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Composite Design Pattern

Composite design pattern is used to handle primitive and composite objects uniformly.

  • Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
The intent is to compose the object to present part-whole hierarchy and handle both primitive and composite object uniformly.

  • The primitive and composite objects have to be handled differently. To distinguish these objects for manipulation increases the complexity.
  • Come up with a way to manipulate the primitive and composite object uniformly.
That means both the primitive and composite object have to be handled in similar manner.

Where it is applicable?
  • When part-whole hierarchy of object has to be represented.
  • Wherever primitive and composite objects have to be handled in similar manner.
  • Directories and Files in File system are the best example.

This is the UML structure of Composite design pattern-

Composite Design Pattern

Participant classes
  • Component class provides the interface Operation().
  • Concrete classes Leaf and Composite implements the method Operation().
How they work together?
  • Composite has instance of the class Component. It has objects which can be Leaf and Composite as well. It traverses them and does the operation on all the elements it has composed, either primitive or composite.
  • The method Add(), Remove() and GetChild() can be provided in Component or Composite, its transparency vs safety. Providing it in Component class makes it transparent and providing it in Composite class makes it safe.
  • So you can see both the primitive and composite objects are handled in similar manner.

Implementation Code

Lets explore the implementation of Composite design pattern in different programming languages.

C++   Composite Pattern in C++   Example in C++
C#   Composite Pattern in C#   Example in C#
Java   Composite Pattern in Java   Example in Java
Python   Composite Pattern in Python   Example in Python
JavaScript   Composite Pattern in JavaScript   Example in JavaScript
PHP   Composite Pattern in PHP   Example in PHP
Ruby   Composite Pattern in Ruby   Example in Ruby
Swift   Composite Pattern in Swift   Example in Swift
Objective-C   Composite Pattern in Objective-C   Example in Objective-C

Suresh Kumar Srivastava is founder of online learning site and author of popular books "C In Depth", "Data Structures Through C In Depth". He has 18+ years experience in industry and worked on architecture and design of multiple products. This article is from his courses on design patterns.

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