What is Composite Pattern?

Composite pattern is used to separate abstraction from its implementation so that both can be modified independently.

Composite pattern is used when we need to treat a group of objects and a single object in the same way. Composite pattern composes objects in term of a tree structure to represent part as well as whole hierarchies. This pattern creates a class contains a group of its own objects. This class provides ways to modify its group of the same objects.

UML class diagram

A visualization of the classes and objects participating in this pattern.

Participants

• Component
  • declares the interface for objects in the composition.
  • implements default behavior for the interface common to all classes, as appropriate.
  • declares an interface for accessing and managing its child components.
  • (optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate.
• Leaf
  • represents leaf objects in the composition. A leaf has no children.
  • defines behavior for primitive objects in the composition.
• Composite
  • defines behavior for components having children.
  • stores child components.
  • implements child-related operations in the Component interface.
• Client
  • manipulates objects in the composition through the Component interface.