Is there more to an interface than having the correct methods

Question

So lets say I have this interface   public interface IBox      public void setSize int size      public int getSize       public int getArea           and so on     And I have a class that implements it   public class Rectangle implements IBox      private int size       Methods here     If I wanted to use the interface IBox  i can t actually create an instance of it  in the way   public static void main String args          Ibox myBox new Ibox        right  So I d actually have to do this   public static void main String args          Rectangle myBox new Rectangle        If that s true  then the only purpose of interfaces is to make sure that the class which implements an interface has got the correct methods in it as described by an interface  Or is there any other use of interfaces

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What makes interfaces useful is not the fact that  you can change your mind and use a different implementation later and only have to change the one place where the object is created   That s a non-issue   The real point is already in the name  they define an interface that anyone at all can implement to use all code that operates on that interface  The best example is java util Collections which provides all kinds of useful methods that operate exclusively on interfaces  such as sort   or reverse   for List  The point here is that this code can now be used to sort or reverse any class that implements the List interfaces - not just ArrayList and LinkedList  but also classes that you write yourself  which may be implemented in a way the people who wrote java util Collections never imagined   In the same way  you can write code that operates on well-known interfaces  or interfaces you define  and other people can use your code without having to ask you to support their classes   Another common use of interfaces is for Callbacks  For example  java swing table TableCellRenderer  which allows you to influence how a Swing table displays the data in a certain column  You implement that interface  pass an instance to the JTable  and at some point during the rendering of the table  your code will get called to do its stuff

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the only purpose of interfaces is to make sure that the class which implements an interface has got the correct methods in it as described by an interface  Or is there any other use of interfaces    I am updating the answer with new features of interface  which have introduced with java 8 version    From oracle documentation page on summary of interface    An interface declaration can contain    method signatures   default methods  static methods  constant definitions     The only methods that have implementations are default and static methods   Uses of interface    To define a contract  To link unrelated classes with has a capabilities  e g  classes implementing Serializable interface may or may not have any relation between them except implementing that interface To provide interchangeable implementation e g  strategy pattern 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 Organize helper methods in your libraries with static methods   you can keep static methods specific to an interface in the same interface rather than in a separate class    Some related SE questions with respect to difference between abstract class and interface and use cases with working examples   What is the difference between an interface and abstract class   How should I have explained the difference between an Interface and an Abstract class   Have a look at documentation page to understand new features added in java 8   default methods and static methods

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Don t forget that at a later date you can take an existing class  and make it implement IBox  and it will then become available to all your box-aware code   This becomes a bit clearer if interfaces are named -able  e g   public interface Saveable         public interface Printable          etc   Naming schemes don t always work e g  I m not sure Boxable is appropriate here

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you could do  Ibox myBox   new Rectangle      that way you are using this object as Ibox and you don t care that its really Rectangle

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Don t forget that at a later date you can take an existing class  and make it implement IBox  and it will then become available to all your box-aware code   This becomes a bit clearer if interfaces are named -able  e g   public interface Saveable         public interface Printable          etc   Naming schemes don t always work e g  I m not sure Boxable is appropriate here

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I think you understand everything Interfaces do  but you re not yet imagining the situations in which an Interface is useful   If you re instantiating  using and releasing an object all within a narrow scope  for example  within one method call   an Interface doesn t really add anything  Like you noted  the concrete class is known   Where Interfaces are useful is when an object needs to be created one place and returned to a caller that may not care about the implementation details  Let s change your IBox example to an Shape  Now we can have implementations of Shape such as Rectangle  Circle  Triangle  etc   The implementations of the getArea   and getSize   methods will be completely different for each concrete class   Now you can use a factory with a variety of createShape params  methods which will return an appropriate Shape depending on the params passed in  Obviously  the factory will know about what type of Shape is being created  but the caller won t have to care about whether it s a circle  or a square  or so on    Now  imagine you have a variety of operations you have to perform on your shapes  Maybe you need to sort them by area  set them all to a new size  and then display them in a UI  The Shapes are all created by the factory and then can be passed to the Sorter  Sizer and Display classes very easily  If you need to add a hexagon class some time in the future  you don t have to change anything but the factory  Without the Interface  adding another shape becomes a very messy process

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Interfaces allow statically typed languages to support polymorphism   An Object Oriented purist would insist that a language should provide inheritance  encapsulation  modularity and polymorphism in order to be a fully-featured Object Oriented language   In dynamically-typed - or duck typed - languages  like Smalltalk   polymorphism is trivial  however  in statically typed languages  like Java or C    polymorphism is far from trivial  in fact  on the surface it seems to be at odds with the notion of strong typing    Let me demonstrate   In a dynamically-typed  or duck typed  language  like Smalltalk   all variables are references to objects  nothing less and nothing more    So  in Smalltalk  I can do this    anAnimal      anAnimal    Pig new  anAnimal makeNoise   anAnimal    Cow new  anAnimal makeNoise    That code    Declares a local variable called anAnimal  note that we DO NOT specify the TYPE of the variable - all variables are references to an object  no more and no less   Creates a new instance of the class named  Pig  Assigns that new instance of Pig to the variable anAnimal  Sends the message makeNoise to the pig  Repeats the whole thing using a cow  but assigning it to the same exact variable as the Pig    The same Java code would look something like this  making the assumption that Duck and Cow are subclasses of Animal   Animal anAnimal   new Pig    duck makeNoise     anAnimal   new Cow    cow makeNoise      That s all well and good  until we introduce class Vegetable   Vegetables have some of the same behavior as Animal  but not all   For example  both Animal and Vegetable might be able to grow  but clearly vegetables don t make noise and animals cannot be harvested   In Smalltalk  we can write this    aFarmObject  aFarmObject    Cow new  aFarmObject grow  aFarmObject makeNoise   aFarmObject    Corn new  aFarmObject grow  aFarmObject harvest    This works perfectly well in Smalltalk because it is duck-typed  if it walks like a duck  and quacks like a duck - it is a duck    In this case  when a message is sent to an object  a lookup is performed on the receiver s method list  and if a matching method is found  it is called   If not  some kind of NoSuchMethodError exception is thrown - but it s all done at runtime   But in Java  a statically typed language  what type can we assign to our variable   Corn needs to inherit from Vegetable  to support grow  but cannot inherit from Animal  because it does not make noise   Cow needs to inherit from Animal to support makeNoise  but cannot inherit from Vegetable because it should not implement harvest   It looks like we need multiple inheritance - the ability to inherit from more than one class   But that turns out to be a pretty difficult language feature because of all the edge cases that pop up  what happens when more than one parallel superclass implement the same method   etc    Along come interfaces     If we make Animal and Vegetable classes  with each implementing Growable  we can declare that our Cow is Animal and our Corn is Vegetable   We can also declare that both Animal and Vegetable are Growable   That lets us write this to grow everything   List lt Growable gt  list   new ArrayList lt Growable gt     list add new Cow     list add new Corn     list add new Pig      for Growable g   list       g grow        And it lets us do this  to make animal noises   List lt Animal gt  list   new ArrayList lt Animal gt     list add new Cow     list add new Pig     for Animal a   list      a makeNoise        The advantage to the duck-typed language is that you get really nice polymorphism  all a class has to do to provide behavior is provide the method   As long as everyone plays nice  and only sends messages that match defined methods  all is good   The downside is that the kind of error below isn t caught until runtime    aFarmObject  aFarmObject    Corn new  aFarmObject makeNoise     No compiler error - not checked until runtime    Statically-typed languages provide much better  programming by contract   because they will catch the two kinds of error below at compile-time      Compiler error  Corn cannot be cast to Animal  Animal farmObject   new Corn      farmObject makeNoise      --     Compiler error  Animal doesn t have the harvest message  Animal farmObject   new Cow    farmObject harvest       So    to summarize    Interface implementation allows you to specify what kinds of things objects can do  interaction  and Class inheritance lets you specify how things should be done  implementation   Interfaces give us many of the benefits of  true  polymorphism  without sacrificing compiler type checking

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Normally Interfaces define the interface you should use  as the name says it  -     Sample  public void foo List l           do something    Now your function foo accepts ArrayLists  LinkedLists      not only one type  The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class  Sample   class Test extends Foo implements Comparable  Serializable  Formattable          is possible but  class Test extends Foo  Bar  Buz          is not   Your code above could also be  IBox myBox   new Rectangle     The important thing is now  that myBox ONLY contains the methods fields from IBox and not the  possibly existing  other methods from Rectangle

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What makes interfaces useful is not the fact that  you can change your mind and use a different implementation later and only have to change the one place where the object is created   That s a non-issue   The real point is already in the name  they define an interface that anyone at all can implement to use all code that operates on that interface  The best example is java util Collections which provides all kinds of useful methods that operate exclusively on interfaces  such as sort   or reverse   for List  The point here is that this code can now be used to sort or reverse any class that implements the List interfaces - not just ArrayList and LinkedList  but also classes that you write yourself  which may be implemented in a way the people who wrote java util Collections never imagined   In the same way  you can write code that operates on well-known interfaces  or interfaces you define  and other people can use your code without having to ask you to support their classes   Another common use of interfaces is for Callbacks  For example  java swing table TableCellRenderer  which allows you to influence how a Swing table displays the data in a certain column  You implement that interface  pass an instance to the JTable  and at some point during the rendering of the table  your code will get called to do its stuff

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Interfaces where a fetature added to java to allow multiple inheritance  The developers of Java though realized that having multiple inheritance was a  dangerous  feature  that is why the came up with the idea of an interface   multiple inheritance is dangerous because you might have a class like the following     class Box      public int getSize           return 0            public int getArea           return 1            class Triangle      public int getSize           return 1            public int getArea           return 0            class FunckyFigure extends Box  Triable        we do not implement the methods we will used the inherited ones      Which would be the method that should be called when we use        FunckyFigure GetArea        All the problems are solved with interfaces  because you do know you can extend the interfaces and that they wont have classing methods    ofcourse the compiler is nice and tells you if you did not implemented a methods  but I like to think that is a side effect of a more interesting idea

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The purpose of interfaces is abstraction  or decoupling from implementation   If you introduce an abstraction in your program  you don t care about the possible implementations  You are interested in what it can do and not how  and you use an interface to express this in Java

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Normally Interfaces define the interface you should use  as the name says it  -     Sample  public void foo List l           do something    Now your function foo accepts ArrayLists  LinkedLists      not only one type  The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class  Sample   class Test extends Foo implements Comparable  Serializable  Formattable          is possible but  class Test extends Foo  Bar  Buz          is not   Your code above could also be  IBox myBox   new Rectangle     The important thing is now  that myBox ONLY contains the methods fields from IBox and not the  possibly existing  other methods from Rectangle

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Interfaces are a way to make your code more flexible  What you do is this   Ibox myBox new Rectangle      Then  later  if you decide you want to use a different kind of box  maybe there s another library  with a better kind of box   you switch your code to   Ibox myBox new OtherKindOfBox      Once you get used to it  you ll find it s a great  actually essential  way to work   Another reason is  for example  if you want to create a list of boxes and perform some operation on each one  but you want the list to contain different kinds of boxes  On each box you could do   myBox close      assuming IBox has a close   method  even though the actual class of myBox changes depending on which box you re at in the iteration

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A great example of how interfaces are used is in the Collections framework   If you write a function that takes a List  then it doesn t matter if the user passes in a Vector or an ArrayList or a HashList or whatever   And you can pass that List to any function requiring a Collection or Iterable interface too   This makes functions like Collections sort List list  possible  regardless of how the List is implemented

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If you have CardboardBox and HtmlBox  both of which implement IBox   you can pass both of them to any method that accepts a IBox  Even though they are both very different and not completely interchangable  methods that don t care about  open  or  resize  can still use your classes  perhaps because they care about how many pixels are needed to display something on a screen

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One of the many uses I have read is where its difficult without multiple-inheritance-using-interfaces in Java     class Animal   void walk                    other methods and finally void chew         concentrate on this      Now  Imagine a case where   class Reptile extends Animal       reptile specific code here     not a problem here   but   class Bird extends Animal            other Bird specific code     now Birds cannot chew so this would a problem in the sense Bird classes can also call chew   method which is unwanted   Better design would be   class Animal   void walk                    other methods       Animal does not have the chew   method and instead is put in an interface as     interface Chewable   void chew        and have Reptile class implement this and not Birds  since Birds cannot chew      class Reptile extends Animal implements Chewable        and incase of Birds simply   class Bird extends Animal

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If you have CardboardBox and HtmlBox  both of which implement IBox   you can pass both of them to any method that accepts a IBox  Even though they are both very different and not completely interchangable  methods that don t care about  open  or  resize  can still use your classes  perhaps because they care about how many pixels are needed to display something on a screen

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I think you understand everything Interfaces do  but you re not yet imagining the situations in which an Interface is useful   If you re instantiating  using and releasing an object all within a narrow scope  for example  within one method call   an Interface doesn t really add anything  Like you noted  the concrete class is known   Where Interfaces are useful is when an object needs to be created one place and returned to a caller that may not care about the implementation details  Let s change your IBox example to an Shape  Now we can have implementations of Shape such as Rectangle  Circle  Triangle  etc   The implementations of the getArea   and getSize   methods will be completely different for each concrete class   Now you can use a factory with a variety of createShape params  methods which will return an appropriate Shape depending on the params passed in  Obviously  the factory will know about what type of Shape is being created  but the caller won t have to care about whether it s a circle  or a square  or so on    Now  imagine you have a variety of operations you have to perform on your shapes  Maybe you need to sort them by area  set them all to a new size  and then display them in a UI  The Shapes are all created by the factory and then can be passed to the Sorter  Sizer and Display classes very easily  If you need to add a hexagon class some time in the future  you don t have to change anything but the factory  Without the Interface  adding another shape becomes a very messy process

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The purpose of interfaces is abstraction  or decoupling from implementation   If you introduce an abstraction in your program  you don t care about the possible implementations  You are interested in what it can do and not how  and you use an interface to express this in Java

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The purpose of interfaces is polymorphism  a k a  type substitution  For example  given the following method   public void scale IBox b  int i       b setSize b getSize     i       When calling the scale method  you can provide any value that is of a type that implements the IBox interface  In other words  if Rectangle and Square both implement IBox  you can provide either a Rectangle or a Square wherever an IBox is expected

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The purpose of interfaces is polymorphism  a k a  type substitution  For example  given the following method   public void scale IBox b  int i       b setSize b getSize     i       When calling the scale method  you can provide any value that is of a type that implements the IBox interface  In other words  if Rectangle and Square both implement IBox  you can provide either a Rectangle or a Square wherever an IBox is expected

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I think you understand everything Interfaces do  but you re not yet imagining the situations in which an Interface is useful   If you re instantiating  using and releasing an object all within a narrow scope  for example  within one method call   an Interface doesn t really add anything  Like you noted  the concrete class is known   Where Interfaces are useful is when an object needs to be created one place and returned to a caller that may not care about the implementation details  Let s change your IBox example to an Shape  Now we can have implementations of Shape such as Rectangle  Circle  Triangle  etc   The implementations of the getArea   and getSize   methods will be completely different for each concrete class   Now you can use a factory with a variety of createShape params  methods which will return an appropriate Shape depending on the params passed in  Obviously  the factory will know about what type of Shape is being created  but the caller won t have to care about whether it s a circle  or a square  or so on    Now  imagine you have a variety of operations you have to perform on your shapes  Maybe you need to sort them by area  set them all to a new size  and then display them in a UI  The Shapes are all created by the factory and then can be passed to the Sorter  Sizer and Display classes very easily  If you need to add a hexagon class some time in the future  you don t have to change anything but the factory  Without the Interface  adding another shape becomes a very messy process

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This is the reason why Factory Patterns and other creational patterns are so popular in Java  You are correct that without them Java doesn t provide an out of the box mechanism for easy abstraction of instantiation  Still  you get abstraction everywhere where you don t create an object in your method  which should be most of your code   As an aside  I generally encourage people to not follow the  IRealname  mechanism for naming interfaces  That s a Windows COM thing that puts one foot in the grave of Hungarian notation and really isn t necessary  Java is already strongly typed  and the whole point of having interfaces is to have them as largely indistinguishable from class types as possible

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Interfaces where a fetature added to java to allow multiple inheritance  The developers of Java though realized that having multiple inheritance was a  dangerous  feature  that is why the came up with the idea of an interface   multiple inheritance is dangerous because you might have a class like the following     class Box      public int getSize           return 0            public int getArea           return 1            class Triangle      public int getSize           return 1            public int getArea           return 0            class FunckyFigure extends Box  Triable        we do not implement the methods we will used the inherited ones      Which would be the method that should be called when we use        FunckyFigure GetArea        All the problems are solved with interfaces  because you do know you can extend the interfaces and that they wont have classing methods    ofcourse the compiler is nice and tells you if you did not implemented a methods  but I like to think that is a side effect of a more interesting idea

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WHY INTERFACE        It starts with a dog  In particular  a pug   The pug has various behaviors   public class Pug    private String name  public Pug String n    name   n     public String getName     return name      public String bark     return   Arf       public boolean hasCurlyTail     return true        And you have a Labrador  who also has a set of behaviors   public class Lab    private String name   public Lab String n    name   n     public String getName     return name     public String bark     return  Woof       public boolean hasCurlyTail     return false        We can make some pugs and labs   Pug pug   new Pug  Spot     Lab lab   new Lab  Fido      And we can invoke their behaviors   pug bark   - gt   Arf    lab bark   - gt   Woof    pug hasCurlyTail   - gt  true  lab hasCurlyTail   - gt  false  pug getName   - gt   Spot    Let s say I run a dog kennel and I need to keep track of all the dogs I m housing  I need to store my pugs and labradors in separate arrays   public class Kennel    Pug   pugs   new Pug 10    Lab   labs   new Lab 10     public void addPug Pug p           public void addLab Lab l           public void printDogs        Display names of all the dogs       But this is clearly not optimal  If I want to house some poodles  too  I have to change my Kennel definition to add an array of Poodles  In fact  I need a separate array for each kind of dog   Insight  both pugs and labradors  and poodles  are types of dogs and they have the same set of behaviors  That is  we can say  for the purposes of this example  that all dogs can bark  have a name  and may or may not have a curly tail  We can use an interface to define what all dogs can do  but leave it up to the specific types of dogs to implement those particular behaviors  The interface says  here are the things that all dogs can do  but doesn t say how each behavior is done   public interface Dog    public String bark     public String getName     public boolean hasCurlyTail        Then I slightly alter the Pug and Lab classes to implement the Dog behaviors  We can say that a Pug is a Dog and a Lab is a dog   public class Pug implements Dog      the rest is the same as before     public class Lab implements Dog       the rest is the same as before      I can still instantiate Pugs and Labs as I previously did  but now I also get a new way to do it   Dog d1   new Pug  Spot     Dog d2   new Lab  Fido      This says that d1 is not only a Dog  it s specifically a Pug  And d2 is also a Dog  specifically a Lab  We can invoke the behaviors and they work as before   d1 bark   - gt   Arf    d2 bark   - gt   Woof    d1 hasCurlyTail   - gt  true  d2 hasCurlyTail   - gt  false  d1 getName   - gt   Spot    Here s where all the extra work pays off  The Kennel class become much simpler  I need only one array and one addDog method  Both will work with any object that is a dog  that is  objects that implement the Dog interface   public class Kennel   Dog   dogs   new Dog 20    public void addDog Dog d           public void printDogs        Display names of all the dogs       Here s how to use it   Kennel k   new Kennel     Dog d1   new Pug  Spot     Dog d2   new Lab  Fido     k addDog d1    k addDog d2    k printDogs      The last statement would display  Spot Fido  An interface give you the ability to specify a set of behaviors that all classes that implement the interface will share in common  Consequently  we can define variables and collections  such as arrays  that don t have to know in advance what kind of specific object they will hold  only that they ll hold objects that implement the interface

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The purpose of interfaces is polymorphism  a k a  type substitution  For example  given the following method   public void scale IBox b  int i       b setSize b getSize     i       When calling the scale method  you can provide any value that is of a type that implements the IBox interface  In other words  if Rectangle and Square both implement IBox  you can provide either a Rectangle or a Square wherever an IBox is expected

User · Answer

Normally Interfaces define the interface you should use  as the name says it  -     Sample  public void foo List l           do something    Now your function foo accepts ArrayLists  LinkedLists      not only one type  The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class  Sample   class Test extends Foo implements Comparable  Serializable  Formattable          is possible but  class Test extends Foo  Bar  Buz          is not   Your code above could also be  IBox myBox   new Rectangle     The important thing is now  that myBox ONLY contains the methods fields from IBox and not the  possibly existing  other methods from Rectangle

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This is the reason why Factory Patterns and other creational patterns are so popular in Java  You are correct that without them Java doesn t provide an out of the box mechanism for easy abstraction of instantiation  Still  you get abstraction everywhere where you don t create an object in your method  which should be most of your code   As an aside  I generally encourage people to not follow the  IRealname  mechanism for naming interfaces  That s a Windows COM thing that puts one foot in the grave of Hungarian notation and really isn t necessary  Java is already strongly typed  and the whole point of having interfaces is to have them as largely indistinguishable from class types as possible

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If you have CardboardBox and HtmlBox  both of which implement IBox   you can pass both of them to any method that accepts a IBox  Even though they are both very different and not completely interchangable  methods that don t care about  open  or  resize  can still use your classes  perhaps because they care about how many pixels are needed to display something on a screen

User · Answer

Don t forget that at a later date you can take an existing class  and make it implement IBox  and it will then become available to all your box-aware code   This becomes a bit clearer if interfaces are named -able  e g   public interface Saveable         public interface Printable          etc   Naming schemes don t always work e g  I m not sure Boxable is appropriate here

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Here is my understanding of interface advantage  Correct me if I am wrong  Imagine we are developing OS and other team is developing the drivers for some devices  So we have developed an interface StorageDevice  We have two implementations of it  FDD and HDD  provided by other developers team   Then we have a OperatingSystem class which can call interface methods such as saveData by just passing an instance of class implemented the StorageDevice interface   The advantage here is that we don t care about the implementation of the interface  The other team will do the job by implementing the StorageDevice interface   package mypack   interface StorageDevice       void saveData  String data       class FDD implements StorageDevice       public void saveData  String data            System out println  Save to floppy drive  Data    data            class HDD implements StorageDevice       public void saveData  String data            System out println  Save to hard disk drive  Data    data            class OperatingSystem       public String name      StorageDevice   devices      public OperatingSystem String name  StorageDevice   devices             this name   name          this devices   devices clone             System out println  Running OS     this name           System out println  List with storage devices available             for  StorageDevice s  devices                System out println s                         public void saveSomeDataToStorageDevice  StorageDevice storage  String data            storage saveData data            public class Main        public static void main String   args             StorageDevice fdd0   new FDD            StorageDevice hdd0   new HDD                 StorageDevice   devs    fdd0  hdd0                   OperatingSystem os   new OperatingSystem  Linux   devs           os saveSomeDataToStorageDevice fdd0   blah  blah  blah

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The purpose of interfaces is polymorphism  a k a  type substitution  For example  given the following method   public void scale IBox b  int i       b setSize b getSize     i       When calling the scale method  you can provide any value that is of a type that implements the IBox interface  In other words  if Rectangle and Square both implement IBox  you can provide either a Rectangle or a Square wherever an IBox is expected

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you could do  Ibox myBox   new Rectangle      that way you are using this object as Ibox and you don t care that its really Rectangle

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Interfaces are a way to make your code more flexible  What you do is this   Ibox myBox new Rectangle      Then  later  if you decide you want to use a different kind of box  maybe there s another library  with a better kind of box   you switch your code to   Ibox myBox new OtherKindOfBox      Once you get used to it  you ll find it s a great  actually essential  way to work   Another reason is  for example  if you want to create a list of boxes and perform some operation on each one  but you want the list to contain different kinds of boxes  On each box you could do   myBox close      assuming IBox has a close   method  even though the actual class of myBox changes depending on which box you re at in the iteration

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This is the reason why Factory Patterns and other creational patterns are so popular in Java  You are correct that without them Java doesn t provide an out of the box mechanism for easy abstraction of instantiation  Still  you get abstraction everywhere where you don t create an object in your method  which should be most of your code   As an aside  I generally encourage people to not follow the  IRealname  mechanism for naming interfaces  That s a Windows COM thing that puts one foot in the grave of Hungarian notation and really isn t necessary  Java is already strongly typed  and the whole point of having interfaces is to have them as largely indistinguishable from class types as possible

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One of the many uses I have read is where its difficult without multiple-inheritance-using-interfaces in Java     class Animal   void walk                    other methods and finally void chew         concentrate on this      Now  Imagine a case where   class Reptile extends Animal       reptile specific code here     not a problem here   but   class Bird extends Animal            other Bird specific code     now Birds cannot chew so this would a problem in the sense Bird classes can also call chew   method which is unwanted   Better design would be   class Animal   void walk                    other methods       Animal does not have the chew   method and instead is put in an interface as     interface Chewable   void chew        and have Reptile class implement this and not Birds  since Birds cannot chew      class Reptile extends Animal implements Chewable        and incase of Birds simply   class Bird extends Animal

User · Answer

This is the reason why Factory Patterns and other creational patterns are so popular in Java  You are correct that without them Java doesn t provide an out of the box mechanism for easy abstraction of instantiation  Still  you get abstraction everywhere where you don t create an object in your method  which should be most of your code   As an aside  I generally encourage people to not follow the  IRealname  mechanism for naming interfaces  That s a Windows COM thing that puts one foot in the grave of Hungarian notation and really isn t necessary  Java is already strongly typed  and the whole point of having interfaces is to have them as largely indistinguishable from class types as possible

User · Answer

Don t forget that at a later date you can take an existing class  and make it implement IBox  and it will then become available to all your box-aware code   This becomes a bit clearer if interfaces are named -able  e g   public interface Saveable         public interface Printable          etc   Naming schemes don t always work e g  I m not sure Boxable is appropriate here

User · Answer

you could do  Ibox myBox   new Rectangle      that way you are using this object as Ibox and you don t care that its really Rectangle

User · Answer

Normally Interfaces define the interface you should use  as the name says it  -     Sample  public void foo List l           do something    Now your function foo accepts ArrayLists  LinkedLists      not only one type  The most important thing in Java is that you can implement multiple interfaces but you can only extend ONE class  Sample   class Test extends Foo implements Comparable  Serializable  Formattable          is possible but  class Test extends Foo  Bar  Buz          is not   Your code above could also be  IBox myBox   new Rectangle     The important thing is now  that myBox ONLY contains the methods fields from IBox and not the  possibly existing  other methods from Rectangle

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Interfaces allow statically typed languages to support polymorphism   An Object Oriented purist would insist that a language should provide inheritance  encapsulation  modularity and polymorphism in order to be a fully-featured Object Oriented language   In dynamically-typed - or duck typed - languages  like Smalltalk   polymorphism is trivial  however  in statically typed languages  like Java or C    polymorphism is far from trivial  in fact  on the surface it seems to be at odds with the notion of strong typing    Let me demonstrate   In a dynamically-typed  or duck typed  language  like Smalltalk   all variables are references to objects  nothing less and nothing more    So  in Smalltalk  I can do this    anAnimal      anAnimal    Pig new  anAnimal makeNoise   anAnimal    Cow new  anAnimal makeNoise    That code    Declares a local variable called anAnimal  note that we DO NOT specify the TYPE of the variable - all variables are references to an object  no more and no less   Creates a new instance of the class named  Pig  Assigns that new instance of Pig to the variable anAnimal  Sends the message makeNoise to the pig  Repeats the whole thing using a cow  but assigning it to the same exact variable as the Pig    The same Java code would look something like this  making the assumption that Duck and Cow are subclasses of Animal   Animal anAnimal   new Pig    duck makeNoise     anAnimal   new Cow    cow makeNoise      That s all well and good  until we introduce class Vegetable   Vegetables have some of the same behavior as Animal  but not all   For example  both Animal and Vegetable might be able to grow  but clearly vegetables don t make noise and animals cannot be harvested   In Smalltalk  we can write this    aFarmObject  aFarmObject    Cow new  aFarmObject grow  aFarmObject makeNoise   aFarmObject    Corn new  aFarmObject grow  aFarmObject harvest    This works perfectly well in Smalltalk because it is duck-typed  if it walks like a duck  and quacks like a duck - it is a duck    In this case  when a message is sent to an object  a lookup is performed on the receiver s method list  and if a matching method is found  it is called   If not  some kind of NoSuchMethodError exception is thrown - but it s all done at runtime   But in Java  a statically typed language  what type can we assign to our variable   Corn needs to inherit from Vegetable  to support grow  but cannot inherit from Animal  because it does not make noise   Cow needs to inherit from Animal to support makeNoise  but cannot inherit from Vegetable because it should not implement harvest   It looks like we need multiple inheritance - the ability to inherit from more than one class   But that turns out to be a pretty difficult language feature because of all the edge cases that pop up  what happens when more than one parallel superclass implement the same method   etc    Along come interfaces     If we make Animal and Vegetable classes  with each implementing Growable  we can declare that our Cow is Animal and our Corn is Vegetable   We can also declare that both Animal and Vegetable are Growable   That lets us write this to grow everything   List lt Growable gt  list   new ArrayList lt Growable gt     list add new Cow     list add new Corn     list add new Pig      for Growable g   list       g grow        And it lets us do this  to make animal noises   List lt Animal gt  list   new ArrayList lt Animal gt     list add new Cow     list add new Pig     for Animal a   list      a makeNoise        The advantage to the duck-typed language is that you get really nice polymorphism  all a class has to do to provide behavior is provide the method   As long as everyone plays nice  and only sends messages that match defined methods  all is good   The downside is that the kind of error below isn t caught until runtime    aFarmObject  aFarmObject    Corn new  aFarmObject makeNoise     No compiler error - not checked until runtime    Statically-typed languages provide much better  programming by contract   because they will catch the two kinds of error below at compile-time      Compiler error  Corn cannot be cast to Animal  Animal farmObject   new Corn      farmObject makeNoise      --     Compiler error  Animal doesn t have the harvest message  Animal farmObject   new Cow    farmObject harvest       So    to summarize    Interface implementation allows you to specify what kinds of things objects can do  interaction  and Class inheritance lets you specify how things should be done  implementation   Interfaces give us many of the benefits of  true  polymorphism  without sacrificing compiler type checking

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I think you understand everything Interfaces do  but you re not yet imagining the situations in which an Interface is useful   If you re instantiating  using and releasing an object all within a narrow scope  for example  within one method call   an Interface doesn t really add anything  Like you noted  the concrete class is known   Where Interfaces are useful is when an object needs to be created one place and returned to a caller that may not care about the implementation details  Let s change your IBox example to an Shape  Now we can have implementations of Shape such as Rectangle  Circle  Triangle  etc   The implementations of the getArea   and getSize   methods will be completely different for each concrete class   Now you can use a factory with a variety of createShape params  methods which will return an appropriate Shape depending on the params passed in  Obviously  the factory will know about what type of Shape is being created  but the caller won t have to care about whether it s a circle  or a square  or so on    Now  imagine you have a variety of operations you have to perform on your shapes  Maybe you need to sort them by area  set them all to a new size  and then display them in a UI  The Shapes are all created by the factory and then can be passed to the Sorter  Sizer and Display classes very easily  If you need to add a hexagon class some time in the future  you don t have to change anything but the factory  Without the Interface  adding another shape becomes a very messy process

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Interfaces where a fetature added to java to allow multiple inheritance  The developers of Java though realized that having multiple inheritance was a  dangerous  feature  that is why the came up with the idea of an interface   multiple inheritance is dangerous because you might have a class like the following     class Box      public int getSize           return 0            public int getArea           return 1            class Triangle      public int getSize           return 1            public int getArea           return 0            class FunckyFigure extends Box  Triable        we do not implement the methods we will used the inherited ones      Which would be the method that should be called when we use        FunckyFigure GetArea        All the problems are solved with interfaces  because you do know you can extend the interfaces and that they wont have classing methods    ofcourse the compiler is nice and tells you if you did not implemented a methods  but I like to think that is a side effect of a more interesting idea

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What makes interfaces useful is not the fact that  you can change your mind and use a different implementation later and only have to change the one place where the object is created   That s a non-issue   The real point is already in the name  they define an interface that anyone at all can implement to use all code that operates on that interface  The best example is java util Collections which provides all kinds of useful methods that operate exclusively on interfaces  such as sort   or reverse   for List  The point here is that this code can now be used to sort or reverse any class that implements the List interfaces - not just ArrayList and LinkedList  but also classes that you write yourself  which may be implemented in a way the people who wrote java util Collections never imagined   In the same way  you can write code that operates on well-known interfaces  or interfaces you define  and other people can use your code without having to ask you to support their classes   Another common use of interfaces is for Callbacks  For example  java swing table TableCellRenderer  which allows you to influence how a Swing table displays the data in a certain column  You implement that interface  pass an instance to the JTable  and at some point during the rendering of the table  your code will get called to do its stuff

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The purpose of interfaces is abstraction  or decoupling from implementation   If you introduce an abstraction in your program  you don t care about the possible implementations  You are interested in what it can do and not how  and you use an interface to express this in Java

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A great example of how interfaces are used is in the Collections framework   If you write a function that takes a List  then it doesn t matter if the user passes in a Vector or an ArrayList or a HashList or whatever   And you can pass that List to any function requiring a Collection or Iterable interface too   This makes functions like Collections sort List list  possible  regardless of how the List is implemented

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WHY INTERFACE        It starts with a dog  In particular  a pug   The pug has various behaviors   public class Pug    private String name  public Pug String n    name   n     public String getName     return name      public String bark     return   Arf       public boolean hasCurlyTail     return true        And you have a Labrador  who also has a set of behaviors   public class Lab    private String name   public Lab String n    name   n     public String getName     return name     public String bark     return  Woof       public boolean hasCurlyTail     return false        We can make some pugs and labs   Pug pug   new Pug  Spot     Lab lab   new Lab  Fido      And we can invoke their behaviors   pug bark   - gt   Arf    lab bark   - gt   Woof    pug hasCurlyTail   - gt  true  lab hasCurlyTail   - gt  false  pug getName   - gt   Spot    Let s say I run a dog kennel and I need to keep track of all the dogs I m housing  I need to store my pugs and labradors in separate arrays   public class Kennel    Pug   pugs   new Pug 10    Lab   labs   new Lab 10     public void addPug Pug p           public void addLab Lab l           public void printDogs        Display names of all the dogs       But this is clearly not optimal  If I want to house some poodles  too  I have to change my Kennel definition to add an array of Poodles  In fact  I need a separate array for each kind of dog   Insight  both pugs and labradors  and poodles  are types of dogs and they have the same set of behaviors  That is  we can say  for the purposes of this example  that all dogs can bark  have a name  and may or may not have a curly tail  We can use an interface to define what all dogs can do  but leave it up to the specific types of dogs to implement those particular behaviors  The interface says  here are the things that all dogs can do  but doesn t say how each behavior is done   public interface Dog    public String bark     public String getName     public boolean hasCurlyTail        Then I slightly alter the Pug and Lab classes to implement the Dog behaviors  We can say that a Pug is a Dog and a Lab is a dog   public class Pug implements Dog      the rest is the same as before     public class Lab implements Dog       the rest is the same as before      I can still instantiate Pugs and Labs as I previously did  but now I also get a new way to do it   Dog d1   new Pug  Spot     Dog d2   new Lab  Fido      This says that d1 is not only a Dog  it s specifically a Pug  And d2 is also a Dog  specifically a Lab  We can invoke the behaviors and they work as before   d1 bark   - gt   Arf    d2 bark   - gt   Woof    d1 hasCurlyTail   - gt  true  d2 hasCurlyTail   - gt  false  d1 getName   - gt   Spot    Here s where all the extra work pays off  The Kennel class become much simpler  I need only one array and one addDog method  Both will work with any object that is a dog  that is  objects that implement the Dog interface   public class Kennel   Dog   dogs   new Dog 20    public void addDog Dog d           public void printDogs        Display names of all the dogs       Here s how to use it   Kennel k   new Kennel     Dog d1   new Pug  Spot     Dog d2   new Lab  Fido     k addDog d1    k addDog d2    k printDogs      The last statement would display  Spot Fido  An interface give you the ability to specify a set of behaviors that all classes that implement the interface will share in common  Consequently  we can define variables and collections  such as arrays  that don t have to know in advance what kind of specific object they will hold  only that they ll hold objects that implement the interface

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A great example of how interfaces are used is in the Collections framework   If you write a function that takes a List  then it doesn t matter if the user passes in a Vector or an ArrayList or a HashList or whatever   And you can pass that List to any function requiring a Collection or Iterable interface too   This makes functions like Collections sort List list  possible  regardless of how the List is implemented

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you could do  Ibox myBox   new Rectangle      that way you are using this object as Ibox and you don t care that its really Rectangle

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the only purpose of interfaces is to make sure that the class which implements an interface has got the correct methods in it as described by an interface  Or is there any other use of interfaces    I am updating the answer with new features of interface  which have introduced with java 8 version    From oracle documentation page on summary of interface    An interface declaration can contain    method signatures   default methods  static methods  constant definitions     The only methods that have implementations are default and static methods   Uses of interface    To define a contract  To link unrelated classes with has a capabilities  e g  classes implementing Serializable interface may or may not have any relation between them except implementing that interface To provide interchangeable implementation e g  strategy pattern 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 Organize helper methods in your libraries with static methods   you can keep static methods specific to an interface in the same interface rather than in a separate class    Some related SE questions with respect to difference between abstract class and interface and use cases with working examples   What is the difference between an interface and abstract class   How should I have explained the difference between an Interface and an Abstract class   Have a look at documentation page to understand new features added in java 8   default methods and static methods

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The purpose of interfaces is abstraction  or decoupling from implementation   If you introduce an abstraction in your program  you don t care about the possible implementations  You are interested in what it can do and not how  and you use an interface to express this in Java

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Interfaces are a way to make your code more flexible  What you do is this   Ibox myBox new Rectangle      Then  later  if you decide you want to use a different kind of box  maybe there s another library  with a better kind of box   you switch your code to   Ibox myBox new OtherKindOfBox      Once you get used to it  you ll find it s a great  actually essential  way to work   Another reason is  for example  if you want to create a list of boxes and perform some operation on each one  but you want the list to contain different kinds of boxes  On each box you could do   myBox close      assuming IBox has a close   method  even though the actual class of myBox changes depending on which box you re at in the iteration

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Interfaces allow statically typed languages to support polymorphism   An Object Oriented purist would insist that a language should provide inheritance  encapsulation  modularity and polymorphism in order to be a fully-featured Object Oriented language   In dynamically-typed - or duck typed - languages  like Smalltalk   polymorphism is trivial  however  in statically typed languages  like Java or C    polymorphism is far from trivial  in fact  on the surface it seems to be at odds with the notion of strong typing    Let me demonstrate   In a dynamically-typed  or duck typed  language  like Smalltalk   all variables are references to objects  nothing less and nothing more    So  in Smalltalk  I can do this    anAnimal      anAnimal    Pig new  anAnimal makeNoise   anAnimal    Cow new  anAnimal makeNoise    That code    Declares a local variable called anAnimal  note that we DO NOT specify the TYPE of the variable - all variables are references to an object  no more and no less   Creates a new instance of the class named  Pig  Assigns that new instance of Pig to the variable anAnimal  Sends the message makeNoise to the pig  Repeats the whole thing using a cow  but assigning it to the same exact variable as the Pig    The same Java code would look something like this  making the assumption that Duck and Cow are subclasses of Animal   Animal anAnimal   new Pig    duck makeNoise     anAnimal   new Cow    cow makeNoise      That s all well and good  until we introduce class Vegetable   Vegetables have some of the same behavior as Animal  but not all   For example  both Animal and Vegetable might be able to grow  but clearly vegetables don t make noise and animals cannot be harvested   In Smalltalk  we can write this    aFarmObject  aFarmObject    Cow new  aFarmObject grow  aFarmObject makeNoise   aFarmObject    Corn new  aFarmObject grow  aFarmObject harvest    This works perfectly well in Smalltalk because it is duck-typed  if it walks like a duck  and quacks like a duck - it is a duck    In this case  when a message is sent to an object  a lookup is performed on the receiver s method list  and if a matching method is found  it is called   If not  some kind of NoSuchMethodError exception is thrown - but it s all done at runtime   But in Java  a statically typed language  what type can we assign to our variable   Corn needs to inherit from Vegetable  to support grow  but cannot inherit from Animal  because it does not make noise   Cow needs to inherit from Animal to support makeNoise  but cannot inherit from Vegetable because it should not implement harvest   It looks like we need multiple inheritance - the ability to inherit from more than one class   But that turns out to be a pretty difficult language feature because of all the edge cases that pop up  what happens when more than one parallel superclass implement the same method   etc    Along come interfaces     If we make Animal and Vegetable classes  with each implementing Growable  we can declare that our Cow is Animal and our Corn is Vegetable   We can also declare that both Animal and Vegetable are Growable   That lets us write this to grow everything   List lt Growable gt  list   new ArrayList lt Growable gt     list add new Cow     list add new Corn     list add new Pig      for Growable g   list       g grow        And it lets us do this  to make animal noises   List lt Animal gt  list   new ArrayList lt Animal gt     list add new Cow     list add new Pig     for Animal a   list      a makeNoise        The advantage to the duck-typed language is that you get really nice polymorphism  all a class has to do to provide behavior is provide the method   As long as everyone plays nice  and only sends messages that match defined methods  all is good   The downside is that the kind of error below isn t caught until runtime    aFarmObject  aFarmObject    Corn new  aFarmObject makeNoise     No compiler error - not checked until runtime    Statically-typed languages provide much better  programming by contract   because they will catch the two kinds of error below at compile-time      Compiler error  Corn cannot be cast to Animal  Animal farmObject   new Corn      farmObject makeNoise      --     Compiler error  Animal doesn t have the harvest message  Animal farmObject   new Cow    farmObject harvest       So    to summarize    Interface implementation allows you to specify what kinds of things objects can do  interaction  and Class inheritance lets you specify how things should be done  implementation   Interfaces give us many of the benefits of  true  polymorphism  without sacrificing compiler type checking

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Here is my understanding of interface advantage  Correct me if I am wrong  Imagine we are developing OS and other team is developing the drivers for some devices  So we have developed an interface StorageDevice  We have two implementations of it  FDD and HDD  provided by other developers team   Then we have a OperatingSystem class which can call interface methods such as saveData by just passing an instance of class implemented the StorageDevice interface   The advantage here is that we don t care about the implementation of the interface  The other team will do the job by implementing the StorageDevice interface   package mypack   interface StorageDevice       void saveData  String data       class FDD implements StorageDevice       public void saveData  String data            System out println  Save to floppy drive  Data    data            class HDD implements StorageDevice       public void saveData  String data            System out println  Save to hard disk drive  Data    data            class OperatingSystem       public String name      StorageDevice   devices      public OperatingSystem String name  StorageDevice   devices             this name   name          this devices   devices clone             System out println  Running OS     this name           System out println  List with storage devices available             for  StorageDevice s  devices                System out println s                         public void saveSomeDataToStorageDevice  StorageDevice storage  String data            storage saveData data            public class Main        public static void main String   args             StorageDevice fdd0   new FDD            StorageDevice hdd0   new HDD                 StorageDevice   devs    fdd0  hdd0                   OperatingSystem os   new OperatingSystem  Linux   devs           os saveSomeDataToStorageDevice fdd0   blah  blah  blah

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If you have CardboardBox and HtmlBox  both of which implement IBox   you can pass both of them to any method that accepts a IBox  Even though they are both very different and not completely interchangable  methods that don t care about  open  or  resize  can still use your classes  perhaps because they care about how many pixels are needed to display something on a screen

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Interfaces allow statically typed languages to support polymorphism   An Object Oriented purist would insist that a language should provide inheritance  encapsulation  modularity and polymorphism in order to be a fully-featured Object Oriented language   In dynamically-typed - or duck typed - languages  like Smalltalk   polymorphism is trivial  however  in statically typed languages  like Java or C    polymorphism is far from trivial  in fact  on the surface it seems to be at odds with the notion of strong typing    Let me demonstrate   In a dynamically-typed  or duck typed  language  like Smalltalk   all variables are references to objects  nothing less and nothing more    So  in Smalltalk  I can do this    anAnimal      anAnimal    Pig new  anAnimal makeNoise   anAnimal    Cow new  anAnimal makeNoise    That code    Declares a local variable called anAnimal  note that we DO NOT specify the TYPE of the variable - all variables are references to an object  no more and no less   Creates a new instance of the class named  Pig  Assigns that new instance of Pig to the variable anAnimal  Sends the message makeNoise to the pig  Repeats the whole thing using a cow  but assigning it to the same exact variable as the Pig    The same Java code would look something like this  making the assumption that Duck and Cow are subclasses of Animal   Animal anAnimal   new Pig    duck makeNoise     anAnimal   new Cow    cow makeNoise      That s all well and good  until we introduce class Vegetable   Vegetables have some of the same behavior as Animal  but not all   For example  both Animal and Vegetable might be able to grow  but clearly vegetables don t make noise and animals cannot be harvested   In Smalltalk  we can write this    aFarmObject  aFarmObject    Cow new  aFarmObject grow  aFarmObject makeNoise   aFarmObject    Corn new  aFarmObject grow  aFarmObject harvest    This works perfectly well in Smalltalk because it is duck-typed  if it walks like a duck  and quacks like a duck - it is a duck    In this case  when a message is sent to an object  a lookup is performed on the receiver s method list  and if a matching method is found  it is called   If not  some kind of NoSuchMethodError exception is thrown - but it s all done at runtime   But in Java  a statically typed language  what type can we assign to our variable   Corn needs to inherit from Vegetable  to support grow  but cannot inherit from Animal  because it does not make noise   Cow needs to inherit from Animal to support makeNoise  but cannot inherit from Vegetable because it should not implement harvest   It looks like we need multiple inheritance - the ability to inherit from more than one class   But that turns out to be a pretty difficult language feature because of all the edge cases that pop up  what happens when more than one parallel superclass implement the same method   etc    Along come interfaces     If we make Animal and Vegetable classes  with each implementing Growable  we can declare that our Cow is Animal and our Corn is Vegetable   We can also declare that both Animal and Vegetable are Growable   That lets us write this to grow everything   List lt Growable gt  list   new ArrayList lt Growable gt     list add new Cow     list add new Corn     list add new Pig      for Growable g   list       g grow        And it lets us do this  to make animal noises   List lt Animal gt  list   new ArrayList lt Animal gt     list add new Cow     list add new Pig     for Animal a   list      a makeNoise        The advantage to the duck-typed language is that you get really nice polymorphism  all a class has to do to provide behavior is provide the method   As long as everyone plays nice  and only sends messages that match defined methods  all is good   The downside is that the kind of error below isn t caught until runtime    aFarmObject  aFarmObject    Corn new  aFarmObject makeNoise     No compiler error - not checked until runtime    Statically-typed languages provide much better  programming by contract   because they will catch the two kinds of error below at compile-time      Compiler error  Corn cannot be cast to Animal  Animal farmObject   new Corn      farmObject makeNoise      --     Compiler error  Animal doesn t have the harvest message  Animal farmObject   new Cow    farmObject harvest       So    to summarize    Interface implementation allows you to specify what kinds of things objects can do  interaction  and Class inheritance lets you specify how things should be done  implementation   Interfaces give us many of the benefits of  true  polymorphism  without sacrificing compiler type checking

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A great example of how interfaces are used is in the Collections framework   If you write a function that takes a List  then it doesn t matter if the user passes in a Vector or an ArrayList or a HashList or whatever   And you can pass that List to any function requiring a Collection or Iterable interface too   This makes functions like Collections sort List list  possible  regardless of how the List is implemented

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What makes interfaces useful is not the fact that  you can change your mind and use a different implementation later and only have to change the one place where the object is created   That s a non-issue   The real point is already in the name  they define an interface that anyone at all can implement to use all code that operates on that interface  The best example is java util Collections which provides all kinds of useful methods that operate exclusively on interfaces  such as sort   or reverse   for List  The point here is that this code can now be used to sort or reverse any class that implements the List interfaces - not just ArrayList and LinkedList  but also classes that you write yourself  which may be implemented in a way the people who wrote java util Collections never imagined   In the same way  you can write code that operates on well-known interfaces  or interfaces you define  and other people can use your code without having to ask you to support their classes   Another common use of interfaces is for Callbacks  For example  java swing table TableCellRenderer  which allows you to influence how a Swing table displays the data in a certain column  You implement that interface  pass an instance to the JTable  and at some point during the rendering of the table  your code will get called to do its stuff

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Interfaces are a way to make your code more flexible  What you do is this   Ibox myBox new Rectangle      Then  later  if you decide you want to use a different kind of box  maybe there s another library  with a better kind of box   you switch your code to   Ibox myBox new OtherKindOfBox      Once you get used to it  you ll find it s a great  actually essential  way to work   Another reason is  for example  if you want to create a list of boxes and perform some operation on each one  but you want the list to contain different kinds of boxes  On each box you could do   myBox close      assuming IBox has a close   method  even though the actual class of myBox changes depending on which box you re at in the iteration

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Interfaces where a fetature added to java to allow multiple inheritance  The developers of Java though realized that having multiple inheritance was a  dangerous  feature  that is why the came up with the idea of an interface   multiple inheritance is dangerous because you might have a class like the following     class Box      public int getSize           return 0            public int getArea           return 1            class Triangle      public int getSize           return 1            public int getArea           return 0            class FunckyFigure extends Box  Triable        we do not implement the methods we will used the inherited ones      Which would be the method that should be called when we use        FunckyFigure GetArea        All the problems are solved with interfaces  because you do know you can extend the interfaces and that they wont have classing methods    ofcourse the compiler is nice and tells you if you did not implemented a methods  but I like to think that is a side effect of a more interesting idea

[java] Is there more to an interface than having the correct methods

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