What is the difference between association aggregation and composition

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What is the difference between association  aggregation  and composition   Please explain in terms of implementation

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Composition  This is where once you destroy an object  School   another object  Classrooms  which is bound to it would get destroyed too  Both of them can t exist independently    Aggregation  This is sorta the exact opposite of the above  Composition  association where once you kill an object  Company   the other object  Employees  which is bound to it can exist on its own   Association  Composition and Aggregation are the two forms of association

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From  Remo H  Jansen book    Beginning React  Learning TypeScript 2 x - Second Edition       We call association those relationships whose objects have an independent life cycle where there is no ownership of the objects  Let s take a look at an example of a teacher and a student  Multiple students can be associated with a single teacher  and a single student can be associated with multiple teachers  but both have independent life cycles  both can create and delete independently   So  when a teacher leaves the school  we don t need to delete any students  and when a student leaves the school  we don t need to delete any teachers   We call aggregation those relationships whose objects have an independent life cycle  but there is ownership  and child objects cannot belong to another parent object  Let s take an example of a cell phone and a cell phone battery  A single battery can belong to a phone  but if the phone stops working  and we delete it from our database  the phone battery will not be deleted because it may still be functional  So  in aggregation  while there is ownership  objects have their life cycle  We use the term composition to refer to relationships whose objects don t have an independent life cycle  and if the parent object is deleted  all child objects will also be deleted  Let s take an example of the relationship between questions and answers  Single questions can have multiple answers  and answers cannot belong to multiple questions  If we delete questions  answers will automatically be deleted

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It s important to understand why we should even bother with using more than once relationship line  The most obvious reason is to describe parent-child relationship between classes  when parent deleted all its child   s are deleted as a result   but more impotently  we want to distinguish between simple association and composition in order to place implicit restrictions on the visibility and propagation of changes to the related classes  a matter which plays an important role in understanding and reducing system complexity      Association   The most abstract way to describe static relationship between classes is using the Association link  which simply states that there is some kind of a link or a dependency between two classes or more   Weak Association  ClassA may be linked to ClassB in order to show that one of its methods includes parameter of ClassB instance  or returns instance of ClassB    Strong Association  ClassA may also be linked to ClassB in order to show that it holds a reference to ClassB instance      Aggregation  Shared Association    In cases where there   s a part-of relationship between ClassA  whole  and ClassB  part   we can be more specific and use the aggregation link instead of the association link  highlighting that ClassB can also be aggregated by other classes in the application  therefore aggregation is also known as shared association       It   s important to note that the aggregation link doesn   t state in any way that ClassA owns ClassB nor that there   s a parent-child relationship  when parent deleted all its child   s are being deleted as a result  between the two  Actually  quite the opposite  The aggregation link usually used to stress the point that ClassA is not the exclusive container of ClassB  as in fact ClassB has another container      Aggregation v s  Association The association link can replace the aggregation link in every situation  while aggregation cannot replace association in situations where there   s only a    weak link    between the classes  i e  ClassA has method s that contain parameter of ClassB but ClassA doesn   t hold reference to ClassB instance   Martin Fowler suggest that the aggregation link should not be used at all because it has no added value and it disturb consistency  Quoting  Jim Rumbaugh  Think of it as a modeling placebo       Composition  Not-Shared Association    We should be more specific and use the composition link in cases where in addition to the part-of relationship between ClassA and ClassB - there   s a strong lifecycle dependency between the two  meaning that when ClassA is deleted then ClassB is also deleted as a result     The composition link shows that a class  container  whole  has exclusive ownership over other class s  parts   meaning that the container object and its parts constitute a parent-child s relationship    Unlike association and aggregation  when using the composition relationship  the composed class cannot appear as a return type or parameter type of the composite class  Thus  changes to the composed class cannot propagate to the rest of the system  Consequently  usage of composition limits complexity growth as the system grows      Measuring system complexity   System complexity can be measured simply by looking at a UML class diagram and evaluating the association  aggregation  and composition relationship lines  The way to measure complexity is to determine how many classes can be affected by changing a particular class  If class A exposes class B  then any given class that uses class A can theoretically be affected by changes to class B  The sum of the number of potentially affected classes for every class in the system is the total system complexity   You can read more on my blog   http   aviadezra blogspot com 2009 05 uml-association-aggregation-composition html

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I d like to illustrate how the three terms are implemented in Rails  ActiveRecord calls any type of relationship between two models an association  One would not find very often the terms composition and aggregation  when reading documentation or articles  related to ActiveRecord  An association is created by adding one of the association class macros to the body of the class  Some of these macros are belongs to  has one  has many etc    If we want to set up a composition or aggregation  we need to add belongs to to the owned model  also called child  and has one or has many to the owning model  also called parent   Wether we set up composition or aggregation depends on the options we pass to the belongs to call in the child model  Prior to Rails 5  setting up belongs to without any options created an aggregation  the child could exist without a parent  If we wanted a composition  we needed to explicitly declare this by adding the option required  true   class Room  lt  ActiveRecord  Base   belongs to  house  required  true end   In Rails 5 this was changed  Now  declaring a belongs to association creates a composition by default  the child cannot exist without a parent  So the above example can be re-written as   class Room  lt  ApplicationRecord   belongs to  house end   If we want to allow the child object to exist without a parent  we need to declare this explicitly via the option optional  class Product  lt  ApplicationRecord   belongs to  category  optional  true end

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It s amazing how much confusion exists about the distinction between the three relationship concepts association  aggregation and composition    Notice that the terms aggregation and composition have been used in the C   community  probably for some time before they have been defined as special cases of association in UML Class Diagrams   The main problem is the widespread and ongoing misunderstanding  even among expert software developers  that the concept of composition implies a life-cycle dependency between the whole and its parts such that the parts cannot exist without the whole  ignoring the fact that there are also cases of part-whole-associations with non-shareable parts where the parts can be detached from  and survive the destruction of  the whole    As far as I can see  this confusion has two roots    In the C   community  the term  aggregation  was used in the sense of a class defining an attribute for referencing objects of another independent class  see  e g    1    which is the sense of association in UML Class Diagrams  The term  composition  was used for classes that define component objects for their objects  such that on destruction of the composite object  these component objects are being destroyed as well  In UML Class Diagrams  both  aggregation  and  composition  have been defined as special cases of associations representing part-whole relationships  which have been discussed in philosophy for a long time   In their definitions  the distinction between an  aggregation  and a  composition  is based on the fact if it allows sharing a part between two or more wholes  They define  compositions  as having non-shareable  exclusive  parts  while  aggregations  may share their parts  In addition they say something like the following  very often  but not in all cases  compositions come with a life-cycle dependency between the whole and its parts such that the parts cannot exist without the whole    Thus  while UML has put the terms  aggregation  and  composition  in the right context  of part-whole relationships   they have not managed to define them in a clear and unambiguous manner  capturing the intuitions of developers  However  this is not surprising because there are so many different properties  and implementation nuances  these relationships can have  and developers do not agree on how to implement them    See also my extended answer to the SO question of Apr 2009 listed below   And the property that was assumed to define  composition  between OOP objects in the C   community  and this belief is still widely held   the run-time life-cycle dependency between the two related objects  the composite and its component   is not really characteristic for  composition  because we can have such dependencies due to referential integrity also in other types of associations   For instance  the following code pattern for  composition  was proposed in an SO answer   final class Car         private final Engine engine     Car EngineSpecs specs        engine   new Engine specs          void move         engine work            The respondent claimed that it would be characteristic for  composition  that no other class could reference know the component  However  this is certainly not true for all possible cases of  composition   In particular  in the case of a car s engine  the maker of the car  possibly implemented with the help of another class  may have to reference the engine for being able to contact the car s owner whenever there is an issue with it    1  http   www learncpp com cpp-tutorial 103-aggregation   Appendix - Incomplete list of repeatedly asked questions about composition versus aggregation on StackOverflow   Apr 2009   Aggregation versus Composition  closed as primarily opinion-based by   Apr 2009   What is the difference between Composition and Association relationship    May 2009   Difference between association  aggregation and composition  May 2009   What is the difference between composition and aggregation   duplicate   Oct 2009  What is the difference between aggregation  composition and dependency   marked as duplicate   Nov 2010   Association vs  Aggregation  marked as duplicate   Aug 2012   Implementation difference between Aggregation and Composition in Java  Feb 2015   UML - association or aggregation  simple code snippets

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Dependency  references  It means there is no conceptual link between two objects  e g  EnrollmentService object references Student  amp  Course objects  as method parameters or return types  public class EnrollmentService       public void enroll Student s  Course c       Association  has-a  It means there is almost always a link between objects  they are associated   Order object has a Customer object public class Order       private Customer customer    Aggregation  has-a   whole-part  Special kind of association where there is whole-part relation between two objects  they might live without each other though  public class PlayList       private List lt Song gt  songs     OR public class Computer       private Monitor monitor     Note  the trickiest part is to distinguish aggregation from normal association  Honestly  I think this is open to different interpretations  Composition  has-a   whole-part   ownership  Special kind of aggregation  An Apartment is composed of some Rooms  A Room cannot exist without an Apartment  when an apartment is deleted  all associated rooms are deleted as well  public class Apartment      private Room bedroom      public Apartment            bedroom   new Room

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Association Association  represents the relationship between two classes It can be unidirectional one way  or bidirectional two way  for example   unidirectional   Customer places orders   bidirectional   A is married to B B is married to A  Aggregation Aggregation is a kind of association But with specific features Aggregation is the relationship in one larger  quot whole quot  class contains one or more smaller  quot parts quot  classes Conversely  a smaller  quot part quot  class is a part of  quot whole quot  larger class  for example   club has members  A club  quot whole quot   is made up of several club members  quot parts quot   Member have life to outside the club  If the club  quot whole quot   were to die  members  quot parts quot   would not die with it  Because member can belong to multiple clubs  quot whole quot    Composition This is a stronger form of aggregation  quot Whole quot  is responsible for the creation or destruction of its  quot parts quot  For example   A school has departments  In this case school  quot whole quot   were to die  department  quot parts quot   would die with it  Because each part can belong to only one  quot whole quot

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Simple rules      A  owns  B   Composition   B has no meaning or purpose in the system      without A     A  uses  B   Aggregation   B exists independently  conceptually  from A     A  belongs Have  B  Association  And B exists just have a relation     Example 1       A Company is an aggregation of Employees      A Company is a composition of Accounts  When a Company ceases to do      business its Accounts cease to exist but its People continue to exist       Employees have association relationship with each other       Example 2   very simplified      A Text Editor owns a Buffer  composition   A Text Editor uses a File       aggregation   When the Text Editor is closed      the Buffer is destroyed but the File itself is not destroyed

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For two objects  Foo and Bar the relationships can be defined  Association - I have a relationship with an object   Foo uses Bar   public class Foo        void Baz Bar bar                Composition - I own an object and I am responsible for its lifetime  When Foo dies  so does Bar  public class Foo       private Bar bar   new Bar         Aggregation - I have an object which I ve borrowed from someone else   When Foo dies  Bar may live on   public class Foo        private Bar bar       Foo Bar bar            this bar   bar

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https   www linkedin com pulse types-relationships-object-oriented-programming-oop-sarah-el-dawody   Composition  is a  part-of  relationship   for example    engine is part of the car        heart is part of the body        Association  is a    has-a    type relationship  For example  suppose we have two classes then these two classes are said to be    has-a    relationships if both of these entities share each other   s object for some work and at the same time they can exist without each other s dependency or both have their own lifetime     The above example showing an association relationship because of both Employee and Manager class using the object of each other and both their own independent life cycle   Aggregation  is based is on  has-a  relationship and it s is   a special form of association  for example     Student    and    address     Each student must have an address so the relationship between Student class and Address class will be    Has-A    type relationship but vice versa is not true

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Composition  If you remove  whole      part    is also removed automatically       Ownership       Create objects of your existing class inside the new class  This is called composition because the new class is composed of objects of existing classes  Typically use normal member variables  Can use pointer values if the composition class automatically handles allocation deallocation responsible for creation destruction of subclasses      Composition in C      include  lt iostream gt  using namespace std                          Engine Class                     class Engine       int nEngineNumber      public      Engine int nEngineNo        Engine void      Engine  Engine int nEngineNo        cout lt  lt   Engine    Constructor    lt  lt endl    Engine   Engine void        cout lt  lt   Engine    Destructor    lt  lt endl                            Car Class                     class Car       int nCarColorNumber      int nCarModelNumber      Engine objEngine      public      Car  int  int int        Car void      Car  Car int nModelNo int nColorNo  int nEngineNo   nCarModelNumber nModelNo  nCarColorNumber nColorNo  objEngine nEngineNo        cout lt  lt   Car    Constructor    lt  lt endl    Car   Car void        cout lt  lt   Car    Destructor    lt  lt endl      Car     Engine     Figure 1   Composition                           Bus Class                     class Bus       int nBusColorNumber      int nBusModelNumber      Engine  ptrEngine      public      Bus int int int        Bus void      Bus  Bus int nModelNo int nColorNo  int nEngineNo   nBusModelNumber nModelNo  nBusColorNumber nColorNo        ptrEngine   new Engine nEngineNo       cout lt  lt   Bus    Constructor    lt  lt endl    Bus   Bus void        cout lt  lt   Bus    Destructor    lt  lt endl      delete ptrEngine                            Main Function                     int main         freopen   InstallationDump Log    w   stdout       cout lt  lt  --------------- Start Of Program --------------------  lt  lt endl         Composition using simple Engine in a car object               cout lt  lt  ------------- Inside Car Block ------------------  lt  lt endl          Car objCar  1  2 3             cout lt  lt  ------------- Out of Car Block ------------------  lt  lt endl         Composition using pointer of Engine in a Bus object               cout lt  lt  ------------- Inside Bus Block ------------------  lt  lt endl          Bus objBus 11  22 33             cout lt  lt  ------------- Out of Bus Block ------------------  lt  lt endl      cout lt  lt  --------------- End Of Program --------------------  lt  lt endl      fclose  stdout       Output  --------------- Start Of Program -------------------- ------------- Inside Car Block ------------------ Engine    Constructor Car    Constructor Car    Destructor Engine    Destructor ------------- Out of Car Block ------------------ ------------- Inside Bus Block ------------------ Engine    Constructor Bus    Constructor Bus    Destructor Engine    Destructor ------------- Out of Bus Block ------------------ --------------- End Of Program --------------------   Aggregation  If you remove  whole      Part    can exist         No Ownership       An aggregation is a specific type of composition where no ownership between the complex object and the subobjects is implied  When an aggregate is destroyed  the subobjects are not destroyed  Typically use pointer variables reference variable that point to an object that lives outside the scope of the aggregate class Can use reference values that point to an object that lives outside the scope of the aggregate class Not responsible for creating destroying subclasses     Aggregation Code in C      include  lt iostream gt   include  lt string gt  using namespace std                          Teacher Class                     class Teacher       private      string m strName      public      Teacher string strName        Teacher void       string GetName       Teacher  Teacher string strName    m strName strName        cout lt  lt   Teacher    Constructor --- Teacher Name      lt  lt m strName lt  lt endl    Teacher   Teacher void        cout lt  lt   Teacher    Destructor --- Teacher Name      lt  lt m strName lt  lt endl    string Teacher  GetName         return m strName                            Department Class                     class Department       private      Teacher  m pcTeacher      Teacher amp  m refTeacher      public      Department Teacher  pcTeacher  Teacher amp  objTeacher        Department void      Department  Department Teacher  pcTeacher  Teacher amp  objTeacher    m pcTeacher pcTeacher   m refTeacher objTeacher        cout lt  lt   Department    Constructor    lt  lt endl    Department   Department void        cout lt  lt   Department    Destructor    lt  lt endl                            Main Function                     int main         freopen   InstallationDump Log    w   stdout       cout lt  lt  --------------- Start Of Program --------------------  lt  lt endl                   Create a teacher outside the scope of the Department         Teacher objTeacher  Reference Teacher            Teacher  pTeacher   new Teacher  Pointer Teacher       create a teacher                       cout lt  lt  ------------- Inside Block ------------------  lt  lt endl                 Create a department and use the constructor parameter to pass the teacher to it              Department cDept pTeacher objTeacher               Department             Teacher             Figure 2  Aggregation              cDept goes out of scope here and is destroyed         cout lt  lt  ------------- Out of Block ------------------  lt  lt endl             pTeacher still exists here because cDept did not destroy it         delete pTeacher            cout lt  lt  --------------- End Of Program --------------------  lt  lt endl      fclose  stdout       Output   --------------- Start Of Program -------------------- Teacher    Constructor --- Teacher Name    Reference Teacher Teacher    Constructor --- Teacher Name    Pointer Teacher ------------- Inside Block ------------------ Department    Constructor Department    Destructor ------------- Out of Block ------------------ Teacher    Destructor --- Teacher Name    Pointer Teacher Teacher    Destructor --- Teacher Name    Reference Teacher --------------- End Of Program --------------------

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As others said  an association is a relationship between objects  aggregation and composition are types of association   From an implementation point of view  an aggregation is obtained by having a class member by reference  For example  if class A aggregates an object of class B  you ll have something like this  in C      class A       B  amp  element       or B   element       The semantics of aggregation is that when an object A is destroyed  the B object it is storing will still exists  When using composition  you have a stronger relationship  usually by storing the member by value   class A       B element       Here  when an A object is destroyed  the B object it contains will be destroyed too  The easiest way to achieve this is by storing the member by value  but you could also use some smart pointer  or delete the member in the destructor   class A       std  auto ptr lt B gt  element      class A       B   element        A             delete B             The important point is that in a composition  the container object owns the contained one  whereas in aggregation  it references it

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Association is generalized concept of relations  It includes both Composition and Aggregation     Composition mixture  is a way to wrap simple objects or data types into a single unit  Compositions are a critical building block of many basic data structures  Aggregation collection  differs from ordinary composition in that it does not imply ownership  In composition  when the owning object is destroyed  so are the contained objects  In aggregation  this is not necessarily true      Both denotes relationship between object and only differ in their strength    Trick to remember the difference   has A -Aggregation and Own - cOmpositoin     Now let observe the following image      Analogy     Composition  The following picture is image composition i e  using individual images making one image       Aggregation   collection of image in single location        For example  A university owns various departments  and each department has a number of professors  If the university closes  the departments will no longer exist  but the professors in those departments will continue to exist  Therefore  a University can be seen as a composition of departments  whereas departments have an aggregation of professors  In addition  a Professor could work in more than one department  but a department could not be part of more than one university

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I know this question is tagged as C  but the concepts are pretty generic questions like this redirect here  So I am going to provide my point of view here  a bit biased from java point of view where I am more comfortable    When we think of Object-oriented nature we always think of Objects  class  objects blueprints  and the relationship between them  Objects are related and interact with each other via methods  In other words the object of one class may use services methods provided by the object of another class  This kind of relationship is termed as association    Aggregation and Composition are subsets of association meaning they are specific cases of association      In both aggregation and composition object of one class  owns  object of another class  But there is a subtle difference  In Composition the object of class that is owned by the object of it s owning class cannot live on it s own Also called  death relationship    It will always live as a part of it s owning object where as in Aggregation the dependent object is standalone and can exist even if the object of owning class is dead   So in composition if owning object is garbage collected the owned object will also be which is not the case in aggregation    Confused    Composition Example   Consider the example of a Car and an engine that is very specific to that car  meaning it cannot be used in any other car   This type of relationship between Car and SpecificEngine class is called Composition  An object of the Car class cannot exist without an object of SpecificEngine class and object of SpecificEngine has no significance without Car class  To put in simple words Car class solely  owns  the SpecificEngine class   Aggregation Example   Now consider class Car and class Wheel  Car needs a Wheel object to function  Meaning the Car object owns the Wheel object but we cannot say the Wheel object has no significance without the Car Object  It can very well be used in a Bike  Truck or different Cars Object   Summing it up -   To sum it up association is a very generic term used to represent when a class uses the functionalities provided by another class  We say it s composition if one parent class object owns another child class object and that child class object cannot meaningfully exist without the parent class object  If it can then it is called Aggregation   More details here   I am the author of http   opensourceforgeeks blogspot in and have added a link above to the relevant post for more context

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Problem with these answers is they are half the story  they explain that aggregation and composition are forms of association  but they don t say if it is possible for an association to be neither of those    I gather based on some brief readings of many posts on SO and some UML docs that there are 4 main concrete forms of class association     composition  A is-composed-of-a B  B doesn t exist without A  like a room in a home aggregation  A has-a B  B can exist without A  like a student in a classroom dependency  A uses-a B  no lifecycle dependency between A and B  like a method call parameter  return value  or a temporary created during a method call generalization  A is-a B   When a relationship between two entities isn t one of these  it can just be called  an association  in the generic sense of the term  and further described other ways  note  stereotype  etc     My guess is that the  generic association  is intended to be used primarily in two circumstances     when the specifics of a relationship are still being worked out  such relationship in a diagram should be converted as soon as possible to what it actually is will be  one of the other 4    when a relationship doesn t match any of those 4 predetermined by UML  the  generic  association still gives you a way of representing a relationship that is  not one of the other ones   so that you aren t stuck using an incorrect relationship with a note  this is not actually aggregation  it s just that UML doesn t have any other symbol we could use

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From a post by Robert Martin in comp object   Association represents the ability of one instance to send a message to another instance  This is typically implemented with a pointer or reference instance variable  although it might also be implemented as a method argument  or the creation of a local variable      Example       A ----------- gt  B   class A     private      B  itsB       Aggregation       is the typical whole part relationship  This is exactly the same as an association with the exception that instances cannot have cyclic aggregation relationships  i e  a part cannot contain its whole       Example       Node  lt  gt -------- gt  Node   class Node     private      vector lt Node  gt  itsNodes       The fact that this is aggregation means that the instances of Node cannot form a cycle  Thus  this is a Tree of Nodes not a graph of Nodes   Composition       is exactly like Aggregation except that the lifetime of the  part  is controlled by the  whole   This control may be direct or transitive  That is  the  whole  may take direct responsibility for creating or destroying the  part   or it may accept an already created part  and later pass it on to some other whole that assumes responsibility for it      Example       Car  lt   gt -------- gt  Carburetor   class Car     public      virtual  Car    delete itsCarb     private      Carburetor  itsCarb

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In a very simple sentence  Aggregation and Composition are subsets of association    A uses B -  this is an aggregation A needs B -  is composition    Read more here

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I think this link will do your homework  http   ootips org uml-hasa html  To understand the terms I remember an example in my early programming days   If you have a  chess board  object that contains  box  objects that is composition because if the  chess board  is deleted there is no reason for the boxes to exist anymore   If you have a  square  object that have a  color  object and the square gets deleted the  color  object may still exist  that is aggregation  Both of them are associations  the main difference is conceptual

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Association is a relationship between two separate classes and the association can be of any type say one to one  one to may etc  It joins two entirely separate entities   Aggregation is a special form of association which is a unidirectional one way relationship between classes  or entities   for e g  Wallet and Money classes  Wallet has Money but money doesn   t need to have Wallet necessarily so its a one directional relationship  In this relationship both the entries can survive if other one ends  In our example if Wallet class is not present  it does not mean that the Money class cannot exist   Composition is a restricted form of Aggregation in which two entities  or you can say classes  are highly dependent on each other  For e g  Human and Heart  A human needs heart to live and a heart needs a Human body to survive  In other words when the classes  entities  are dependent on each other and their life span are same  if one dies then another one too  then its a composition  Heart class has no sense if Human class is not present

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Association  Aggregation  Composition are about Has a relationship  Aggregation and Composition are subsets of Association  which describe relationship more accurately  Aggregation - independent relationship  An object can be passed and saved inside class via constructor  method  setter    Composition - dependent relationship  An object is created by owner object  Association is an alternative for sybtyping

[oop] What is the difference between association, aggregation and composition?

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