  So the new code will look like below   abstract class Car       string color      int speed    class HondaAccord   Car  IInflateAir  IRefueling       MusicPlayer musicPlayer      And the English will read like this    Honda Accord is a Car that supports inflating tire and refueling

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The topic of abstract classes vs interfaces is mostly about semantics   Abstract classes act in different programming languages often as a superset of interfaces  except one thing and that is  that you can implement multiple interfaces  but inherit only one class   An interface defines what something must be able to do  like a contract  but does not provide an implementation of it   An abstract class defines what something is and it commonly hosts shared code between the subclasses   For example a Formatter should be able to format   something  The common semantics to describe something like that would be to create an interface IFormatter with a declaration of format   that acts like a contract  But IFormatter does not describe what something is  but just what it should be able to to  The common semantics to describe what something actually is  is to create a class  In this case we create an abstract class    So we create an abstract class Formatter which implements the interface  That is a very descriptive code  because we now know we have a Formatter and we now know what every Formatter must be able to do   Also one very important topic is documentation  at least for some people      In your documentation you probably want to explain within your subclasses what a Formatter actually is  It is very convenient to have an abstract class Formatter to which documentation you can link to within your subclasses  That is very convenient and generic  On the other hand if you do not have an abstract class Formatter and only an interface IFormatter you would have to explain in each of your subclasses what a Formatter actucally is  because an interface is a contract and you would not describe what a Formatter actually is within the documentation of an interface     at least it would be not something common to do and you would break the semantics that most developers consider to be correct   Note  It is a very common pattern to make an abstract class implement an interface

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Not really the answer to the original question  but once you have the answer to the difference between them  you will enter the when-to-use-each dilemma  When to use interfaces or abstract classes  When to use both   I ve limited knowledge of OOP  but seeing interfaces as an equivalent of an adjective in grammar has worked for me until now  correct me if this method is bogus    For example  interface names are like attributes or capabilities you can give to a class  and a class can have many of them  ISerializable  ICountable  IList  ICacheable  IHappy

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In an interface all methods must be only definitions  not single one should be implemented    But in an abstract class there must an abstract method with only definition  but other methods can be also in the abstract class with implementation

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An abstract class is a class whose object cannot be created or a class which cannot be instantiated  An abstract method makes a class abstract   An abstract class needs to be inherited in order to override the methods that are declared in the abstract class   No restriction on access specifiers   An abstract class can have constructor and other concrete non abstarct methods   methods in them but interface cannot have   An interface is a blueprint template of methods  eg  A house on a paper is given interface house  and different architects will use their ideas to build it the classes of architects implementing the house interface     It is a collection of abstract methods   default methods   static methods   final variables and nested classes  All members will be either final or public   protected and private access specifiers are not allowed No object creation is allowed   A class has to be made in order to use the implementing interface and also to  override the abstract method declared in the interface  An interface is a good example of loose coupling dynamic polymorphism dynamic binding  An interface implements polymorphism and abstraction It tells what to do but how to do is defined by the implementing class   For Eg  There s a car company and it wants that some features to be same for all the car it is manufacturing so for that the company would be making an interface vehicle which will have those features and different classes of car like Maruti Suzkhi   Maruti 800  will override those features functions    Why interface when we already have abstract class  Java supports only multilevel and hierarchal inheritance but with the help of interface we can implement multiple inheritance

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Some important differences   In the form of a table     As stated by Joe from javapapers       1 Main difference is methods of a Java interface are implicitly abstract and cannot have implementations  A Java abstract class can   have instance methods that implements a default behavior       2 Variables declared in a Java interface is by default final  An  abstract class may contain non-final variables       3 Members of a Java interface are public by default  A Java abstract class can have the usual flavors of class members like private    protected  etc        4 Java interface should be implemented using keyword    implements     A Java abstract class should be extended using keyword    extends          5 An interface can extend another Java interface only  an abstract class can extend another Java class and implement multiple Java   interfaces       6 A Java class can implement multiple interfaces but it can extend only one abstract class       7 Interface is absolutely abstract and cannot be instantiated  A Java abstract class also cannot be instantiated  but can be invoked if a   main   exists       8 In comparison with java abstract classes  java interfaces are slow as it requires extra indirection

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The only difference is that one can participate in multiple inheritance and other cannot   The definition of an interface has changed over time  Do you think an interface just has method declarations only and are just contracts  What about static final variables and what about default definitions after Java 8   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   See Why does Java allow static final variables in interfaces when they are only intended to be contracts

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Interface  Turn   Turn Left  Turn Right    Abstract Class  Wheel   Class  Steering Wheel  derives from Wheel  exposes Interface Turn  One is for categorizing behavior that can be offered across a diverse range of things  the other is for modelling an ontology of things

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We have various structural syntactical difference between interface and abstract class  Some more differences are   1  Scenario based difference   Abstract classes are used in scenarios when we want to restrict the user to create object of parent class AND we believe there will be more abstract methods will be added in future   Interface has to be used when we are sure there can be no more abstract method left to be provided  Then only an interface is published    2  Conceptual difference    Do we need to provide more abstract methods in future  if YES make it abstract class and if NO make it Interface    Most appropriate and valid till java 1 7

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Interfaces  An interface is a contract  The person writing the interface says   hey  I accept things looking that way   and the person using the interface says  OK  the class I write looks that way    An interface is an empty shell  There are only the signatures of the methods  which implies that the methods do not have a body  The interface can t do anything  It s just a pattern   For example  pseudo code       I say all motor vehicles should look like this  interface MotorVehicle       void run         int getFuel          My team mate complies and writes vehicle looking that way class Car implements MotorVehicle        int fuel       void run                 print  Wrroooooooom                int getFuel                 return this fuel            Implementing an interface consumes very little CPU  because it s not a class  just a bunch of names  and therefore there isn t any expensive look-up to do  It s great when it matters  such as in embedded devices     Abstract classes  Abstract classes  unlike interfaces  are classes  They are more expensive to use  because there is a look-up to do when you inherit from them   Abstract classes look a lot like interfaces  but they have something more  You can define a behavior for them  It s more about a person saying   these classes should look like that  and they have that in common  so fill in the blanks     For example      I say all motor vehicles should look like this  abstract class MotorVehicle        int fuel          They ALL have fuel  so lets implement this for everybody      int getFuel                  return this fuel                That can be very different  force them to provide their        own implementation      abstract void run          My teammate complies and writes vehicle looking that way class Car extends MotorVehicle       void run                 print  Wrroooooooom                Implementation  While abstract classes and interfaces are supposed to be different concepts  the implementations make that statement sometimes untrue  Sometimes  they are not even what you think they are   In Java  this rule is strongly enforced  while in PHP  interfaces are abstract classes with no method declared   In Python  abstract classes are more a programming trick you can get from the ABC module and is actually using metaclasses  and therefore classes  And interfaces are more related to duck typing in this language and it s a mix between conventions and special methods that call descriptors  the   method   methods    As usual with programming  there is theory  practice  and practice in another language  -

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If you have some common methods that can be used by multiple classes go for abstract classes  Else if you want the classes to follow some definite blueprint go for interfaces  Following examples demonstrate this  Abstract class in Java  abstract class Animals          They all love to eat  So let s implement them for everybody     void eat                 System out println  quot Eating    quot                 The make different sounds  They will provide their own implementation      abstract void sound        class Dog extends Animals       void sound                 System out println  quot Woof Woof quot              class Cat extends Animals       void sound                 System out println  quot Meoww quot             Following is an implementation of interface in Java  interface Shape       void display        double area        class Rectangle implements Shape        int length  width      Rectangle int length  int width                this length   length          this width   width             Override     public void display                  System out println  quot      n    n    n     quot                Override     public double area                  return  double  length width              class Circle implements Shape        double pi   3 14      int radius      Circle int radius                this radius   radius             Override     public void display                  System out println  quot O quot        P            Override     public double area                   return  double   pi radius radius  2            Some Important Key points in a nutshell   The variables declared in Java interface are by default final  Abstract classes can have non-final variables   The variables declared in Java interface are by default static  Abstract classes can have non-static variables   Members of a Java interface are public by default  A Java abstract class can have the usual flavors of class members like private  protected  etc

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To give a simple but clear answer  it helps to set the context   you use both when you do not want to provide full implementations   The main difference then is an interface has no implementation at all  only methods without a body  while abstract classes can have members and methods with a body as well  i e  can be partially implemented

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The key technical differences between an abstract class and an interface are    Abstract classes can have constants  members  method stubs  methods without a body  and defined methods  whereas interfaces can only have constants and methods stubs  Methods and members of an abstract class can be defined with any visibility  whereas all methods of an interface must be defined as public  they are defined public by default   When inheriting an abstract class  a concrete child class must define the abstract methods  whereas an abstract class can extend another abstract class and abstract methods from the parent class don t have to be defined  Similarly  an interface extending another interface is not responsible for implementing methods from the parent interface  This is because interfaces cannot define any implementation  A child class can only extend a single class  abstract or concrete   whereas an interface can extend or a class can implement multiple other interfaces  A child class can define abstract methods with the same or less restrictive visibility  whereas a class implementing an interface must define the methods with the exact same visibility  public

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Differences between abstract class and interface on behalf of real implementation   Interface  It is a keyword and it is used to define the template or blue print of an object and it forces all the sub classes would follow the same prototype as for as implementation  all the sub classes are free to implement the functionality as per it s requirement   Some of other use cases where we should use interface   Communication between two external objects Third party integration in our application  done through Interface here Interface works as Contract   Abstract Class  Abstract it is a keyword and when we use this keyword before any class then it becomes abstract class It is mainly used when we need to define the template as well as some default functionality of an object that is followed by all the sub classes and this way it removes the redundant code and one more use cases where we can use abstract class  such as we want no other classes can directly instantiate an object of the class  only derived classes can use the functionality   Example of Abstract Class    public abstract class DesireCar         It is an abstract method that defines the prototype       public abstract void Color          It is a default implementation of a Wheel method as all the desire cars have the same no  of wheels         and hence no need to define this in all the sub classes in this way it saves the code duplicasy         public void Wheel                               Console WriteLine  Car has four wheel                                           Here is the sub classes          public class DesireCar1   DesireCar                       public override void Color                                 Console WriteLine  This is a red color Desire car                                     public class DesireCar2   DesireCar                       public override void Color                                 Console WriteLine  This is a red white Desire car                              Example Of Interface     public interface IShape                        Defines the prototype template               void Draw                     All the sub classes follow the same template but implementation can be different       public class Circle   IShape               public void Draw                         Console WriteLine  This is a Circle                         public class Rectangle   IShape               public void Draw                         Console WriteLine  This is a Rectangle

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An Interface contains only the definition   signature of functionality  and if we have some common functionality as well as common signatures  then we need to use an abstract class  By using an abstract class  we can provide behavior as well as functionality both in the same time  Another developer inheriting abstract class can use this functionality easily  as they would only need to fill in the blanks    Taken from   http   www dotnetbull com 2011 11 difference-between-abstract-class-and html  http   www dotnetbull com 2011 11 what-is-abstract-class-in-c-net html http   www dotnetbull com 2011 11 what-is-interface-in-c-net html

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A simple yet effective explanation of abstract class and interface on php net      An Interface is like a protocol  It doesn t designate the behavior of the object  it designates how your code tells that object to act  An interface would be like the English Language  defining an interface defines how your code communicates with any object implementing that interface       An interface is always an agreement or a promise  When a class says  I implement interface Y   it is saying  I promise to have the same public methods that any object with interface Y has        On the other hand  an Abstract Class is like a partially built class  It is much like a document with blanks to fill in  It might be using English  but that isn t as important as the fact that some of the document is already written       An abstract class is the foundation for another object  When a class says  I extend abstract class Y   it is saying  I use some methods or properties already defined in this other class named Y        So  consider the following PHP    lt  php class X implements Y        this is saying that  X  agrees to speak language  Y  with your code   class X extends Y        this is saying that  X  is going to complete the partial class  Y     gt        You would have your class implement a particular interface if you were distributing a class to be used by other people  The interface is an agreement to have a specific set of public methods for your class       You would have your class extend an abstract class if you  or someone else  wrote a class that already had some methods written that you want to use in your new class       These concepts  while easy to confuse  are specifically different and distinct  For all intents and purposes  if you re the only user of any of your classes  you don t need to implement interfaces

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When you want to provide polymorphic behaviour in an inheritance hierarchy  use abstract classes    When you want polymorphic behaviour for classes which are completely unrelated  use an interface

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Here is a very basic understanding over interface vs abstract class

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The comparison of interface vs  abstract class is wrong  There should be two other comparisons instead  1  interface vs  class and 2  abstract vs  final class   Interface vs Class  Interface is a contract between two objects  E g   I m a Postman and you re a Package to deliver  I expect you to know your delivery address  When someone gives me a Package  it has to know its delivery address   interface Package     String address        Class is a group of objects that obey the contract  E g   I m a box from  Box  group and I obey the contract required by the Postman  At the same time I obey other contracts   class Box implements Package  Property      Override   String address         return  5th Street  New York  NY          Override   Human owner            this method is part of another contract         Abstract vs Final  Abstract class is a group of incomplete objects  They can t be used  because they miss some parts  E g   I m an abstract GPS-aware box - I know how to check my position on the map   abstract class GpsBox implements Package      Override   public abstract String address      protected Coordinates whereAmI            connect to GPS and return my current position         This class  if inherited extended by another class  can be very useful  But by itself - it is useless  since it can t have objects  Abstract classes can be building elements of final classes   Final class is a group of complete objects  which can be used  but can t be modified  They know exactly how to work and what to do  E g   I m a Box that always goes to the address specified during its construction   final class DirectBox implements Package     private final String to    public DirectBox String addr        this to   addr         Override   public String address         return this to          In most languages  like Java or C    it is possible to have just a class  neither abstract nor final  Such a class can be inherited and can be instantiated  I don t think this is strictly in line with object-oriented paradigm  though   Again  comparing interfaces with abstract classes is not correct

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Key Points    Abstract class can have property  Data fields  Methods  complete   incomplete  both  If method or Properties define in abstract keyword that must override in derived class  its work as a tightly coupled functionality  If define abstract keyword for method or properties in abstract class you can not define body of method and get set value for properties and that must override in derived class  Abstract class does not support multiple inheritance  Abstract class contains Constructors  An abstract class can contain access modifiers for the subs  functions  properties  Only Complete Member of abstract class can be Static  An interface can inherit from another interface only and cannot inherit from an abstract class  where as an abstract class can inherit from another abstract class or another interface    Advantage    It is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards  If various implementations are of the same kind and use common behavior or status then abstract class is better to use  If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly  Its allow fast execution than interface  interface Requires more time to find the actual method in the corresponding classes   It can use for tight and loosely coupling    find details here    http   pradeepatkari wordpress com 2014 11 20 interface-and-abstract-class-in-c-oops

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usually Abstract class used for core of something but interface used for appending peripheral   when you want to create base type for vehicle you should use abstract class but if you want to add some functionality or property that is not part of base concept of vehicle you should use interface for example you want to add  ToJSON    function   interface has wide range of abstraction rather than abstract class  you can see this in passing arguments look this example     if you use vehicle as argument you just can use one of its derived type  bus or car-same category-just vehicle category   but when you use IMoveable interface as argument you have more choices

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An explanation can be found here  http   www developer com lang php article php 3604111 PHP-5-OOP-Interfaces-Abstract-Classes-and-the-Adapter-Pattern htm     An abstract class is a class that is   only partially implemented by the   programmer  It may contain one or more   abstract methods  An abstract method   is simply a function definition that   serves to tell the programmer that the   method must be implemented in a child   class       An interface is similar to an abstract   class  indeed interfaces occupy the   same namespace as classes and abstract   classes  For that reason  you cannot   define an interface with the same name   as a class  An interface is a fully   abstract class  none of its methods   are implemented and instead of a class   sub-classing from it  it is said to   implement that interface    Anyway I find this explanation of interfaces somewhat confusing  A more common definition is  An interface defines a contract that implementing classes must fulfill  An interface definition consists of signatures of public members  without any implementing code

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I am constructing a building of 300 floors  The building s blueprint interface   For example  Servlet I    Building constructed up to 200 floors - partially completed---abstract   Partial implementation  for example  generic and HTTP servlet   Building construction completed-concrete   Full implementation  for example  own servlet    Interface   We don t know anything about implementation  just requirements  We can go for an interface  Every method is public and abstract by default  It is a 100  pure abstract class If we declare public we cannot declare private and protected If we declare abstract we cannot declare final  static  synchronized  strictfp and native Every interface has public  static and final Serialization and transient is not applicable  because we can t create an instance for in interface Non-volatile because it is final Every variable is static When we declare a variable inside an interface we need to initialize variables while declaring Instance and static block not allowed   Abstract   Partial implementation It has an abstract method  An addition  it uses concrete No restriction for abstract class method modifiers No restriction for abstract class variable modifiers We cannot declare other modifiers except abstract No restriction to initialize variables   Taken from DurgaJobs Website

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I d like to add one more difference which makes sense  For example  you have a framework with thousands of lines of code  Now if you want to add a new feature throughout the code using a method enhanceUI    then it s better to add that method in abstract class rather in interface  Because  if you add this method in an interface then you should implement it in all the implemented class but it s not the case if you add the method in abstract class

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In practicality terms JAVA   the major difference between abstract class and interface is Abstract class can hold state  Other than holding state we can achieve rest operations with Interface also

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It s pretty simple actually   You can think of an interface as a class which is only allowed to have abstract methods and nothing else    So an interface can only  declare  and not define the behavior you want the class to have   An abstract class allows you to do both declare  using abstract methods  as well as define  using full method implementations  the behavior you want the class to have   And a regular class only allows you to define  not declare  the behavior actions you want the class to have   One last thing   In Java  you can implement multiple interfaces  but you can only extend one  Abstract Class or Class      This means inheritance of defined behavior is restricted to only allow one per class    ie if you wanted a class that encapsulated behavior from Classes A B amp C you would need to do the following  Class A extends B  Class C extends A    its a bit of a round about way to have multiple inheritance     Interfaces on the other hand  you could simply do  interface C implements A  B  So in effect Java supports multiple inheritance only in  declared behavior  ie interfaces  and only single inheritance with defined behavior   unless you do the round about way I described     Hopefully that makes sense

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Let s work on this question again    The first thing to let you know is that 1 1 and 1 1 results in the same  but it does not mean that multiplication and division are same  Obviously  they hold some good relationship  but mind you both are different    I will point out main differences  and the rest have already been explained   Abstract classes are useful for modeling a class hierarchy  At first glance of any requirement  we are partially clear on what exactly is to be built  but we know what to build  And so your abstract classes are your base classes   Interfaces are useful for letting other hierarchy or classes to know that what I am capable of doing  And when you say I am capable of something  you must have that capacity  Interfaces will mark it as compulsory for a class to implement the same functionalities

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The shortest way to sum it up is that an interface is    Fully abstract  apart from default and static methods  while it has definitions  method signatures   implementations  for default and static methods  it only has declarations  method signatures  for other methods  Subject to laxer rules than classes  a class can implement multiple interfaces  and an interface can inherit from multiple interfaces    All variables are implicitly constant  whether specified as public static final or not   All members are implicitly public  whether specified as such or not  Generally used as a guarantee that the implementing class will have the specified features and or be compatible with any other class which implements the same interface    Meanwhile  an abstract class is    Anywhere from fully abstract to fully implemented  with a tendency to have one or more abstract methods   Can contain both declarations and definitions  with declarations marked as abstract  A full-fledged class  and subject to the rules that govern other classes  can only inherit from one class   on the condition that it cannot be instantiated  because there s no guarantee that it s fully implemented    Can have non-constant member variables   Can implement member access control  restricting members as protected  private  or private package  unspecified   Generally used either to provide as much of the implementation as can be shared by multiple subclasses  or to provide as much of the implementation as the programmer is able to supply    Or  if we want to boil it all down to a single sentence  An interface is what the implementing class has  but an abstract class is what the subclass is

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Inheritance is used for two purposes    To allow an object to regard parent-type data members and method implementations as its own  To allow a reference to an objects of one type to be used by code which expects a reference to supertype object    In languages frameworks which support generalized multiple inheritance  there is often little need to classify a type as either being an  interface  or an  abstract class    Popular languages and frameworks  however  will allow a type to regard one other type s data members or method implementations as its own even though they allow a type to be substitutable for an arbitrary number of other types   Abstract classes may have data members and method implementations  but can only be inherited by classes which don t inherit from any other classes   Interfaces put almost no restrictions on the types which implement them  but cannot include any data members or method implementations   There are times when it s useful for types to be substitutable for many different things  there are other times when it s useful for objects to regard parent-type data members and method implementations as their own   Making a distinction between interfaces and abstract classes allows each of those abilities to be used in cases where it is most relevant

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The main point is that    Abstract is object oriented  It offers the basic data an  object  should have and or functions it should be able to do  It is concerned with the object s basic characteristics  what it has and what it can do  Hence objects which inherit from the same abstract class share the basic characteristics  generalization   Interface is functionality oriented  It defines functionalities an object should have  Regardless what object it is  as long as it can do these functionalities  which are defined in the interface  it s fine  It ignores everything else  An object class can contain several  groups of  functionalities  hence it is possible for a class to implement multiple interfaces

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Interfaces are generally the classes without logic just a signature  Whereas abstract classes are those having logic  Both support contract as interface all method should be implemented in the child class but in abstract only the abstract method should be implemented  When to use interface and when to abstract  Why use Interface   class Circle    protected  radius   public function   construct  radius          this- gt radius    radius    public function area         return 3 14159   pow 2  this- gt radius      simply pie r2  square           Our area calculator class would look like  class Areacalculator     protected  circle   public function   construct Circle  circle         this- gt circle    circle     public function areaCalculate         return  circle- gt area      returns the circle area now        We would simply do   areacalculator   new Areacalculator new Circle 7       Few days later we would need the area of rectangle  Square  Quadrilateral and so on  If so do we have to change the code every time and check if the instance is of square or circle or rectangle  Now what OCP says is CODE TO AN INTERFACE NOT AN IMPLEMENTATION   Solution would be   Interface Shape    public function area      Defining contract for the classes     Class Square implements Shape     protected length   public function   construct  length          settter for length like we did on circle class    public function area           return l square for area of square    Class Rectangle implements Shape     protected length   protected breath   public function   construct  length  breath          settter for length  breath like we did on circle square class    public function area           return l b for area of rectangle        Now for area calculator  class Areacalculator     protected  shape   public function   construct Shape  shape         this- gt shape    shape     public function areaCalculate         return  shape- gt area      returns the circle area now        areacalculator   new Areacalculator new Square 1     areacalculator- gt areaCalculate      areacalculator   new Areacalculator new Rectangle 1 2     areacalculator- gt  areaCalculate      Isn t that more flexible  If we would code without interface we would check the instance for each shape redundant code   Now when to use abstract   Abstract Animal    public function breathe       all animals breathe inhaling o2 and exhaling co2     public function hungry        every animals do feel hungry      abstract function communicate        different communication style some bark  some meow  human talks etc      Now abstract should be used when one doesn t need instance of that class  having similar logic  having need for the contract

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The general idea of abstract classes and interfaces is to be extended implemented by other classes  cannot be constructed alone  that use these general  settings   some kind of a template   making it simple to set a specific-general behaviour for all the objects that later extend it   An abstract class has regular methods set AND abstract methods  Extended classes can include unset methods after being extended by an abstract class  When setting abstract methods - they are defined by the classes that are extending it later   Interfaces have the same properties as an abstract class  but includes only abstract methods  which could be implemented in an other class es  and can be more than one interface to implement   this creates a more permanent-solid definishion of methods static variables  Unlike the abstract class  you cannot add custom  regular  methods

[oop] What is the difference between an interface and abstract class?

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