Difference between Inheritance and Composition

Question

Are Composition and Inheritance the same  If I want to implement the composition pattern  how can I do that in Java

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Inheritance  between two classes  where one class extends another class establishes  IS A  relationship    Composition on the other end contains an instance of another class in your class establishes   Has A  relationship  Composition in java is is useful since it technically facilitates multiple inheritance

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They are absolutely different  Inheritance is an  is-a  relationship  Composition is a  has-a    You do composition by having an instance of another class C as a field of your class  instead of extending C  A good example where composition would ve been a lot better than inheritance is java util Stack  which currently extends java util Vector  This is now considered a blunder  A stack  is-NOT-a  vector  you should not be allowed to insert and remove elements arbitrarily  It should ve been composition instead   Unfortunately it s too late to rectify this design mistake  since changing the inheritance hierarchy now would break compatibility with existing code  Had Stack used composition instead of inheritance  it can always be modified to use another data structure without violating the API   I highly recommend Josh Bloch s book Effective Java 2nd Edition   Item 16  Favor composition over inheritance Item 17  Design and document for inheritance or else prohibit it   Good object-oriented design is not about liberally extending existing classes  Your first instinct should be to compose instead     See also    Composition versus Inheritance  A Comparative Look at Two Fundamental Ways to Relate Classes

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Composition is where something is made up of distinct parts and it has a strong relationship with those parts  If the main part dies so do the others  they cannot have a life of their own  A rough example is the human body  Take out the heart and all the other parts die away   Inheritance is where you just take something that already exists and use it  There is no strong relationship  A person could inherit his fathers estate but he can do without it    I don t know Java so I cannot provide an example but I can provide an explanation of the concepts

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Composition is just as it sounds - you create an object by plugging in parts   EDIT the rest of this answer is erroneously based on the following premise  This is accomplished with Interfaces  For example  using the Car example above   Car implements iDrivable  iUsesFuel  iProtectsOccupants Motorbike implements iDrivable  iUsesFuel  iShortcutThroughTraffic House implements iProtectsOccupants Generator implements iUsesFuel   So with a few standard theoretical components you can build up your object    It s then your job to fill in how a House protects its occupants  and how a Car protects its occupants   Inheritance is like the other way around   You start off with a complete  or semi-complete  object and you replace or Override the various bits you want to change   For example  MotorVehicle may come with a Fuelable method and Drive method   You may leave the Fuel method as it is because it s the same to fill up a motorbike and a car  but you may override the Drive method because the Motorbike drives very differently to a Car   With inheritance  some classes are completely implemented already  and others have methods that you are forced to override   With Composition nothing s given to you   but you can Implement the interfaces by calling methods in other classes if you happen to have something laying around    Composition is seen as more flexible  because if you have a method such as iUsesFuel  you can have a method somewhere else  another class  another project  that just worries about dealing with objects that can be fueled  regardless of whether it s a car  boat  stove  barbecue  etc   Interfaces mandate that classes that say they implement that interface actually have the methods that that interface is all about   For example     iFuelable Interface     void AddSomeFuel      void UseSomeFuel      int  percentageFull     then you can have a method somewhere else  private void FillHerUp iFuelable   objectToFill        Do while  objectToFill percentageFull    lt   100              objectToFill AddSomeFuel           Strange example  but it s shows that this method doesn t care what it s filling up  because the object implements iUsesFuel  it can be filled  End of story   If you used Inheritance instead  you would need different FillHerUp methods to deal with MotorVehicles and Barbecues  unless you had some rather weird   ObjectThatUsesFuel  base object from which to inherit

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Composition means HAS A Inheritance means IS A  Example  Car has a Engine and Car is a Automobile  In programming this is represented as   class Engine       The Engine class   class Automobile       Automobile class which is parent to Car class   class Car extends Automobile      Car is an Automobile  so Car class extends Automobile class    private Engine engine     Car has an Engine so  Car class has an instance of Engine class as its member

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Though both Inheritance and Composition provides code reusablility  main difference between Composition and Inheritance in Java is that Composition allows reuse of code without extending it but for Inheritance you must extend the class for any reuse of code or functionality  Another difference which comes from this fact is that by using Composition you can reuse code for even final class which is not extensible but Inheritance cannot reuse code in such cases  Also by using Composition you can reuse code from many classes as they are declared as just a member variable  but with Inheritance you can reuse code form just one class because in Java you can only extend one class  because multiple Inheritance is not supported in Java  You can do this in C   though because there one class can extend more than one class  BTW  You should always prefer Composition over Inheritance in Java  its not just me but even Joshua Bloch has suggested in his book

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as another example  consider a car class  this would be a good use of composition  a car would  have  an engine  a transmission  tires  seats  etc   It would not extend any of those classes

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No   Both are  different   Composition follow  HAS-A  relationship and inheritance follow  IS-A  relationship   Best Example for composition was Strategic pattern

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How inheritance can be dangerous    Lets take an example  public class X         public void do                       Public Class Y extends X     public void work               do                 1  As clear in above code   Class Y has very strong coupling with class X  If anything changes in superclass X   Y may break dramatically  Suppose In future class X implements a method work with below signature  public int work        Change is done in class X but it will make class Y uncompilable  SO this kind of dependency can go up to any level and it can be very dangerous  Every time superclass might not have full visibility to code inside all its subclasses and subclass may be keep noticing what is happening in superclass all the time  So we need to avoid this strong and unnecessary coupling   How does composition solves this issue   Lets see by revising the same example  public class X      public void do             Public Class Y      X x   new X            public void work                x do              Here we are creating reference of X class in Y class and invoking method of X class by creating an instance of X class  Now all that strong coupling is gone  Superclass and subclass are highly independent of each other now  Classes can freely make changes which were dangerous in inheritance situation   2  Second very good advantage of composition in that It provides method calling flexibility  for example    class X implements R    class Y implements R     public class Test          R r          In Test class using r reference I can invoke methods of X class as well as Y class  This flexibility was never there in inheritance  3  Another great advantage   Unit testing  public class X       public void do             Public Class Y       X x   new X            public void work                x do                      In above example  if state of x instance is not known  it can easily be mocked up by using some test data and all methods can be easily tested  This was not possible at all in inheritance as you were heavily dependent on superclass to get the state of instance and execute any method   4  Another good reason why we should avoid inheritance is that Java does not support multiple inheritance   Lets take an example to understand this    Public class Transaction       Banking b      public static void main String a                          b   new Deposit                if b deposit                     b   new Credit                c credit                            Good to know      composition is easily achieved at runtime while inheritance provides its features at                      compile time  composition is also know as HAS-A relation and inheritance is also known as IS-A                     relation   So make it a habit of always preferring composition over inheritance for various above reasons

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The answer given by  Michael Rodrigues is not correct  I apologize  I m not able to comment directly   and could lead to some confusion    Interface implementation is a form of inheritance    when you implement an interface  you re not only inheriting all the constants  you are committing your object to be of the type specified by the interface  it s still an  is-a  relationship  If a car implements Fillable  the car  is-a  Fillable  and can be used in your code wherever you would use a Fillable   Composition is fundamentally different from inheritance  When you use composition  you are  as the other answers note  making a  has-a  relationship between two objects  as opposed to the  is-a  relationship that you make when you use inheritance    So  from the car examples in the other questions  if I wanted to say that a car  has-a  gas tank  I would use composition  as follows   public class Car    private GasTank myCarsGasTank       Hopefully that clears up any misunderstanding

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Inheritances Vs Composition   Inheritances and composition both are used to re-usability and extension of class behavior    Inheritances mainly use in a family algorithm programming model such as IS-A relation type means similar kind of object  Example    Duster is a Car  Safari is a Car   These are belongs to Car family   Composition represents HAS-A relationship Type It shows the ability of an object such as Duster has Five Gears   Safari has four Gears etc  Whenever we need to extend the ability of an existing class then use composition Example we need to add one more gear in Duster object then we have to create one more gear object and compose it to the duster object   We should not make the changes in base class until unless all the derived classes needed those functionality For this scenario we should use Composition Such as  class A Derived by Class B  Class A Derived by Class C  Class A Derived by Class D   When we add any functionality in class A then it is available to all sub classes even when Class C and D don t required those functionality For this scenario we need to create a separate class for those functionality and compose it to the required class here is class B     Below is the example                This is a base class                  public abstract class Car                                                Define prototype                        public abstract void color                           public void Gear                                Console WriteLine  Car has a four Gear                                                                   Here is the use of inheritence               This Desire class have four gears                But we need to add one more gear that is Neutral gear             public class Desire   Car                                             Neutral obj   null                         public Desire                                    Here we are incorporating neutral gear It is the use of composition            Now this class would have five gear                               obj   new Neutral                               obj NeutralGear                                                     public override void color                                                       Console WriteLine  This is a white color car                                                                     This Safari class have four gears and it is not required the neutral                  gear and hence we don t need to compose here                      public class Safari  Car                         public Safari                                                      public override void color                                                       Console WriteLine  This is a red color car                                                           This class represents the neutral gear and it would be used as a composition      public class Neutral                         public void NeutralGear                                Console WriteLine  This is a Neutral Gear

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In Simple Word  Aggregation means Has A Relationship      Composition is a special case of aggregation  In a more specific manner  a restricted aggregation is called composition  When an object contains the other object  if the contained object cannot exist without the existence of container object  then it is called composition   Example  A class contains students  A student cannot exist without a class  There exists composition between class and students    Why Use Aggregation  Code Reusability  When Use Aggregation  Code reuse is also best achieved by aggregation when there is no is a Relation ship  Inheritance  Inheritance is a Parent Child Relationship Inheritance Means Is A RelationShip  Inheritance in java is a mechanism in which one object acquires all the properties and behaviors of parent object   Using inheritance in Java 1 Code Reusability  2 Add Extra Feature in Child Class as well as Method Overriding  so runtime polymorphism can be achieved

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Inheritence means reusing the complete functionality of a class  Here my class have to use all the methods of the super class and my class will be titely coupled with the super class and code will be duplicated in both the classes in case of inheritence   But we can overcome from all these problem when we use composition to talk with another class   composition is declaring an attribute of another class into my class to which we want to talk  and what functionality we want from that class we can get by using that attribute

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Are Composition and Inheritance the same    They are not same    Composition   It enables a group of objects have to be treated in the same way as a single instance of an object  The intent of a composite is to  compose  objects into tree structures to represent part-whole hierarchies  Inheritance  A class inherits fields and methods from all its superclasses  whether direct or indirect  A subclass can override methods that it inherits  or it can hide fields or methods that it inherits      If I want to implement the composition pattern  how can I do that in Java    Wikipedia article is good enough to implement composite pattern in java     Key Participants   Component    Is the abstraction for all components  including composite ones Declares the interface for objects in the composition   Leaf    Represents leaf objects in the composition Implements all Component methods   Composite    Represents a composite Component  component having children  Implements methods to manipulate children Implements all Component methods  generally by delegating them to its children   Code example to understand Composite pattern   import java util List  import java util ArrayList   interface Part      public double getPrice        public String getName      class Engine implements Part      String name      double price      public Engine String name double price           this name   name          this price   price            public double getPrice            return price            public String getName            return name          class Trunk implements Part      String name      double price      public Trunk String name double price           this name   name          this price   price            public double getPrice            return price            public String getName            return name          class Body implements Part      String name      double price      public Body String name double price           this name   name          this price   price            public double getPrice            return price            public String getName            return name          class Car implements Part      List lt Part gt  parts      String name       public Car String name           this name   name          parts   new ArrayList lt Part gt               public void addPart Part part           parts add part             public String getName            return name            public String getPartNames            StringBuilder sb   new StringBuilder            for   Part part  parts               sb append part getName    append                         return sb toString              public double getPrice            double price   0          for   Part part  parts               price    part getPrice                      return price              public class CompositeDemo      public static void main String args             Part engine   new Engine  DiselEngine  15000           Part trunk   new Trunk  Trunk  10000           Part body   new Body  Body  12000            Car car   new Car  Innova            car addPart engine           car addPart trunk           car addPart body            double price   car getPrice             System out println  Car name   car getName             System out println  Car parts   car getPartNames             System out println  Car price   car getPrice                output   Car name Innova Car parts DiselEngine Trunk Body Car price 37000 0   Explanation    Part is a leaf Car contains many Parts Different Parts of the car have been added to Car The price of Car   sum of   Price of each Part     Refer to below question for Pros and Cons of Composition and Inheritance   Prefer composition over inheritance

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Inheritance brings out IS-A relation  Composition brings out HAS-A relation  Strategy pattern explain that Composition should be used in cases where there are families of algorithms defining a particular behaviour Classic example being of a duck class which implements a flying behaviour   public interface Flyable   public void fly       public class Duck    Flyable fly    public Duck      fly   new BackwardFlying           Thus we can have multiple classes which implement flying eg   public class BackwardFlying implements Flyable    public void fly        Systemout println  Flies backward           public class FastFlying implements Flyable    public void fly        Systemout println  Flies 100 miles sec            Had it been for inheritance  we would have two different classes of birds which implement the fly function over and over again  So inheritance and composition are completely different

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I think this example explains clearly the differences between inheritance and composition   In this exmple  the problem is solved using inheritance and composition  The author pays attention to the fact that   in inheritance  a change in superclass might cause problems in derived class  that inherit it   There you can also see the difference in representation when you use a UML for inheritance or composition   http   www javaworld com article 2076814 core-java inheritance-versus-composition--which-one-should-you-choose- html

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Composition means creating an object to a class which has relation with that particular class  Suppose Student has relation with Accounts   An Inheritance is  this is the previous class with the extended feature  That means this new class is the Old class with some extended feature  Suppose Student is Student but All Students are Human  So there is a relationship with student and human  This is Inheritance

[java] Difference between Inheritance and Composition

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