What is Composition (Has-A Relation)?
It is one of the fundamental concepts of Object-Oriented Programming. In this concept, we will describe a class that references to one or more objects of other classes as an Instance variable. Here, by using the class name or by creating the object we can access the members of one class inside another class. It enables creating complex types by combining objects of different classes. It means that a class Composite can contain an object of another class Component. This type of relationship is known as Has-A Relation.
composition – diagrammatic representation
In the above figure Classes are represented as boxes with the class name Composite and Component representing Has-A relation between both of them.
class A :
# variables of class A
# methods of class A
...
...
class B :
# by using "obj" we can access member's of class A.
obj = A()
# variables of class B
# methods of class B
...
...
Example :
Python3
class Component: # composite class constructor def __init__(self): print('Component class object created...') # composite class instance method def m1(self): print('Component class m1() method executed...') class Composite: # composite class constructor def __init__(self): # creating object of component class self.obj1 = Component() print('Composite class object also created...') # composite class instance method def m2(self): print('Composite class m2() method executed...') # calling m1() method of component class self.obj1.m1() # creating object of composite class obj2 = Composite() # calling m2() method of composite class obj2.m2() |
Component class object created... Composite class object also created... Composite class m2() method executed... Component class m1() method executed...
Explanation:
- In the above example, we created two classes Composite and Component to show the Has-A Relation among them.
- In the Component class, we have one constructor and an instance method m1().
- Similarly, in Composite class, we have one constructor in which we created an object of Component Class. Whenever we create an object of Composite Class, the object of the Component class is automatically created.
- Now in m2() method of Composite class we are calling m1() method of Component Class using instance variable obj1 in which reference of Component Class is stored.
- Now, whenever we call m2() method of Composite Class, automatically m1() method of Component Class will be called.
Inheritance and Composition in Python
Prerequisite – Classes and Objects in Python
This article will compare and highlight the features of is-a relation and has-a relation in Python.
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