Interface or an Abstract Class which one to use

Question

Please explain when I should use a PHP interface and when I should use an abstract class   How I can change my abstract class in to an interface

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Also  just would like to add here that just because any other OO language has some kind of interfaces and abstraction too doesn t mean they have the same meaning and purpose as in PHP  The use of abstraction interfaces is slightly different while interfaces in PHP actually don t have a real function  They merely are used for semantic and scheme-related reasons  The point is to have a project as much flexible as possible  expandable and safe for future extensions regardless whether the developer later on has a totally different plan of use or not    If your English is not native you might lookup what Abstraction and Interfaces actually are  And look for synonyms too   And this might help you as a metaphor   INTERFACE  Let s say  you bake a new sort of cake with strawberries and you made up a recipe describing the ingredients and steps   Only you know why it s tasting so well and your guests like it   Then you decide to publish your recipe so other people can try that cake as well   The point here is      - to make it right      - to be careful      - to prevent things which could go bad  like too much strawberries or something       - to keep it easy for the people who try it out      - to tell you how long is what to do  like stiring       - to tell which things you CAN do but don t HAVE to    Exactly THIS is what describes interfaces  It is a guide  a set of instructions which observe the content of the recipe  Same as if you would create a project in PHP and you want to provide the code on GitHub or with your mates or whatever  An interface is what people can do and what you should not  Rules that hold it - if you disobey one  the entire construct will be broken     ABSTRACTION  To continue with this metaphor here    imagine  you are the guest this time eating that cake  Then you are trying that cake using the recipe now  But you want to add new ingredients or change skip the steps described in the recipe  So what comes next  Plan a different version of that cake   This time with black berries and not straw berries and more vanilla cream   yummy   This is what you could consider an extension of the original cake  You basically do an abstraction of it by creating a new recipe because it s a lil different  It has a few new steps and other ingredients  However  the black berry version has some parts you took over from the original - these are the base steps that every kind of that cake must have  Like ingredients just as milk - That is what every derived class has    Now you want to exchange ingredients and steps and these MUST be defined in the new version of that cake  These are abstract methods which have to be defined for the new cake  because there should be a fruit in the cake but which  So you take the black berries this time  Done   There you go  you have extended the cake  followed the interface and abstracted steps and ingredients from it

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Just wanted to add an example of when you may need to use both  I am currently writing a file handler bound to a database model in a general purpose ERP solution     I have multiple abstract classes which handle the standard crud and also some specialty functionality like conversion and streaming for different categories of files   The file access interface defines a common set of methods which are needed to get  store and delete a file     This way  I get to have multiple templates for different files and a common set of interface methods with clear distinction  The interface gives the correct analogy to the access methods rather than what would have been with a base abstract class    Further down the line when I will make adapters for different file storage services  this implementation will allow the interface to be used elsewhere in totally different contexts

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The main difference is an abstract class can contain default implementation whereas an interface cannot   An interface is a contract of behaviour without any implementation

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Use an interface when you want to force developers working in your system  yourself included  to implement a set number of methods on the classes they ll be building   Use an abstract class when you want to force developers working in your system  yourself included  to implement a set numbers of methods and you want to provide some base methods that will help them develop their child classes     Another thing to keep in mind is client classes can only extend one abstract class  whereas they can implement multiple interfaces   So  if you re defining your behavior contracts in abstract classes  that means each child class may only conform to a single contract   Sometimes this a good thing  when you want to force your user-programmers along a particular path  Other times it would be bad  Imagine if PHP s Countable and Iterator interfaces were abstract classes instead of interfaces     One approach that s common when you re uncertain which way to go  as mentioned by cletus below  is to create an interface  and then have your abstract class implement that interface

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Best practice is to use an interface to specify the contract and an abstract class as just one implementation thereof  That abstract class can fill in a lot of the boilerplate so you can create an implementation by just overriding what you need to or want to without forcing you to use a particular implementation

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The technical differences between an abstract class and an interface are already listed in the other answers precisely  I want to add an explanation to choose between a class and an interface while writing the code for the sake of object oriented programming    A class should represent an entity whereas an interface should represent the behavior    Let s take an example  A computer monitor is an entity and should be represented as a class   class Monitor      private int monitorNo      It is designed to provide a display interface to you  so the functionality should be defined by an interface   interface Display      void display        There are many other things to consider as explained in the other answers  but this is the most basic thing which most of the people ignore while coding

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Just to throw this into the mix  but as Cletus mentioned using an interface in conjunction with an abstract class  I often use the interface to clarify my design thinking   For instance    lt  php class parser implements parserDecoratorPattern                 That way  anyone reading my code  and who knows what a Decorator Pattern is  will know right away a  how I build my parser and b  be able to see what methods are used to implement the decorator pattern   Also  and I may be off base here not being a Java C   etc programmer  but data types can come into play here  Your objects are of a type  and when you pass them around the type matters programmatically  Moving your contractable items into the interface only dictates the types that the methods return  but not the base type of the class that implements it   It s late and I can t think of a better psudo-code example  but here goes    lt  php interface TelevisionControls     class Remote implements TelevisionControls     class Spouse implements TelevisionControls     Spouse spouse   new Spouse    Remote remote   new Remote    isSameType    bool  remote    spouse

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The differences between an Abstract Class and an Interface   Abstract Classes  An abstract class can provide some functionality and leave the rest for derived class    The derived class may or may not override the concrete functions defined in the base class  A child class extended from an abstract class should logically be related    Interface  An interface cannot contain any functionality  It only contains definitions of the methods    The derived class MUST provide code for all the methods defined in the interface  Completely different and non-related classes can be logically grouped together using an interface

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Why to use abstract classes  The following is a simple example   Lets say we have the following code    lt  php   class Fruit       private  color       public function eat                chew            public function setColor  c             this- gt color    c           class Apple extends Fruit       public function eat                chew until core          class Orange extends Fruit       public function eat                peeling            chew           Now I give you an apple and you eat it  What does it taste like  It tastes like an apple    lt  php   apple   new Apple     apple- gt eat        Now I give you a fruit   fruit   new Fruit     fruit- gt eat      What does that taste like  Well  it doesn t make much sense  so you shouldn t be able to do that  This is accomplished by making the Fruit class abstract as well as the eat method inside of it    lt  php  abstract class Fruit       private  color       abstract public function eat          public function setColor  c             this- gt color    c            gt    An abstract class is just like an interface  but you can define methods in an abstract class whereas in an interface they are all abstract   Abstract classes can have both empty and working concrete methods   In interfaces  functions defined there cannot have a body  In abstract classes  they can   A real world example    lt  php  abstract class person        public  LastName      public  FirstName      public  BirthDate       abstract protected function write info       final class employee extends person       public  EmployeeNumber      public  DateHired       public function write info              sql codes here         echo  Writing     this- gt LastName     s info to emloyee dbase table  lt br gt                final class student extends person       public  StudentNumber      public  CourseName       public function write info              sql codes here         echo  Writing     this- gt LastName     s info to student dbase table  lt br gt                ----------  personA   new employee   personB   new student    personA- gt FirstName  Joe    personA- gt LastName  Sbody     personB- gt FirstName  Ben    personB- gt LastName  Dover     personA- gt write info       Writing Sbody s info to emloyee dbase table  personB- gt write info       Writing Dover s info to student dbase table

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From a phylosophic point of view      An abstract class represents an  is a  relationship  Lets say I have fruits  well I would have a Fruit abstract class that shares common responsabilities and common behavior  An interface represents a  should do  relationship  An interface  in my opinion  which is the opinion of a junior dev   should be named by an action  or something close to an action   Sorry  can t find the word  I m not an english native speaker  lets say IEatable  You know it can be eaten  but you don t know what you eat    From a coding point of view      If your objects have duplicated code  it is an indication that they have common behavior  which means you might need an abstract class to reuse the code  which you cannot do with an interface  Another difference is that an object can implement as many interfaces as you need  but you can only have one abstract class because of the  diamond problem   check out here to know why  http   en wikipedia org wiki Multiple inheritance The diamond problem    I probably forget some points  but I hope it can clarify things   PS   The  is a   should do  is brought by Vivek Vermani s answer  I didn t mean to steal his answer  just to reuse the terms because I liked them

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To add to some of the already excellent answers    Abstract classes let you provide some degree of implementation  interfaces are pure templates  An interface can only define functionality  it can never implement it  Any class that implements the interface commits to implementing all the methods it defines or it must be declared abstract  Interfaces can help to manage the fact that  like Java  PHP does not support multiple inheritance  A PHP class can only extend a single parent  However  you can make a class promise to implement as many interfaces as you want  type  for each interface it implements  the class takes on the corresponding type  Because any class can implement an interface  or more interfaces   interfaces effectively join types that are otherwise unrelated  a class can both extend a superclass and implement any number of interfaces   class SubClass extends ParentClass implements Interface1  Interface2                      Please explain when I should use an interface and when I should use abstract class    Use an interface when you need to provide only a template with no implementation what so ever  and you want to make sure any class that implements that interface will have the same methods as any other class that implements it  at least    Use an abstract class when you want to create a foundation for other objects  a partially built class   The class that extends your abstract class will use some properties or methods defined implemented    lt  php    interface class X implements Y        this is saying that  X  agrees to speak language  Y  with your code      abstract class class X extends Y        this is saying that  X  is going to complete the partial class  Y     gt       How I can change my abstract class in to an interface    Here is a simplified case example  Take out any implementation details out  For example  change your abstract class from   abstract class ClassToBuildUpon       public function doSomething               echo  Did something              to   interface ClassToBuildUpon       public function doSomething

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