Composition

Describing Composition

• Composition refers to declaring a class within a class
• Composition relates to the composite design pattern
• Specifically, a class known as a composite contains an object of another class known as a component
• Composition is considered as lossely coupled
• Meaning, changes to the component class rarely affect the composite class
• Composition is typically more flexible than inheritance

Comparing Composition, Inheritance, and Aggregation

• In inheritance, class A is a class B
• In composition, class A owns class B
• In aggregation, class A uses class B
• In composition, class B has no meaning or purpose in the system without class A
• In aggregation, class B exists independently from class A

• For example, suppose we have a Company, People, and Account class

  • Company is an aggregation of People
  • Company is a composition of Accounts
  • When a Company ceases to do business, its Accounts cease to exist
  • However, its People continue to exist in this case

References

• Defining Inheritance and Aggregation
• Python Essential Reference
• Comparing Composition and Inheritance in Python