Implements vs extends When to use What s the difference

Question

Please explain in an easy to understand language or a link to some article

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As shown in the figure given below  a class extends another class  an interface extends another interface but a class implements an interface    For more details

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I notice you have some C   questions in your profile   If you understand the concept of multiple-inheritance from C    referring to classes that inherit characteristics from more than one other class   Java does not allow this  but it does have keyword interface  which is sort of like a pure virtual class in C     As mentioned by lots of people  you extend a class  and you can only extend from one   and you implement an interface -- but your class can implement as many interfaces as you like   Ie  these keywords and the rules governing their use delineate the possibilities for multiple-inheritance in Java  you can only have one super class  but you can implement multiple interfaces

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Those two keywords are directly attached with Inheritance it is a core concept of OOP  When we inherit some class to another class we can use extends but when we are going to inherit some interfaces to our class we can t use extends we should use implements and we can use extends keyword to inherit interface from another interface

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We use SubClass extends SuperClass only when the subclass wants to use some functionality  methods or instance variables  that is already declared in the SuperClass  or if I want to slightly modify the functionality of the SuperClass  Method overriding   But say  I have an Animal class SuperClass  and a Dog class  SubClass  and there are few methods that I have defined in the Animal class eg  doEat      doSleep        and many more  Now  my Dog class can simply extend the Animal class  if i want my dog to use any of the methods declared in the Animal class I can invoke those methods by simply creating a Dog object  So this way I can guarantee that I have a dog that can eat and sleep and do whatever else I want the dog to do  Now  imagine  one day some Cat lover comes into our workspace and she tries to extend the Animal class cats also eat and sleep   She makes a Cat object and starts invoking the methods  But  say  someone tries to make an object of the Animal class  You can tell how a cat sleeps  you can tell how a dog eats  you can tell how an elephant drinks  But it does not make any sense in making an object of the Animal class  Because it is a template and we do not want any general way of eating  So instead  I will prefer to make an abstract class that no one can instantiate but can be used as a template for other classes  So to conclude  Interface is nothing but an abstract class a pure abstract class  which contains no method implementations but only the definitions the templates   So whoever implements the interface just knows that they have the templates of doEat    and doSleep    but they have to define their own doEat    and doSleep    methods according to their need  You extend only when you want to reuse some part of the SuperClass but keep in mind  you can always override the methods of your SuperClass according to your need  and you implement when you want the templates and you want to define them on your own according to your need   I will share with you a piece of code  You try it with different sets of inputs and look at the results  class AnimalClass    public void doEat              System out println  quot Animal Eating    quot       public void sleep              System out println  quot Animal Sleeping    quot          public class Dog extends AnimalClass implements AnimalInterface  Herbi   public static void main String   args             AnimalInterface a   new Dog        Dog obj   new Dog        obj doEat        a eating             obj eating        obj herbiEating       public void doEat         System out println  quot Dog eating    quot        Override public void eating              System out println  quot Eating through an interface    quot           TODO Auto-generated method stub          Override public void herbiEating              System out println  quot Herbi eating through an interface    quot           TODO Auto-generated method stub             Defined Interfaces   public interface AnimalInterface    public void eating         interface Herbi    public void herbiEating

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Generally implements used for implementing an interface and extends used for extension of base class behaviour or abstract class    extends  A derived class can extend a base class  You may redefine the behaviour of an established relation  Derived class  is a  base class type  implements  You are implementing a contract  The class implementing the interface  has a  capability    With java 8 release  interface can have default methods in interface  which provides implementation in interface itself   Refer to this question for when to use each of them   Interface vs Abstract Class  general OO   Example to understand things   public class ExtendsAndImplementsDemo      public static void main String args              Dog dog   new Dog  Tiger  16           Cat cat   new Cat  July  20            System out println  Dog   dog           System out println  Cat   cat            dog remember            dog protectOwner            Learn dl   dog          dl learn             cat remember            cat protectOwner             Climb c   cat          c climb             Man man   new Man  Ravindra  40           System out println man            Climb cm   man          cm climb            Think t   man          t think            Learn l   man          l learn            Apply a   man          a apply              abstract class Animal      String name      int lifeExpentency      public Animal String name int lifeExpentency            this name   name          this lifeExpentency lifeExpentency            public void remember            System out println  Define your own remember              public void protectOwner            System out println  Define your own protectOwner               public String toString            return this getClass   getSimpleName       name     lifeExpentency          class Dog extends Animal implements Learn       public Dog String name int age           super name age             public void remember            System out println this getClass   getSimpleName     can remember for 5 minutes              public void protectOwner            System out println this getClass   getSimpleName      will protect owner              public void learn            System out println this getClass   getSimpleName      can learn             class Cat extends Animal implements Climb       public Cat String name int age           super name age             public void remember            System out println this getClass   getSimpleName       can remember for 16 hours              public void protectOwner            System out println this getClass   getSimpleName      won t protect owner              public void climb            System out println this getClass   getSimpleName      can climb            interface Climb      public void climb      interface Think       public void think       interface Learn       public void learn      interface Apply      public void apply       class Man implements Think Learn Apply Climb      String name      int age       public Man String name int age           this name   name          this age   age            public void think            System out println  I can think   this getClass   getSimpleName               public void learn            System out println  I can learn   this getClass   getSimpleName               public void apply            System out println  I can apply   this getClass   getSimpleName               public void climb            System out println  I can climb   this getClass   getSimpleName               public String toString            return  Man    name   Age   age            output   Dog Dog Tiger 16 Cat Cat July 20 Dog can remember for 5 minutes Dog will protect owner Dog can learn  Cat can remember for 16 hours Cat won t protect owner Cat can climb Man  Ravindra Age 40 I can climb Man I can think Man I can learn Man I can apply Man   Important points to understand    Dog and Cat are animals and they extended remember   and protectOwner   by sharing name lifeExpentency from Animal Cat can climb   but Dog does not  Dog can think   but Cat does not  These specific capabilities are added to Cat and Dog by implementing that capability  Man is not an animal but he can Think Learn Apply Climb   By going through these examples  you can understand that  Unrelated classes can have capabilities through interface but related classes override behaviour through extension of base classes

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extends is for when you re inheriting from a base class  i e  extending its functionality    implements is for when you re implementing an interface   Here is a good place to start  Interfaces and Inheritance

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extends    class extends only one class  interface extends one or more interfaces   implements   class implements one or more interfaces interfaces  can not  implements anything   abstract classes also act like class  with extends and implements

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Both keywords are used when creating your own new class in the Java language   Difference    implements means you are using the elements of a Java Interface in your class   extends means that you are creating a subclass of the base class you are extending  You can only extend one class in your child class  but you can implement as many interfaces as you would like    Refer to oracle documentation page on interface for more details    This can help to clarify what an interface is  and the conventions around using them

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extends is for extending a class  implements is for implementing an interface The difference between an interface and a regular class is that in an interface you can not implement any of the declared methods   Only the class that  quot implements quot  the interface can implement the methods   The C   equivalent of an interface would be an abstract class  not EXACTLY the same but pretty much   Also java doesn t support multiple inheritance for classes  This is solved by using multiple interfaces   public interface ExampleInterface       public void doAction        public String doThis int number        public class sub implements ExampleInterface        public void doAction              specify what must happen              public String doThis int number             specfiy what must happen            now extending a class  public class SuperClass       public int getNb                specify what must happen         return 1               public int getNb2                specify what must happen         return 2              public class SubClass extends SuperClass           you can override the implementation        Override       public int getNb2             return 3             in this case   Subclass s   new SubClass      s getNb      returns 1   s getNb2      returns 3    SuperClass sup   new SuperClass      sup getNb      returns 1   sup getNb2      returns 2  Also  note that an  Override tag is not required for implementing an interface  as there is nothing in the original interface methods to be overridden I suggest you do some more research on dynamic binding  polymorphism and in general inheritance in Object-oriented programming

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In the most simple terms extends is used to inherit from a class and implements is used to apply an interface in your class  extends   public class Bicycle         properties and methods   public class MountainBike extends Bicycle         new properties and methods     implements   public interface Relatable         stuff you want to put   public class RectanglePlus implements Relatable         your class code     if you still have confusion read this  https   docs oracle com javase tutorial java IandI subclasses html https   docs oracle com javase tutorial java IandI usinginterface html

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Implements is used for Interfaces and extends is used to extend a class   To make it more clearer in easier terms an interface is like it sound - an interface - a model  that you need to apply follow  along with your ideas to it   Extend is used for classes here you are extending something that already exists by adding more functionality to it   A few more notes   an interface can extend another interface   And when you need to choose between implementing an interface or extending a class for a particular scenario  go for implementing an interface  Because a class can implement multiple interfaces but extend only one class

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An interface is a description of the actions that an object can do    for example when you flip a light switch  the light goes on  you don t care how  just that it does  In Object Oriented Programming  an Interface is a description of all functions that an object must have in order to be an  X   Again  as an example  anything that  ACTS LIKE  a light  should have a turn on   method and a turn off   method  The purpose of interfaces is to allow the computer to enforce these properties and to know that an object of TYPE T  whatever the interface is   must have functions called X Y Z  etc   An interface is a programming structure syntax that allows the computer to enforce certain properties on an object  class   For example  say we have a car class and a scooter class and a truck class  Each of these three classes should have a start engine   action  How the  engine is started  for each vehicle is left to each particular class  but the fact that they must have a start engine action is the domain of the interface

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Extends is used when you want attributes of parent class interface in your child class interface and implements is used when you want attributes of an interface in your class   Example    Extends using class  class Parent      class Child extends Parent     Extends using interface  interface Parent      interface Child extends Parent     Implements   interface A      class B implements A      Combination of extends and implements  interface A      class B        class C implements A extends B

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When a subclass extends a class  it allows the subclass to inherit  reuse  and override code defined in the supertype  When a class implements an interface  it allows an object created from the class to be used in any context that expects a value of the interface   The real catch here is that while we are implementing anything it simply means we are using those methods as it is  There is no scope for change in their values and return types    But when we are extending anything then it becomes an extension of your class  You can change it  use it  reuse use it and it does not necessarily need to return the same values as it does in superclass

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Classes and Interfaces are both contracts  They provide methods and properties other parts of an application relies on   You define an interface when you are not interested in the implementation details of this contract  The only thing to care about is that the contract  the interface  exists   In this case you leave it up to the class which implements the interface to care about the details how the contract is fulfilled  Only classes can implement interfaces   extends is used when you would like to replace details of an existing contract  This way you replace one way to fulfill a contract with a different way  Classes can extend other classes  and interfaces can extend other interfaces

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In Java a class sub class  extends another class super class  and can override the methods defined in the super class  While implements is used when a class seeks to declare the methods defined in the Interface the said class is extending

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Extends   This is used to get attributes of a parent class into base class and may contain already defined methods that can be overridden in the child class   Implements   This is used to implement an interface  parent class with functions signatures only but not their definitions  by defining it in the child class   There is one special condition   What if I want a new Interface to be the child of an existing interface    In the above condition  the child interface extends the parent interface

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A extends B   A and B are both classes or both interfaces A implements B  A is a class and B is an interface The remaining case where A is an interface and B is a class is not legal in Java

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A class can only  implement  an interface  A class only  extends  a class  Likewise  an interface can extend another interface   A class can only extend one other class  A class can implement several interfaces    If instead you are more interested in knowing when to use abstract classes and interfaces  refer to this thread  Interface vs Abstract Class  general OO

[java] Implements vs extends: When to use? What's the difference?

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