How should I have explained the difference between an Interface and an Abstract class

Question

In one of my interviews  I have been asked to explain the difference between an Interface and an Abstract class   Here s my response      Methods of a Java interface are implicitly abstract    and cannot have implementations  A Java abstract class can have    instance methods that implements a default behaviour       Variables declared in a Java interface are by default final  An     abstract class may contain non-final variables       Members of a Java interface are public by default  A Java abstract    class can have the usual flavours of class members like private     protected  etc       A Java interface should be implemented using keyword    implements     A   Java abstract class should be extended using keyword    extends          An interface can extend another Java interface only  an abstract class    can extend another Java class and implement multiple Java interfaces       A Java class can implement multiple interfaces but it can extend only    one abstract class    However  the interviewer was not satisfied  and told me that this description represented  bookish knowledge    He asked me for a more practical response  explaining when I would choose an abstract class over an interface  using practical examples   Where did I go wrong

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Yes  your responses were technically correct but where you went wrong was not showing them you understand the upsides and downsides of choosing one over the other  Additionally  they were probably concerned freaked out about compatibility of their codebase with upgrades in the future  This type of response may have helped  in addition to what you said         Choosing an Abstract Class over an Interface Class depends on what we   project the future of the code will be        Abstract classes allow better forward-compatibility because you can   continue adding behavior to an Abstract Class well into the future   without breaking your existing code --  this is not possible with an   Interface Class       On the other hand  Interface Classes are more flexible than Abstract   Classes  This is because they can implement multiple interfaces  The   thing is Java does not have multiple inheritances so using abstract   classes won t let you use any other class hierarchy structure         So  in the end a good general rule of thumb is  Prefer using Interface   Classes when there are no existing default implementations in your   codebase  And  use Abstract Classes to preserve compatibility if you   know you will be updating your class in the future     Good luck on your next interview

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An interface is a  contract  where the class that implements the contract promises to implement the methods  An example where I had to write an interface instead of a class was when I was upgrading a game from 2D to 3D  I had to create an interface to share classes between the 2D and the 3D version of the game    package adventure  import java awt    public interface Playable       public void playSound String s       public Image loadPicture String s           Then I can implement the methods based on the environment  while still being able to call those methods from an object that doesn t know which version of the game that is loading   public class Adventure extends JFrame implements Playable  public class Dungeon3D extends SimpleApplication implements Playable  public class Main extends SimpleApplication implements AnimEventListener          ActionListener  Playable  Typically  in the gameworld  the world can be an abstract class that performs methods on the game   public abstract class World         public Playable owner       public Playable getOwner             return owner             public void setOwner Playable owner            this owner   owner

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You choose Interface in Java to avoid the Diamond Problem in multiple inheritance   If you want all of your methods to be implemented by your client you go for interface  It means you design the entire application at abstract    You choose abstract class if you already know what is in common  For example Take an abstract class Car  At higher level you implement the common car methods like calculateRPM    It is a common method and you let the client implement his own behavior like  calculateMaxSpeed   etc  Probably you would have explained by giving few real time examples which you have encountered in your day to day job

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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    These two points above is what I m looking for when interviewing  and is a compact enough summary  Read on for more details   Alternative summary   An interface is for defining public APIs An abstract class is for internal use  and for defining SPIs   By example  To put it differently  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   Another perspective  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     Book knowledge   The technical details provided in the OP s answer are considered  book knowledge  because this is usually the approach used in school and in most technology books about a language  what a thing is  not how to use it in practice  especially in large scale applications   Here s an analogy  supposed the question was      What is better to rent for prom night  a car or a hotel room    The technical answer sounds like      Well  in a car you can do it sooner  but in a hotel room you can do it more comfortably  On the other hand  the hotel room is in only one place  while in the car you can do it in more places  like  let s say you can go to the vista point for a nice view  or in a drive-in theater  or many other places  or even in more than one place  Also  the hotel room has a shower    That is all true  but completely misses the points that they are two completely different things  and both can be used at the same time for different purposes  and the  doing it  aspect is not the most important thing about either of the two options  The answer lacks perspective  it shows an immature way of thinking  while correctly presenting true  facts

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From what I understand  an Interface  which is comprised of final variables and methods with no implementations  is implemented by a class to obtain a group of methods or methods that are related to each other  On the other hand  an abstract class  which can contain non-final variables and methods with implementations  is usually used as a guide or as a superclass from which all related or similar classes inherits from  In other words  an abstract class contains all the methods variables that are shared by all its subclasses

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1 1 Difference between Abstract class and interface 1 1 1  Abstract classes versus interfaces in Java 8 1 1 2  Conceptual Difference   1 2 Interface Default Methods in Java 8 1 2 1  What is Default Method  1 2 2  ForEach method compilation error solved using Default Method 1 2 3  Default Method and Multiple Inheritance Ambiguity Problems 1 2 4  Important points about java interface default methods   1 3 Java Interface Static Method 1 3 1  Java Interface Static Method  code example  static method vs default method 1 3 2  Important points about java interface static method   1 4 Java Functional Interfaces   1 1 1  Abstract classes versus interfaces in Java 8  Java 8 interface changes include static methods and default methods in interfaces  Prior to Java 8  we could have only method declarations in the interfaces  But from Java 8  we can have default methods and static methods in the interfaces  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   1 1 2  Conceptual Difference  Abstract classes are valid for skeletal  i e  partial  implementations of interfaces but should not exist without a matching interface  So when abstract classes are effectively reduced to be low-visibility  skeletal implementations of interfaces  can default methods take this away as well  Decidedly  No  Implementing interfaces almost always requires some or all of those class-building tools which default methods lack  And if some interface doesn   t  it is clearly a special case  which should not lead you astray  1 2 Interface Default Methods in Java 8 Java 8 introduces    Default Method    or  Defender methods  new feature  which allows developer to add new methods to the Interfaces without breaking the existing implementation of these Interface  It provides flexibility to allow Interface define implementation which will use as default in the situation where a concrete Class fails to provide an implementation for that method  Let consider small example to understand how it works  public interface OldInterface           public void existingMethod               default public void newDefaultMethod                     System out println  quot New default method quot                                   quot  is added in interface quot                 The following Class will compile successfully in Java JDK 8  public class OldInterfaceImpl implements OldInterface           public void existingMethod                  existing implementation is here                 If you create an instance of OldInterfaceImpl  OldInterfaceImpl obj   new OldInterfaceImpl        print    New default method add in interface    obj newDefaultMethod        1 2 1  Default Method   Default methods are never final  can not be synchronized and can not override Object   s methods  They are always public  which severely limits the ability to write short and reusable methods   Default methods can be provided to an Interface without affecting implementing Classes as it includes an implementation  If each added method in an Interface defined with implementation then no implementing Class is affected  An implementing Class can override the default implementation provided by the Interface   Default methods enable to add new functionality to existing Interfaces without breaking older implementation of these Interfaces   When we extend an interface that contains a default method  we can perform following   Not override the default method and will inherit the default method  Override the default method similar to other methods we override in subclass  Redeclare default method as abstract  which force subclass to override it    1 2 2  ForEach method compilation error solved using Default Method For Java 8  the JDK collections have been extended and forEach method is added to the entire collection  which work in conjunction with lambdas   With conventional way  the code looks like below  public interface Iterable lt T gt            public void forEach Consumer lt   super T gt  consumer      Since this result each implementing Class with compile errors therefore  a default method added with a required implementation in order that the existing implementation should not be changed  The Iterable Interface with the Default method is below  public interface Iterable lt T gt            public default void forEach Consumer                                        lt   super T gt  consumer                    for  T t   this                            consumer accept t                                  The same mechanism has been used to add Stream in JDK Interface without breaking the implementing Classes   1 2 3  Default Method and Multiple Inheritance Ambiguity Problems Since java Class can implement multiple Interfaces and each Interface can define default method with same method signature  therefore  the inherited methods can conflict with each other  Consider below example  public interface InterfaceA                    default void defaultMethod                             System out println  quot Interface A default method quot                         public interface InterfaceB         default void defaultMethod                  System out println  quot Interface B default method quot                 public class Impl implements InterfaceA  InterfaceB        The above code will fail to compile with the following error   java  class Impl inherits unrelated defaults for defaultMethod   from types InterfaceA and InterfaceB  In order to fix this class  we need to provide default method implementation  public class Impl implements InterfaceA  InterfaceB           public void defaultMethod                 Further  if we want to invoke default implementation provided by any of super Interface rather than our own implementation  we can do so as follows  public class Impl implements InterfaceA  InterfaceB           public void defaultMethod                       existing code here                   InterfaceA super defaultMethod                 We can choose any default implementation or both as part of our new method  1 2 4  Important points about java interface default methods   Java interface default methods will help us in extending interfaces without having the fear of breaking implementation classes  Java interface default methods have bridge down the differences between interfaces and abstract classes  Java 8 interface default methods will help us in avoiding utility classes  such as all the Collections class method can be provided in the interfaces itself  Java interface default methods will help us in removing base implementation classes  we can provide default implementation and the implementation classes can chose which one to override  One of the major reason for introducing default methods in interfaces is to enhance the Collections API in Java 8 to support lambda expressions  If any class in the hierarchy has a method with same signature  then default methods become irrelevant  A default method cannot override a method from java lang Object  The reasoning is very simple  it   s because Object is the base class for all the java classes  So even if we have Object class methods defined as default methods in interfaces  it will be useless because Object class method will always be used  That   s why to avoid confusion  we can   t have default methods that are overriding Object class methods  Java interface default methods are also referred to as Defender Methods or Virtual extension methods   Resource Link   When to use  Java 8  interface default method  vs  abstract method Abstract class versus interface in the JDK 8 era Interface evolution via virtual extension methods   1 3 Java Interface Static Method 1 3 1  Java Interface Static Method  code example  static method vs default method Java interface static method is similar to default method except that we can   t override them in the implementation classes  This feature helps us in avoiding undesired results incase of poor implementation in implementation classes  Let   s look into this with a simple example  public interface MyData        default void print String str            if   isNull str               System out println  quot MyData Print   quot    str              static boolean isNull String str            System out println  quot Interface Null Check quot             return str    null   true    quot  quot  equals str    true   false           Now let   s see an implementation class that is having isNull   method with poor implementation  public class MyDataImpl implements MyData        public boolean isNull String str            System out println  quot Impl Null Check quot             return str    null   true   false                 public static void main String args             MyDataImpl obj   new MyDataImpl            obj print  quot  quot            obj isNull  quot abc quot             Note that isNull String str  is a simple class method  it   s not overriding the interface method  For example  if we will add  Override annotation to the isNull   method  it will result in compiler error  Now when we will run the application  we get following output   Interface Null Check Impl Null Check  If we make the interface method from static to default  we will get following output   Impl Null Check MyData Print   Impl Null Check  Java interface static method is visible to interface methods only  if we remove the isNull   method from the MyDataImpl class  we won   t be able to use it for the MyDataImpl object  However like other static methods  we can use interface static methods using class name  For example  a valid statement will be  boolean result   MyData isNull  quot abc quot     1 3 2  Important points about java interface static method   Java interface static method is part of interface  we can   t use it for implementation class objects  Java interface static methods are good for providing utility methods  for example null check  collection sorting etc  Java interface static method helps us in providing security by not allowing implementation classes to override them  We can   t define interface static method for Object class methods  we will get compiler error as    This static method cannot hide the instance method from Object     This is because it   s not allowed in java  since Object is the base class for all the classes and we can   t have one class level static method and another instance method with same signature  We can use java interface static methods to remove utility classes such as Collections and move all of it   s static methods to the corresponding interface  that would be easy to find and use    1 4 Java Functional Interfaces Before I conclude the post  I would like to provide a brief introduction to Functional interfaces  An interface with exactly one abstract method is known as Functional Interface  A new annotation  FunctionalInterface has been introduced to mark an interface as Functional Interface   FunctionalInterface annotation is a facility to avoid accidental addition of abstract methods in the functional interfaces  It   s optional but good practice to use it  Functional interfaces are long awaited and much sought out feature of Java 8 because it enables us to use lambda expressions to instantiate them  A new package java util function with bunch of functional interfaces are added to provide target types for lambda expressions and method references  We will look into functional interfaces and lambda expressions in the future posts  Resource Location   Java 8 Interface Changes     static method  default method

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Here   s an explanation centred around Java 8  that tries to show the key differences between abstract classes and interfaces  and cover all the details needed for the Java Associate Exam   Key concepts    A class can extend only one class  but it can implement any number of interfaces Interfaces define what a class does  abstract classes define what it is Abstract classes are classes  They can   t be instantiated  but otherwise behave like normal classes Both can have abstract methods and static methods Interfaces can have default methods  amp  static final constants  and can extend other interfaces All interface members are public  until Java 9    Interfaces define what a class does  abstract classes define what it is  Per Roedy Green      Interfaces are often used to describe the abilities of a class  not its central identity  e g  An Automobile class might implement the Recyclable interface  which could apply to many unrelated objects   An abstract class defines the core identity of its descendants  If you defined a Dog abstract class then Dalmatian descendants are Dogs  they are not merely dogable    Pre Java 8   Daniel Lerps   s answer was spot on  that interfaces are like a contract that the implementing class has to fulfil   Now  with default methods  they are more like a Mixin  that still enforces a contract  but can also give code to do the work   This has allowed interfaces to take over some of the use cases of abstract classes   The point of an abstract class is that it has missing functionality  in the form of abstract methods  If a class doesn   t have any abstract behaviour  which changes between different types  then it could be a concrete class instead   Abstract classes are classes  Here are some of the normal features of classes which are available in abstract classes  but not in interfaces    Instance variables   non-final variables   And therefore    Methods which can access and modify the state of the object  Private   protected members  but see note on Java 9  Ability to extend abstract or concrete classes Constructors   Points to note about abstract classes    They cannot be final  because their whole purpose is to be extended  An abstract class that extends another abstract class inherits all of its abstract methods as its own abstract methods   Abstract methods  Both abstract classes and interfaces can have zero to many abstract methods  Abstract methods    Are method signatures without a body  i e  no     Must be marked with the abstract keyword in abstract classes  In interfaces this keyword is unnecessary Cannot be private  because they need to be implemented by another class  Cannot be final  because they don   t have bodies yet  Cannot be static  because reasons    Note also that    Abstract methods can be called by non-abstract methods in the same class interface  The first concrete class that extends an abstract class or implements an interface must provide an implementation for all the abstract methods   Static methods  A static method on an abstract class can be called directly with MyAbstractClass method     i e  just like for a normal class  and it can also be called via a class that extends the abstract class    Interfaces can also have static methods   These can only be called via the name of the interface  MyInterface method       These methods    Cannot be abstract  i e  must have a body  see    because reasons    above   Are not default  see below    Default methods  Interfaces can have default methods which must have the default keyword and a method body   These can only reference other interface methods  and can   t refer to a particular implementation s state   These methods    Are not static Are not abstract  they have a body  Cannot be final  the name    default    indicates that they may be overridden    If a class implements two interfaces with default methods with the same signatures this causes a compilation error  which can be resolved by overriding the method   Interfaces can have static final constants  Interfaces can only contain the types of methods describe above  or constants   Constants are assumed to be static and final  and can be used without qualification in classes that implement the interface    All interface members are public  In Java 8 all members of interfaces  and interfaces themselves  are assumed to be public  and cannot be protected or private  but Java 9 does allow private methods in interfaces    This means that classes implementing an interface must define the methods with public visibility  in line with the normal rule that a method cannot be overridden with lower visibility

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All your statements are valid except your first statement  after the Java 8 release       Methods of a Java interface are implicitly abstract and cannot have implementations   From the documentation page      An interface is a reference type  similar to a class  that can contain only    constants  method signatures  default methods  static methods and nested types      Method bodies exist only for default methods and static methods    Default methods   An interface can have default methods  but are different than abstract methods in abstract classes      Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces    When you extend an interface that contains a default method  you can do the following    Not mention the default method at all  which lets your extended interface inherit the default method  Redeclare the default method  which makes it abstract  Redefine the default method  which overrides it    Static Methods   In addition to default methods  you can define static methods in interfaces   A static method is a method that is associated with the class in which it is defined rather than with any object  Every instance of the class shares its static methods     This makes it easier for you to organize helper methods in your libraries   Example code from documentation page about interface having static and default methods   import java time     public interface TimeClient       void setTime int hour  int minute  int second       void setDate int day  int month  int year       void setDateAndTime int day  int month  int year                                 int hour  int minute  int second       LocalDateTime getLocalDateTime         static ZoneId getZoneId  String zoneString            try               return ZoneId of zoneString             catch  DateTimeException e                System err println  Invalid time zone      zoneString                      using default time zone instead                 return ZoneId systemDefault                         default ZonedDateTime getZonedDateTime String zoneString            return ZonedDateTime of getLocalDateTime    getZoneId zoneString              Use the below guidelines to chose whether to use an interface or abstract class   Interface    To define a contract   preferably stateless - I mean no variables   To link unrelated classes with has a capabilities  To declare public constant variables  immutable state    Abstract class    Share code among several closely related classes  It establishes is a relation  Share common state among related classes   state can be modified in concrete classes    Related posts    Interface vs Abstract Class  general OO   Implements vs extends  When to use  What  39 s the difference   By going through these examples  you can understand that   Unrelated classes can have capabilities through interface but related classes change the behaviour through extension of base classes

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Even I have faced the same question in multiple interviews and believe me it makes your time miserable to convince the interviewer  If I inherent all the answers from above then I need to add one more key point to make it more convincing and utilizing OO at its best  In case you are not  planning any modification in the rules   for the subclass to be followed  for a long future  go for the interface  as you wont be able to modify in it and if you do so  you need to go for the changes in all the other sub classes  whereas  if you think   you want to reuse the functionality  set some rules and also make it open for modification  go for Abstract class     Think in this way  you had used a consumable service or you had provided some code to world and You have a chance to modify something  suppose a security check And If I am being a consumer of the code and One morning after a update   I find all read marks in my Eclipse  entire application is down  So to prevent such nightmares  use Abstract over Interfaces   I think this might convince the Interviewer to a extent   Happy Interviews Ahead

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When I am trying to share behavior between 2 closely related classes  I create an abstract class that holds the common behavior and serves as a parent to both classes   When I am trying to define a Type  a list of methods that a user of my object can reliably call upon  then I create an interface     For example  I would never create an abstract class with 1 concrete subclass because abstract classes are about sharing behavior   But I might very well create an interface with only one implementation   The user of my code won t know that there is only one implementation   Indeed  in a future release there may be several implementations  all of which are subclasses of some new abstract class that didn t even exist when I created the interface   That might have seemed a bit too bookish too  though I have never seen it put that way anywhere that I recall    If the interviewer  or the OP  really wanted more of my personal experience on that  I would have been ready with anecdotes of an interface has evolved out of necessity and visa versa   One more thing   Java 8 now allows you to put default code into an interface  further blurring the line between interfaces and abstract classes    But from what I have seen  that feature is overused even by the makers of the Java core libraries   That feature was added  and rightly so  to make it possible to extend an interface without creating binary incompatibility    But if you are making a brand new Type by defining an interface  then the interface should be JUST an interface   If you want to also provide common code  then by all means make a helper class  abstract or concrete    Don t be cluttering your interface from the start with functionality that you may want to change

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From what I understand and how I approach   Interface is like a specification contract  any class that implements an interface class have to implement all the methods defined in the abstract class  except default methods  introduced in Java 8    Whereas I define a class abstract when I know the implementation required for some methods of the class and some methods I still do not know what will be the implementation  we might know the function signature but not the implementation   I do this so that later in the part of development when I know how these methods are to be implemented  I can just extend this abstract class and implement these methods   Note  You cannot have function body in interface methods unless the method is static or default

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The main difference what i have observed was that abstract class  provides us with some common behaviour implemented already and subclasses only needs to implement specific functionality corresponding to them  where as for an interface will only specify what tasks needs to be done and no implementations will be given by interface  I can say it specifies the contract between itself and implemented classes

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To keep it down to a simple  reasonable response you can provide in an interview  I offer the following    An interface is used to specify an API for a family of related classes - the relation being the interface   Typically used in a situation that has multiple implementations  the correct implementation being chosen either by configuration or at runtime    Unless using Spring  at which point an interface is basically a Spring Bean    Interfaces are often used to solve the multiple inheritance issue  An abstract class is a class designed specifically for inheritance   This also implies multiple implementations  with all implementations having some commonality  that found in the abstract class   If you want to nail it  then say that an abstract class often implements a portion of an interface - job is yours

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In abstract class  you can write default implementation of methods  But in Interface you can not  Basically  In interface there exist pure virtual methods which have to be implemented by the class which implements the interface

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I will try to answer using practical scenario to show the distinction between the two   Interfaces come with zero payload i e  no state has to be maintained and thus are better choice to just associate a contract  capability  with a class   For example  say I have a Task class that performs some action  now to execute a task in separate thread I don t really need to extend Thread class rather better choice is to make Task implement Runnable interface  i e  implement its run   method  and then pass object of this Task class to a Thread instance and call its start   method      Now you can ask what if Runnable was a abstract class    Well technically that was possible but design wise that would have been a poor choice reason being    Runnable has no state associated with it and neither it  offers  any default implementation for the run   method Task would have to extend it thus it couldn t extend any other class Task has nothing to offer as specialization to Runnable class  all it needs is to override run   method   In other words  Task class needed a capability to be run in a thread which it achieved by implementing Runnable interface verses extending the Thread class that would make it a thread      Simply put us interface to define a capability  contract   while use a   abstract class to define skeleton  common partial  implementation of   it    Disclaimer  silly example follows  try not to judge  -P  interface Forgiver       void forgive       abstract class GodLike implements Forgiver       abstract void forget        final void forgive             forget              Now you have been given a choice to be GodLike but you may choose to be Forgiver only  i e  not GodLike  and do   class HumanLike implements Forgiver       void forgive               forgive but remember               Or you may may choose to be GodLike and do   class AngelLike extends GodLike       void forget               forget to forgive                P S  with java 8 interface can also have static as well default  overridable implementation  methods and thus difference b w interface and abstract class is even more narrowed down

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I do interviews for work and i would look unfavourably on your answer aswell  sorry but im very honest   It does sound like you ve read about the difference and revised an answer but perhaps you have never used it in practice    A good explanation as to why you would use each can be far better than having a precise explanation of the difference  Employers ultimatley want programers to do things not know them which can be hard to demonstrate in an interview  The answer you gave would be good if applying for a technical or documentation based job but not a developers role   Best of luck with interviews in the future   Also my answer to this question is more about interview technique rather than the technical material youve provided  Perhaps consider reading about it  https   workplace stackexchange com  can be an excellent place for this sort of thing

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An interface consists of singleton variables  public static final  and public abstract methods  We normally prefer to use an interface in real time when we know what to do but don t know how to do it   This concept can be better understood by example   Consider a Payment class  Payment can be made in many ways  such as PayPal  credit card etc  So we normally take Payment as our interface which contains a makePayment   method and CreditCard and PayPal are the two implementation classes   public interface Payment       void makePayment     by default it is a abstract method   public class PayPal implements Payment       public void makePayment                   some logic for PayPal payment           e g  Paypal uses username and password for payment         public class CreditCard implements Payment       public void makePayment                   some logic for CreditCard payment           e g  CreditCard uses card number  date of expiry etc              In the above example CreditCard and PayPal are two implementation classes  strategies  An Interface also allows us the concept of multiple inheritance in Java which cannot be accomplished by an abstract class   We choose an abstract class when there are some features for which we know what to do  and other features that we know how to perform   Consider the following example   public abstract class Burger       public void packing                   some logic for packing a burger           public abstract void price      price is different for different categories of burgers   public class VegBerger extends Burger       public void price                   set price for a veg burger          public class NonVegBerger extends Burger       public void price                   set price for a non-veg burger            If we add methods  concrete abstract  in the future to a given abstract class  then the implementation class will not need a change its code  However  if we add methods in an interface in the future  we must add implementations to all classes that implemented that interface  otherwise compile time errors occur   There are other differences but these are major ones which may have been what your interviewer expected   Hopefully this was helpful

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Almost everything seems to be covered here already   Adding just one more point on practical implementation of abstract class      abstract keyword is also used just prevent a class from being instantiated  If you have a concrete class which you do not want to be instantiated - Make it abstract

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hmm now the people are hungery practical approach  you are quite right but most of interviewer looks as per their current requirment and want a practical approach   after finishing your answer you should jump on the example   Abstract   for example we have salary function which have some parametar common to all employee  then we can have a abstract class called CTC with partialy defined method body and it will got extends by all type of employee and get redeined as per their extra beefits  For common functonality   public abstract class CTC        public int salary int hra  int da  int extra                int total          total   hra da extra            incentive for specific performing employee           total   hra da extra incentive          return total           class Manger extends CTC       class CEO extends CTC      class Developer extends CTC          Interface  interface in java allow to have interfcae functionality without extending that one and you have to be clear with the implementation of signature of functionality that you want to introduce in your application  it will force you to have definiton  For different functionality   public interface EmployeType        public String typeOfEmployee       class ContarctOne implements EmployeType         Override     public String typeOfEmployee             return  contract             class PermanentOne implements EmployeType         Override     public String typeOfEmployee             return  permanent              you can have such forced activity with abstract class too by defined methgos as a abstract one  now a class tha extends abstract class remin abstract one untill it override that abstract function

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What about thinking the following way    A relationship between a class and an abstract class is of type  is-a  A relationship between a class and an interface is of type  has-a    So when you have an abstract class Mammals  a subclass Human  and an interface Driving  then you can say   each Human is-a Mammal each Human has-a Driving  behavior    My suggestion is that the book knowledge phrase indicates that he wanted to hear the semantic difference between both  like others here already suggested

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your answer is right but the interviewer needs you to differentiate according to software engineering perspective not according to the details of Java   Simple words   An Interface is like the interface of a shop anything that is shown on it should be there in the shop  so any method in the Interface must be there implemented in the concrete class  Now what if some classes share some exact methods and varies in others  Suppose the Interface is about a shop that contains two things and suppose we have two shops both contain sport equipment but one has clothes extra and the other has shoes extra  So what you do is making an abstract class for Sport that implements the Sports method and leave the other method unimplemented  Abstract class here means that this shop doesn t exist itself but it is the base for other classes shops  This way you are organising the code  avoiding errors of replicating the code  unifying the code  and ensuring re-usability by some other class

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Your explanation looks decent  but may be it looked like you were reading it all from a textbook   -   What I m more bothered about is  how solid was your example  Did you bother to include almost all the differences between abstract and interfaces    Personally  I would suggest this link  http   mindprod com jgloss interfacevsabstract html TABLE  for an exhaustive list of differences     Hope it helps you and all other readers in their future interviews

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Nothing is perfect in this world  They may have been expecting more of a practical approach   But after your explanation you could add these lines with a slightly different approach      Interfaces are rules  rules because you must give an implementation to them that you can t ignore or avoid  so that they are imposed like rules  which works as a common understanding document among various teams in software development   Interfaces give the idea what is to be done but not how it will be done  So implementation completely depends on developer by following the given rules  means given signature of methods    Abstract classes may contain abstract declarations  concrete implementations  or both  Abstract declarations are like rules to be followed and concrete implementations are like guidelines  you can use it as it is or you can ignore it by overriding and giving your own implementation to it   Moreover which methods with same signature may change the behaviour in different context are provided as interface declarations as rules to implement accordingly in different contexts    Edit  Java 8 facilitates to define default and static methods in interface   public interface SomeInterfaceOne        void usualAbstractMethod String inputString        default void defaultMethod String inputString           System out println  Inside SomeInterfaceOne defaultMethod    inputString             Now when a class will implement SomeInterface  it is not mandatory to provide implementation for default methods of interface   If we have another interface with following methods   public interface SomeInterfaceTwo        void usualAbstractMethod String inputString        default void defaultMethod String inputString           System out println  Inside SomeInterfaceTwo defaultMethod    inputString              Java doesn   t allow extending multiple classes because it results in the    Diamond Problem    where compiler is not able to decide which superclass method to use  With the default methods  the diamond problem will arise for interfaces too  Because if a class is implementing both   SomeInterfaceOne and SomeInterfaceTwo   and doesn   t implement the common default method  compiler can   t decide which one to chose  To avoid this problem  in java 8 it is mandatory to implement common default methods of different interfaces  If any class is implementing both the above interfaces  it has to provide implementation for defaultMethod   method otherwise compiler will throw compile time error

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You made a good summary of the practical differences in use and implementation but did not say anything about the difference in meaning   An interface is a description of the behaviour an implementing class will have  The implementing class ensures  that it will have these methods that can be used on it  It is basically a contract or a promise the class has to make   An abstract class is a basis for different subclasses that share behaviour which does not need to be repeatedly created  Subclasses must complete the behaviour and have the option to override predefined behaviour  as long as it is not defined as final or private    You will find good examples in the java util package which includes interfaces like List and abstract classes like AbstractList which already implements the interface  The official documentation describes the AbstractList as follows      This class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a  random access  data store  such as an array

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An interface is purely abstract  we dont have any implementation code in interface   Abstract class contains both methods and its implementation   click here to watch tutorial on interfaces and abstract classes

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I will give you an example first   public interface LoginAuth     public String encryptPassword String pass      public void checkDBforUser        Suppose you have 3 databases in your application  Then each and every implementation for that database needs to define the above 2 methods    public class DBMySQL implements LoginAuth               Needs to implement both methods   public class DBOracle implements LoginAuth               Needs to implement both methods   public class DBAbc implements LoginAuth               Needs to implement both methods     But what if encryptPassword   is not database dependent  and it s the same for each class  Then the above would not be a good approach   Instead  consider this approach   public abstract class LoginAuth     public String encryptPassword String pass                  Implement the same default behavior here                 that is shared by all subclasses              Each subclass needs to provide their own implementation of this only     public abstract void checkDBforUser        Now in each child class  we only need to implement one method - the method that is database dependent

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I believe what the interviewer was trying to get at was probably the difference between interface and implementation   The interface - not a Java interface  but  interface  in more general terms - to a code module is  basically  the contract made with client code that uses the interface   The implementation of a code module is the internal code that makes the module work   Often you can implement a particular interface in more than one different way  and even change the implementation without client code even being aware of the change   A Java interface should only be used as an interface in the above generic sense  to define how the class behaves for the benefit of client code using the class  without specifying any implementation   Thus  an interface includes method signatures - the names  return types  and argument lists - for methods expected to be called by client code  and in principle should have plenty of Javadoc for each method describing what that method does   The most compelling reason for using an interface is if you plan to have multiple different implementations of the interface  perhaps selecting an implementation depending on deployment configuration   A Java abstract class  in contrast  provides a partial implementation of the class  rather than having a primary purpose of specifying an interface   It should be used when multiple classes share code  but when the subclasses are also expected to provide part of the implementation   This permits the shared code to appear in only one place - the abstract class - while making it clear that parts of the implementation are not present in the abstract class and are expected to be provided by subclasses

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The basic difference between interface and abstract class is  interface supports multiple inheritance but abstract class not   In abstract class also you can provide all abstract methods like interface   why abstract class is required   In some scenarios  while processing user request  the abstract class doesn t know what user intention  In that scenario  we will define one abstract method in the class and ask the user who extending this class  please provide your intention in the abstract method  In this case abstract classes are very useful  Why interface is required   Let s say  I have a work which I don t have experience in that area  Example   if you want to construct a building or dam  then what you will do in that scenario    you will identify what are your requirements and make a contract with that requirements  Then call the Tenders to construct your project Who ever construct the project  that should satisfy your requirements  But the construction logic is different from one vendor to other vendor    Here I don t bother about the logic how they constructed  The final object satisfied my requirements or not  that only my key point   Here your requirements called interface and constructors are called implementor

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In a few words  I would answer this way     inheritance via class hierarchy implies a state inheritance  whereas inheritance via interfaces stands for behavior inheritance    Abstract classes can be treated as something between these two cases  it introduces some state but also obliges you to define a behavior   a fully-abstract class is an interface  this is a further development of classes consist from virtual methods only in C   as far as I m aware of its syntax     Of course  starting from Java 8 things got slightly changed  but the idea is still the same   I guess this is pretty enough for a typical Java interview  if you are not being interviewed to a compiler team

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Abstract classes are not pure abstraction bcz its collection of concrete implemented methods  as well as unimplemented methods  But  Interfaces are pure abstraction bcz there are only unimplemented methods not concrete methods    Why Abstract classes    If user want write common functionality for all objects  Abstract classes are best choice for reimplementation in future that to add more functionality without affecting of end user    Why Interfaces    If user want to write different functionality that would be different functionality on objects  Interfaces are best choice that if not need to modify the requirements once interface has been published

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An interface is like a set of genes that are publicly documented to have some kind of effect  A DNA test will tell me whether I ve got them - and if I do  I can publicly make it known that I m a  carrier  and part of my behavior or state will conform to them   But of course  I may have many other genes that provide traits outside this scope    An abstract class is like the dead ancestor of a single-sex species     She can t be brought to life but a living  i e  non-abstract  descendant inherits all her genes       To stretch this metaphor  let s say all members of the species live to the same age  This means all ancestors of a dead ancestor must also be dead - and likewise  all descendants of a living ancestor must be alive

[java] How should I have explained the difference between an Interface and an Abstract class?

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