What is the difference between association, aggregation, and composition?
Please explain in terms of implementation.
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 {
private Bar bar;
};
NB: See Fowler's definition - the key is that Bar is semantically related to Foo rather than just a dependency (like an int or string).
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;
}
}
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.
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(The formation of a number of things into a cluster) 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.
Trick to remember the difference :
"Has-A": Aggregation
"Part-Of": comPOsitoin
"Is-a": Inheritance
context
Aggregation
Composition
Life time
objects have their own lifetime and there is no owner
controlled by whole or parent that owns it
Scope
parent objects and child objects are independent
parent object also means the death of its children.
Relationship
Has-a
Part-of
Strength
weak relationship
strong relationship.
Real-life example
Car and Driver
Car and wheels
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.
Dependency (references)
It means there is no conceptual link between two objects. e.g. EnrollmentService object references Student & 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<Song> 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();
}
}
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|----------->|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|<>-------->|Node|
class Node
{
private:
vector<Node*> 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|<#>-------->|Carburetor|
class Car
{
public:
virtual ~Car() {delete itsCarb;}
private:
Carburetor* itsCarb
};
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 & 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<B> 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.
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)
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 "whole" class contains one or more smaller "parts" classes.Conversely, a smaller "part" class is a part of "whole" larger class.
for example:
club has members
A club("whole") is made up of several club members("parts").Member have life to outside the club. If the club("whole") were to die, members("parts") would not die with it. Because member can belong to multiple clubs("whole").
Composition
This is a stronger form of aggregation."Whole" is responsible for the creation or destruction of its "parts"
For example:
A school has departments
In this case school("whole") were to die, department("parts") would die with it.
Because each part can belong to only one "whole".
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
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 <iostream>
using namespace std;
/********************** Engine Class ******************/
class Engine
{
int nEngineNumber;
public:
Engine(int nEngineNo);
~Engine(void);
};
Engine::Engine(int nEngineNo)
{
cout<<" Engine :: Constructor " <<endl;
}
Engine::~Engine(void)
{
cout<<" Engine :: Destructor " <<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<<" Car :: Constructor " <<endl;
}
Car::~Car(void)
{
cout<<" Car :: Destructor " <<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<<" Bus :: Constructor " <<endl;
}
Bus::~Bus(void)
{
cout<<" Bus :: Destructor " <<endl;
delete ptrEngine;
}
/********************** Main Function ******************/
int main()
{
freopen ("InstallationDump.Log", "w", stdout);
cout<<"--------------- Start Of Program --------------------"<<endl;
// Composition using simple Engine in a car object
{
cout<<"------------- Inside Car Block ------------------"<<endl;
Car objCar (1, 2,3);
}
cout<<"------------- Out of Car Block ------------------"<<endl;
// Composition using pointer of Engine in a Bus object
{
cout<<"------------- Inside Bus Block ------------------"<<endl;
Bus objBus(11, 22,33);
}
cout<<"------------- Out of Bus Block ------------------"<<endl;
cout<<"--------------- End Of Program --------------------"<<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 <iostream>
#include <string>
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<<" Teacher :: Constructor --- Teacher Name :: "<<m_strName<<endl;
}
Teacher::~Teacher(void)
{
cout<<" Teacher :: Destructor --- Teacher Name :: "<<m_strName<<endl;
}
string Teacher::GetName()
{
return m_strName;
}
/********************** Department Class ******************/
class Department
{
private:
Teacher *m_pcTeacher;
Teacher& m_refTeacher;
public:
Department(Teacher *pcTeacher, Teacher& objTeacher);
~Department(void);
};
Department::Department(Teacher *pcTeacher, Teacher& objTeacher)
: m_pcTeacher(pcTeacher), m_refTeacher(objTeacher)
{
cout<<" Department :: Constructor " <<endl;
}
Department::~Department(void)
{
cout<<" Department :: Destructor " <<endl;
}
/********************** Main Function ******************/
int main()
{
freopen ("InstallationDump.Log", "w", stdout);
cout<<"--------------- Start Of Program --------------------"<<endl;
{
// Create a teacher outside the scope of the Department
Teacher objTeacher("Reference Teacher");
Teacher *pTeacher = new Teacher("Pointer Teacher"); // create a teacher
{
cout<<"------------- Inside Block ------------------"<<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<<"------------- Out of Block ------------------"<<endl;
// pTeacher still exists here because cDept did not destroy it
delete pTeacher;
}
cout<<"--------------- End Of Program --------------------"<<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 --------------------
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"
Association, Aggregation, Composition
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
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.
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.
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
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.
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.
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.
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.
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 < 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 < 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 < ApplicationRecord
belongs_to :category, optional: true
end
in OOP, classes are related to each other. It means their instances call methods from each other. So, if instances of a class call methods from another class, they are related and generally we model this relationship with ASSOCIATION.
For example in the following code snippet, the Customer class is associated with the Order class. she/he cancels the orders.
class Customer {
private Order[] orders;
public boolean removeCart() {
for (int i = 0 ; i < orders.length ; i++) {
orders[i].cancel();
}
}
}
AGGREGATION means a class has some instances of another class. it's nothing more than association and Martin Fowler suggests not using it. Because when a class is associated with another class it has a reference to that class to invoke the methods on it.
But COMPOSITION is a meaningful subset of association. It means a class is composed of some other classes. For example we have a Student class composed of some other classes like ReportCard. We know that the report card is strongly dependent to the student and if we remove the student from the system, their report card should be removed too.
I'm studying the GoF design patterns, in particular the Facade pattern. I understand its use and implementation, but I have a doubt about its UML model.
The solution proposed by my professor, summarized, is the following:
public class Facade{
private ClassA c1;
private ClassB c2;
private ClassC c3;
public Facade(){
this.c1 = new ClassA;
this.c2 = new ClassB;
this.c3 = new ClassC;
}
public void FacadeMethod(){
...
c1.operationA();
c2.operationB();
c3.operationC();
...
}
}
The UML model proposed is like this:
The Facade Class has an association relationship with the classes ClassA, ClassB, ClassC. However should these be aggregation relationships? The Facade Class has reference c1 to ClassA, c2 to ClassB and c3 to ClassC, so i think it's a "HAS-A" relationship. Any Idea?
Preliminary remark
Many sources tend to use UML-aggregation for graphically representing object-composition. A popular source encouraging this trend is for example wikipedia. This is however not to be recommended.
Your professor is (almost) correct for notation and facade example
Your professor uses object-composition in the code of the facade implementation, and represents this with a navigable association which is correct. Some experts claim that this would be much better to use the dot notation of the association end ownership
Your professor uses object-composition and forward facade calls to objects. This is a valid implementation of a facade:
GoF explicitly states page 187 that "[the facade] delegate client requests to appropriate subsystem objets" which clearly allow object-composition.
While this is not the most commonly used way to implement a facade (often class-methods are used instead), GoF further describes implementation alternatives page 188:
An alternative to subclassing [ of an abstract facade class ] is to configure a facade object with different subsystem objects. To customize the facade, simply replace one or more of its subsystem objects.
Additional arguments
Can you use UML-aggregation for representing object-composition?
In apparence, using UML-aggregation for modelling object-composition does not seem fundamentally wrong: UML does not define aggregation semantic very well and leaves room for interpretation. On page 110 of UML specs it's explained that:
an aggregation is when one instance is used to "group together a set of instances" - but nothing forbids the set to be limited to one member.
"Precise semantics of shared aggregation (aka white diamond) varies by application area and modeler" - so why not (mis)use it for object composition
While this is a valid interpretation, it comes with some flaws:
Many modellers migh misunderstand the aggregation to have a * multiplicity by default, in view of the wording "a set of instances", and sets are not by default singletons. When UML-aggregation is used for object composition the multiplicity of 1 should be made explicit to avoid misunderstandings.
A more carefull reading of page 110 shows that aggregation is in reality meant to model a part-whole relationship. Using it for object-composition in other cases is therefore a misuse of the UML aggregation (not wrong, but not the intent):
a Property has an aggregation property (...); the instance representing the whole group is classified by the owner of the Property, and the instances representing the grouped individuals are classified by the type of the Property.
This interpretation of groups of objects is reinforced page 198, being understood that the main difference between UML composite aggregation and shared aggregation is the ownership of the aggregated items:
A binary Association may represent a composite aggregation (i.e., a whole/part relationship).
Booch, Rumbaugh and Jacobson, the founders of UML, confirm this in their non-normative but much more readable book "The UML User's guide":
(...) aggregation, which represents a “has-a” relationship, meaning
that an object of the whole has objects of the part. Aggregation is
really just a special kind of association and is specified by adorning
a plain association with an unfilled diamond at the whole end.
Taking into account this weak semantic, we can summarize: UML-aggregation may be implemented using object-composition. But not all object-composition implement UML-aggregates. It's not a one-to-one mapping between both concepts.
Should you use aggregation at all?
We can conclude with Martin Fowler's quote out of his excellent book "UML Distilled" in which he analyses the difficulty to explain the difference between aggregation and a normal association, independently of any implementation considerations:
Aggregation is strictly meaningless; as a result I recommend that you
ignore it in your own diagrams. If you see it in other people's
diagrams, you'll need to dig deeper to find out what they mean by it.
Different authors use it for different purpose.
However should these be aggregation relationships?
no, a Facade is not composed of ClassA ClassB and ClassC, e.g. these classes are not parts of Facade
Aggregation is a more specific form of association so the professor's solution is not wrong, but I think yours can be fine too
Can someone give a code example of an association UML relationship (one way arrow ->) that is neither an aggregation nor a composition?
I understand that aggregation and composition are types of associations, but I cannot think of an association relationship that is not an aggregation or a composition.
Can the following code be just an association relationship from A->B, but not aggregation or composition under certain conditions?
import B;
public class A {
private B b;
}
Sure, just by changing what your symbols represent, a Person (A) drives (b) a Car (B), without composing it or aggregating it.
According to Robert Martin:
An 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.
The code:
public class A {
private B b;
}
can represent either association, aggregation or composition. It represents a mere association as long as A doesn't have a "HAS-A" relationship with B. For instance, the following could be an association, but not an aggregation or composition relationship.
public class Vehicle {
private Person owner;
}
It appeared to me this morning an Eureka moment :
Association means itself plus aggregation and composition,
sometimes we see the 3 represented like this :
Association
aggregation
composition
There is another kind of relationship in UML called inheritance or derivation.
So the whole hierarchie of concepts or things is :
relationship : 2 kinds (where relationship means putting things of two or more sets in relation to each others, or even of two things of the same set...inner relationship hence the innerjoin for the thing made of two associated things in SQL)
Inheritance : isA kind
Association : hasA kind
Association : (kind of relationship)
aggregation : kind of association
composition : kind of association
Inheritance kind means top down specialization and bottom up generalization
where both aggregation and composition concepts have a notion of ownership embedded... Which means for a while(temporarily: case aggregation) or during the whole lifecycle (case : composition) elements of one set are or is owned, technically the other has a ref on it, by the compositor or the aggregator.
Said otherwise, with composition the composite never exist outside the context of the compositor and "dies" with it... that is the creation ref is owned by the compositor.
and there is an hidden inheritance relationship between the concepts of aggregation and composition vis à vis Association...
That is, both are a kind of association... hence the representation with indentation, parent child relationship...
But there's a term that was often misleading to me, we call relationships of the association's kind : hasA kind !
but sometimes it makes no sense. With the driver driving a car it is an association that is not of the aggregation nor the composition kind. No ownership, thats the looser kind of assocation between the 3. And in this case we can still say that the car has a driver...
But now take an association of persons. No one has any other one, it makes no sense!
Its the association itself that has refs about the associated persons... its like template tag in vue.js or <> in react (called fragment) ... Potentially, the relationship is materialized without any one of its participants knowing about it... A driver that drive a car no matter what, owner or not, is responsible...
The whole is the association and it forms a reified entity... that has refs on its participants... In legal terms it forms one single legal entity... its the archetype of a mediator while aggregation and composition are prototypes of the oberver pattern...
Why is it not written anywhere like this ? its so fuzzy...
One last word, composition is also misleading, in the spoken language a composition has not this exclusive acception, a floral composition for example... any of the constituant live independently of the others, its an association... but it could always be viewed as an aggregation where the aggregator is outside ! thats what an association is and why, precisely we can say Has a !
I'm just trying to find some examples of Martin Fowler's Domain Model pattern and I can't.
From what I found on the Internet Domain Model is just adding some "logic" methods to classes. For example
public class Income {
private String title;
private String comment;
private String date;
private Double amount;
private Integer category;
public ExIn(String title, String comment, Double amount, Integer category, String date) {
this.title = title;
this.comment = comment;
this.date = date;
this.amount = amount;
this.category = category;
}
public Integer getCategory() {
return category;
}
public void setCategory(Integer category) {
this.category = category;
}
// more getters and setters
// Domain Model part starts
public boolean isPositive()
{
return amount > 0 ? true : false;
}
// more methods like that
}
Did I understand it correctly? If not, I'd be grateful for a little example of Domain Model Pattern usage.
Did I understand it correctly? If not, I'd be grateful for a little
example.
Broadly, yes.
From Martin Fowler, domain model is an object model of the domain that incorporates both behavior and data.
The domain model is frequently opposed to a model where you have specific classes to bear data and some other specific classes to bear behavior/processings.
If I take your Income class, it looks like more as a class that holds properties/data than an domain model with a real behavior.
public boolean isPositive(){
return amount > 0 ? true : false;
}
is a kind of utility function that has no relation with the model.
You could put that in a Math class.
I will try to give you a domain model example and then the version where the model separates data and processing.
Suppose in the requirements of the domain of the application you are modeling, we need to add a bonus for incomes. This bonus may take place in winter for Christmas for example (but why not for other events)
Rather than having a service class to do this processing, we let domain model objects perform the task.
Incomes, a high level object could iterate on Income instances and apply the bonus and we could have a bonus rule class that defines the bonus according to some input values.
I introduce multiple classes since the idea is to allow each objects to collaborate according to their responsibilities.
Incomes :
public class Incomes {
List<Income> incomes = ...
....
public void applyBonus(BonusRule bonusRule){
for (Income income : incomes){
income.applyBonus(bonusRule);
}
}
Income :
public class Income {
private float amount;
...
public void applyBonus(BonusRule bonusRule){
float bonus = bonusRule.compute(this);
amount += bonus;
}
...
}
ChristmasRule :
public class ChristmasBonusRule implements BonusRule {
...
#Override
public float compute(Income income){
float bonus = ...
return bonus;
}
...
}
And finally, we could apply the processing in this way :
void foo(){
// create a domain object that has both behavior and data
Incomes incomes = ...;
// invoke a functional method on the object by passing another domain object
incomes.applyBonus(new ChristmasBonusRule());
}
In a design where you separate data and logic in distinct classes, it could look like more like that :
public class IncomeBonusService {
// stateless : no incomes data inside it
....
public void applyChristmasBonus(List<Income> incomes){
for (Income income : incomes){
// Christmas bonus computation here
float bonus = ...
income.setAmount(bonus + income.getAmount());
}
}
}
And we could apply the processing in this way :
// inject the service
#Autowired
IncomeBonusService incomeBonusService;
void foo(){
// create a domain object that has only data
List<Income> incomes = ...;
// invoke a service method by passing data as parameter
incomeBonusService.applyChristmasBonus(incomes);
}
A model design where the objects have no behavior (only getter/setter) is called Anemic Domain Model.
Big differences between the two ways illustrated by this example :
Domain model :
The objects are meaningful.
Behavioral responsibility finely defined between classes.
So good isolation, testability and maintainability.
For example, adding/removing/unit-testing a BonusRule is easy.
Objects responsible of their state.
Indeed, no need to provide setters as the object can itself update its state after collaborating with other objects.
We could see that in Amount.applyBonus() :
float bonus = bonusRule.compute(this);
amount += bonus;
Anemic Domain Model :
All the logic is in the service class.
So a single place to get the code.
With few lines, it is fine.
But note that this advantage has a certain limit because as the logic becomes big or complex, the best thing is often splitting the logic in multiple service classes.
But whatever the number of Service classes you need, the whole logic is located in the service classes and not somewhere else. Which may ease development norms if we compare it to the domain model where the logic may be exploded in some different "types" of classes.
Necessity to provide getter/setter for domain classes.
The domain is not responsible of its state and its invariant rules either.
So any class that depends on the domain class can "break" its state.
As a side note, some frameworks (for persistence, mapping, serialization, ...) rely by default on getter/setter.
That's why this model, despite its drawbacks, leads in some projects.
Fowler's book that you cite refers to Larman's book for introductory understanding and examples.
Interestingly, Larman's approach to domain modeling doesn't ever add behavior to domain classes.
There is a notion in a domain model that a class is conceptual and is not a software class, but that software classes are based on domain (conceptual) classes. Larman's approach to implementing behavior follows responsibility driven design, and GoF design patterns.
The domain model remains a separate element in the software development process. The good aspect of this way of modeling is you separate the problem from the solution. A domain model is supposed to be true to the problem (capture requirements from the problem domain without addressing implementation details).
Larman presents "operation contracts" as a way to assure behaviors are consistent inside a domain model. Again, contracts are supposed to be independent of a solution (an implementation). Contracts have postconditions that describe a constraint in the domain model after an operation has taken place. An example of a postcondition would be that when a customer completes a purchase in a store, the sale object is associated with each of the items the customer purchased.
The implementation of the business logic should respect the contracts (postconditions) defined for the domain model. Larman's approach with the Controller GRASP pattern as well as other GRASP patterns ends up putting this logic in various classes (usually the domain layer, which is software classes inspired by conceptual classes in the domain model) or Façade (Controller) classes that handle the system operations.
Larman's approach is more complicated than this explanation, but the point is that behavior is never only defined alone in the domain model as methods. Larman says many times that domain (conceptual) classes do not have methods, as they are not software classes.
Fowler's book also refers to another book that he wrote on Analysis Patterns for examples.
The patterns come from various domains, including health care, financial trading, and accounting. Each of the patterns is described both textually and in a simple pre-UML notation (this book was written before the UML had stabilized into a usable form).
None of the examples in that book show software classes, that is with methods defined in a programming language (that I could find).
I know of at least one book where domain models in fields such as molecular biology have been published in UML. Here's an example (note the UML is modified -- sub-type boxes are shown in super-type boxes -- to save space):
The book above does not model behaviors, probably because they really depend on the requirements of the software application. These models capture some business rules, such as:
Each Chemical Formulation must be composed of 2 or more of either Chemical Elements or Chemical Compounds, or both.
But the models in that book are mostly data models.
Googling will find you this huge model for the Biomedical Research Integrated Domain Group (BRIDG). Drilling down to the Molecular Biology sub-domain, and to the class Gene, for example, you'll see it has no behavior, nor do any of the other (conceptual) classes in this domain model.
Are domain models in a programming language or not?
Larman's philosophy clearly shows them as programming language-independent (conceptual as opposed to software classes), as a separate artifact from code, to tie them explicitly to the problem domain (requirements).
On the other hand, you'll find Fowler saying, "I prefer POJO domain models.", which is pretty much saying domain models are defined in code.
Eric Evans' DDD makes the assumption that an important degree of complexity in much of software development comes from the domain, and so a model of such complex domains is essential to managing the complexity. Therefore, Domain Modeling is necessary when domains are complex. DDD suggests using a domain modeling language that is ubiquitous; that is, common to the domain experts and the developers. This would imply that in at least some cases, a domain model would not be defined in a programming language.
There is a related question that might shed some light (although it has generated a lot of heat). Some have criticized the question's example as being too trivial (not complex enough) for a justified domain model.
A "domain model" is simply an object which represents some discernible concept in your business domain. A "Customer", an "Order", etc. Whatever the business logic is, the tangible entities which make up that logic are the models in your domain.
Some will have lots of business logic (perhaps worth breaking up into other classes), some will have very little (or even none).
The difference between a "domain model" and any other class isn't a construct of the Java language itself, it's mainly a semantic construct of the business logic that you define.