When to use Java 8 interface default method vs abstract method

Question

Java 8 allows for default implementation of methods in interfaces called Default Methods   I am confused between when would I use that sort of interface default method  instead of an abstract class  with abstract method s     So when should interface with default methods be used and when should an abstract class  with abstract method s   be used  Are the abstract classes still useful in that scenario

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As mentioned in other answers  the ability to add implementation to an interface was added in order to provide backward compatibility in the Collections framework  I would argue that providing backward compatibility is potentially the only good reason for adding implementation to an interface   Otherwise  if you add implementation to an interface  you are breaking the fundamental law for why interfaces were added in the first place  Java is a single inheritance language  unlike C   which allows for multiple inheritance  Interfaces provide the typing benefits that come with a language that supports multiple inheritance without introducing the problems that come with multiple inheritance   More specifically  Java only allows single inheritance of an implementation  but it does allow multiple inheritance of interfaces  For example  the following is valid Java code   class MyObject extends String implements Runnable  Comparable           MyObject inherits only one implementation  but it inherits three contracts   Java passed on multiple inheritance of implementation because multiple inheritance of implementation comes with a host of thorny problems  which are outside the scope of this answer  Interfaces were added to allow multiple inheritance of contracts  aka interfaces  without the problems of multiple inheritance of implementation   To support my point  here is a quote from Ken Arnold and James Gosling from the book The Java Programming Language  4th edition      Single inheritance precludes some useful and correct designs  The   problems of multiple inheritance arise from multiple inheritance of   implementation  but in many cases multiple inheritance is used to   inherit a number of abstract contracts and perhaps one concrete   implementation  Providing a means to inherit an abstract contract   without inheriting an implementation allows the typing benefits of   multiple inheritance without the problems of multiple implementation   inheritance  The inheritance of an abstract contract is termed   interface inheritance  The Java programming language supports interface inheritance by allowing you to declare an interface type

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Whenever we have a choice between abstract class and interface we should always  almost  prefer default  also known as defender or virtual extensions  methods   Default methods have put an end to classic pattern of interface and a companion class that implements most or all of the methods in that interface  An example is Collection and AbstractCollection  Now we should implement the methods in the interface itself to provide default functionality  The classes which implement the interface has choice to override the methods or inherit the default implementation   Another important use of default methods is interface evolution  Suppose I had a class Ball as  public class Ball implements Collection               Now in Java 8 a new feature streams in introduced  We can get a stream by using stream method added to the interface  If stream were not a default method all the implementations for Collection interface would have broken as they would not be implementing this new method  Adding a non-default method to an interface is not source-compatible  But suppose we do not recompile the class and use an old jar file which contains this class Ball  The class will load fine without this missing method  instances can be created and it seems everything is working fine  BUT if program invokes stream method on instance of Ball we will get AbstractMethodError  So making method default solved both the problems  Java 9 has got even private methods in interface which can be used to encapsulate the common code logic that was used in the interface methods that provided a default implementation

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In Java 8  an interface looks like an abstract class although their might be some differences such as    1  Abstract classes are classes  so they are not restricted to other restrictions of the interface in Java e g  abstract class can have the state  but you cannot have the state on the interface in Java   2  Another semantic difference between interface with default methods and abstract class is that you can define constructors inside an abstract class  but you cannot define constructor inside interface in Java

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Although its an old question let me give my input on it as well    abstract class  Inside abstract class we can declare instance     variables  which are required to the child class  Interface  Inside interface every variables is always public static   and final we cannot declare instance variables abstract class  Abstract class can talk about state of object  Interface  Interface can never talk about state of object abstract class  Inside Abstract class we can declare constructors  Interface  Inside interface we cannot declare constructors as purpose of   constructors is to initialize instance variables  So what   is the need of constructor there if we cannot have instance   variables in interfaces  abstract class  Inside abstract class we can declare instance and static blocks  Interface  Interfaces cannot have instance and static blocks  abstract class  Abstract class cannot refer lambda expression  Interfaces  Interfaces with single abstract method can refer lambda expression abstract class  Inside abstract class we can override OBJECT CLASS methods  Interfaces  We cannot override OBJECT CLASS methods inside interfaces    I will end on the note that   Default method concepts static method concepts in interface came just to save implementation classes but not to provide meaningful useful implementation  Default methods static methods are kind of dummy implementation   if you want you can use them  or you can override them  in case of default methods  in implementation class  Thus saving us from implementing new methods in implementation classes whenever new methods in interfaces are added  Therefore interfaces can never be equal to abstract classes

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There s a lot more to abstract classes than default method implementations  such as private state   but as of Java 8  whenever you have the choice of either  you should go with the defender  aka  default  method in the interface   The constraint on the default method is that it can be implemented only in the terms of calls to other interface methods  with no reference to a particular implementation s state  So the main use case is higher-level and convenience methods   The good thing about this new feature is that  where before you were forced to use an abstract class for the convenience methods  thus constraining the implementor to single inheritance  now you can have a really clean design with just the interface and a minimum of implementation effort forced on the programmer   The original motivation to introduce default methods to Java 8 was the desire to extend the Collections Framework interfaces with lambda-oriented methods without breaking any existing implementations  Although this is more relevant to the authors of public libraries  you may find the same feature useful in your project as well  You ve got one centralized place where to add new convenience and you don t have to rely on how the rest of the type hierarchy looks

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Please think first of open closed principle  The default methods in interfaces DO VIOLATE it  This is a bad feature in Java  It encourages bad design  bad architecture  low software quality  I would suggest to avoid using default methods completely   Ask yourself a few questions  Why can t you put your methods to the abstract class  Would you need then more than one abstract class  Then think about what is your class responsible for  Are you sure all methods you are going to put to the single class really fulfill the same purpose  May be you will distinguish several purposes and will then split your class into several classes  for each purpose its own class

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There are a few technical differences  Abstract classes can still do more in comparison to Java 8 interfaces    Abstract class can have a constructor  Abstract classes are more structured and can hold a state    Conceptually  main purpose of defender methods is a backward compatibility after introduction of new features  as lambda-functions  in Java 8

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Default methods in Java interface enables interface evolution   Given an existing interface  if you wish to add a method to it without breaking the binary compatibility with older versions of the interface  you have two options at hands  add a default or a static method  Indeed  any abstract method added to the interface would have to be impleted by the classes or interfaces implementing this interface   A static method is unique to a class  A default method is unique to an instance of the class   If you add a default method to an existing interface  classes and interfaces which implement this interface do not need to implement it  They can   implement the default method  and it overrides the implementation in implemented interface  re-declare the method  without implementation  which makes it abstract  do nothing  then the default method from implemented interface is simply inherited     More on the topic here

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Regarding your query of  So when should interface with default methods be used and when should an abstract class be used  Are the abstract classes still useful in that scenario   java documentation  provides perfect answer  Abstract Classes Compared to Interfaces   Abstract classes are similar to interfaces  You cannot instantiate them  and they may contain a mix of methods declared with or without an implementation  However  with abstract classes  you can declare fields that are not static and final  and define public  protected  and private concrete methods  With interfaces  all fields are automatically public  static  and final  and all methods that you declare or define  as default methods  are public  In addition  you can extend only one class  whether or not it is abstract  whereas you can implement any number of interfaces   Use cases for each of them have been explained in below SE post  What is the difference between an interface and abstract class   Are the abstract classes still useful in that scenario   Yes  They are still useful  They can contain non-static  non-final methods   and attributes  protected  private in addition to public   which is not possible even with Java-8 interfaces

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As described in this article   Abstract classes versus interfaces in Java 8     After introducing Default Method  it seems that interfaces and   abstract classes are same  However  they are still different concept   in Java 8       Abstract class can define constructor  They are more structured and   can have a state associated with them  While in contrast  default   method can be implemented only in the terms of invoking other   interface methods  with no reference to a particular implementation s   state  Hence  both use for different purposes and choosing between two   really depends on the scenario context

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when should interface with default methods be used and when should an   abstract class be used    Backward compatibility  Imagine that your interface is implemented by hundreds of classes  modifying that interface will force all the users to implement the newly added method  even though it could be not essential for many other classes that implements your interface  Plus it allows your  interface to be a functional interface  Facts  amp  Restrictions    1-May only be declared within an interface and not within a class or abstract class   2-Must provide a body  3-It is not assumed to be abstract as other normal methods used in an interface

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This is being described in this article  Think about forEach of Collections   List lt   gt  list       list forEach            The forEach isn   t declared by java util List nor the   java util Collection interface yet  One obvious solution would be to   just add the new method to the existing interface and provide the   implementation where required in the JDK  However  once published  it   is impossible to add methods to an interface without breaking the   existing implementation       The benefit that default methods bring is that now it   s possible to   add a new default method to the interface and it doesn   t break the   implementations

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These two are quite different   Default methods are to add external functionality to existing classes without changing their state   And abstract classes are a normal type of inheritance  they are normal classes which are intended to be extended

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Default methods in Java Interface are to be used more for providing dummy implementation of a function thus saving any implementing class of that interface from the pain of declaring all the abstract methods even if they want to deal with only one  Default methods in interface are thus in a way more a replacement for the concept of adapter classes   The methods in abstract class are however supposed to give a meaningful implementation which any child class should override only if needed to override a common functionality

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Remi Forax rule is You don t design with Abstract classes  You design your app with interfaces  Watever is the version of Java  whatever is the language  It is backed by the Interface segregation principle in SOLID principles   You can later use Abstract classes to factorize code  Now with Java 8 you can do it directly in the interface  This is a facility  not more

[java] When to use: Java 8+ interface default method, vs. abstract method

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