When do I have to use interfaces instead of abstract classes

Question

I was wondering when I should use interfaces   Lets think about the following   public abstract class Vehicle      abstract float getSpeed        and    public interface IVehicle     float getSpeed        I can easily implement both of them  they have the same functionality    BUT I also can add some variables to my vehicle class  which probably should be used in an vehicle  maxSpeed  carType      What is the reason to use interfaces   Thanks   EDIT  I found a nice link about it in another thread  http   www thecoldsun com en content 01-2009 abstract-classes-and-interfaces

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The answer of this question is very simple Whatever we can do with interface can be done with abstract class Agree   so when to use interfaces the answer lies in C  restriction of multiple inheritance  When you have only contracts abstracts  to declare and want your sub-classes implement it go with interfaces  because if you use abstract class in this case  you can not inherit from one more class and you are stuck if want to inherit from one more class but you can implement as many interfaces

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Interfaces and abstracted classes seem very similar  however  there are important differences between them   Abstraction is based on a good  is-a  relationship   Meaning that you would say that a car is a Honda  and a Honda is a car   Using abstraction on a class means you can also have abstract methods   This would require any subclass extended from it to obtain the abstract methods and override them   Using the example below  we can create an abstract howToStart    method that will require each class to implement it   Through abstraction  we can provide similarities between code so we would still have a base class  Using an example of the Car class idea we could create   public abstract class Car      private String make      private String model      protected Car           Default constructor      protect Car String make  String model             Assign values to             public abstract void howToStart        Then with the Honda class we would have   public class Honda extends implements Engine        public Honda           Default constructor      public Honda String make  String model             Assign values             Override     public static void howToStart               Code on how to start            Interfaces are based on the  has-a  relationship   This would mean you could say a car has-a engine  but an engine is not a car   In the above example  Honda has implements Engine   For the engine interface we could create   public interface Engine       public void startup        The interface will provide a many-to-one instance   So we could apply the Engine interface to any type of car   We can also extend it to other object   Like if we were to make a boat class  and have sub classes of boat types  we could extend Engine and have the sub classes of boat require the startup    method   Interfaces are good for creating framework to various classes that have some similarities   We can also implement multiple instances in one class  such as   public class Honda extends implements Engine  Transmission  List lt Parts gt    Hopefully this helps

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From the Oracle tutorials     Unlike interfaces  abstract classes can contain fields that are not static and final  and they can contain implemented methods  Such abstract classes are similar to interfaces  except that they provide a partial implementation  leaving it to subclasses to complete the implementation  If an abstract class contains only abstract method declarations  it should be declared as an interface instead   Multiple interfaces can be implemented by classes anywhere in the class hierarchy  whether or not they are related to one another in any way  Think of Comparable or Cloneable  for example   By comparison  abstract classes are most commonly subclassed to share pieces of implementation  A single abstract class is subclassed by similar classes that have a lot in common  the implemented parts of the abstract class   but also have some differences  the abstract methods

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Considering Java   Interfaces    Are a fundamental OOP abstraction  Will often  but not always  yield clearer code than abstract classes  Can be implemented by multiple concrete classes to suit different situations  Can directly implement scenarios calling for multiple inheritance  Can be more easily mocked out for testing purposes  Are useful for JDK proxies  see java lang reflect Proxy     These are just the beginning of a very long list of pros cons for interfaces vs  abstract classes

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This is an direct excerpt from the excellent book  Thinking in Java  by Bruce Eckel           Should you use an interface or an abstract class         Well  an interface gives you the benefits of an abstract class and the benefits of an interface  so if it   s possible to create your base class without any method definitions or member variables you should always prefer interfaces to abstract classes         In fact  if you know something is going to be a base class  your first choice should be to make it an interface  and only if you   re forced to have method definitions or member variables should you change to an abstract class

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With support of default methods in interface since launch of Java 8  the gap  between interface and abstract classes has been reduced but still they have major differences    Variables in interface are public static final  But abstract class can have other type of variables like private  protected etc Methods in interface are public or public static but methods in abstract class can be  private and protected too Use abstract class to establish relation between interrelated objects  Use interface to establish relation between unrelated classes    Have a look at this article for special properties of interface in java 8  static modifier for default methods in interface causes compile time error in derived error if you want to use  override   This article explains why default methods have been introduced in java 8    To enhance the Collections API in Java 8 to support lambda expressions   Have a look at oracle documentation too to understand the differences in better way   Have a look at this related SE questions with code example to understand things in better way    How should I have explained the difference between an Interface and an Abstract class

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Many cases can be implemented in both class types   Interfaces are usefull when you want to define a class that has to have at least basic functions  Like a real interface for example USB   interface USB       public function sendPower      charge iphone for example     public function sendData      itunes     public function recieveData        Use abstract classes when there are several ways to to implement an object   abstract class MobilePhone       public function isIphone         public function charge               get some power  all phones need that          class iPhone extends MobilePhone       public function isIphone     return true

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interface is basically used when two parties works together and one party want to hide something from other or only want to show some part of his class  than we use interface eg  in jdbc jdbc vendors provide us some interfaces bcoz they want to hide everything from us   abstract class is only used in that case when we want to support a common behavior in more than one classes   or want to provide some preimplemented implementation with some unimplemented methods methods should be abstract   eg  http servlet in servlet interface is an abstract class bcoz this class implements servlet interface except it s service method   so this class help us to get some preimplemetation of interface method

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If you are using JDK 8  there is no reason to use abstract classes because whatever we do with abstract classes we can now do it with interfaces because of default methods   If you use abstract class you have to extend it and there is a restriction that you can extends only once  But if you use interface you can implements as many as you want    Interface is by default an abstract class an all methods and constructors are public

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Abstract Class   Use it when there is strong is-a relation between  super class and sub class and all subclass share some common behavior    Interface   It define just protocol which all subclass need to follow

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From Java How to Program about abstract classes      Because they   re used only as superclasses in inheritance hierarchies    we refer to them as abstract superclasses  These classes cannot be   used to instantiate objects  because abstract classes are incomplete     Subclasses must declare the    missing pieces    to become    concrete    classes     from which you can instantiate objects  Otherwise  these subclasses  too     will be abstract    To answer your question  What is the reason to use interfaces        An abstract class   s purpose is to provide an appropriate superclass   from which other classes can inherit and thus share a common design    As opposed to an interface      An interface describes a set of methods that can be called on an   object  but does not provide concrete implementations for all the   methods    Once a class implements an interface  all objects of that class have   an is-a relationship with the interface type  and all objects of the   class are guaranteed to provide the functionality described by the   interface  This is true of all subclasses of that class as well    So  to answer your question  I was wondering when I should use interfaces   I think you should use interfaces when you want a full implementation and use abstract classes when you want partial pieces for your design  for reusability

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Abstract classes can contain methods that are not abstract  whereas in interfaces all your methods are abstract and have to be implemented   You should use interfaces instead when you know you will always implement those certain methods  Also you can inherit from multiple interfaces  it is java s way of dealing with multiple inheritance

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Actually Interface and abstract class are used to just specify some contract rules which will just show  how their sub classes will be    Mostly we know that interface is a pure abstract Means there you cant specify a single method with body This particular point is the advantages of abstract class Means in abstract class u have right to specify method with body and without body as-well    So if u want to specify something about ur subclass  then u may go for interface   But if u also want to specify something for ur sub classes and u want also ur class should also have some own method Then in that case u may go for abstract class

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Interface and Abstract Class are the two different ways to achieve Abstraction in  OOP Languages   Interface provides 100  abstraction  i e all methods are abstract   Abstract class provides 0 to 100  abstraction  i e it may have or may not have abstract methods   We can use Interface when we want all the functionality of a type to be implemented by the client   We can use Abstract Class when some common functionality can be provided by Abstract Class implementer and client will be given chance to implement what he needs actually

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Use an abstract class when you want to define a template for a group of sub-classes   and you have at least some implementation code that call sub-classes could use   Use an interface when you want to define the role that other classes can play  regardless of where those classes are in the inheritance tree  you extend an abstract class  you implement an interface     in an interface all the fields are automatically public static final and all the methods are public whereas an abstract class allows you a bit of flexibility here

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You can t achieve multiple inheritance with abstract class  that is why Sun Microsystems provide interfaces    You cannot extend two classes but you can implement multiple interfaces

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From The Java    Tutorials - Abstract Classes Compared to Interfaces     Which should you use  abstract classes or interfaces          Consider using abstract classes if any of these statements apply to your situation          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  This enables you to define methods that can access and modify the state of the object to which they belong       Consider using interfaces if any of these statements apply to your situation          You expect that unrelated classes would implement your interface  For example  the interfaces Comparable and Cloneable are implemented by many unrelated classes    You want to specify the behavior of a particular data type  but not concerned about who implements its behavior    You want to take advantage of multiple inheritance of type             An example of an abstract class in the JDK is AbstractMap  which is part of the Collections Framework  Its subclasses  which include HashMap  TreeMap  and ConcurrentHashMap  share many methods  including get  put  isEmpty  containsKey  and containsValue  that AbstractMap defines

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There are a number of times you might consider using an interface over an abstract implementation   When the available abstract implementation doesn t do what you want and you want to create your own when you have an existing class  that extends from other class  and you want to implement the functionality of the interface   Generally speaking  interfaces were introduced to overcome the lack of multiple inheritancy  amongst other things

[java] When do I have to use interfaces instead of abstract classes?

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