This post gives a brief overview about the Composite Pattern. The post is part of a series about software design patterns and their UML representations with the help of PlantUML.
The article aims at providing a very short description of the general idea of the pattern in the first part. This also involves a descriptive UML diagram. Then, the second part provides the PlantUML code for the diagram so that these posts can also be used as a source of design patterns in PlantUML syntax.
What is the Composite Pattern?
According to Wikipedia, the Composite pattern describes a group of objects that is treated the same way as a single instance of the same type of object. The intent of a composite is to “compose” objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.
What problems can the Composite design pattern solve?
- A part-whole hierarchy should be represented so that clients can treat part and whole objects uniformly.
- A part-whole hierarchy should be represented as tree structure.
When defining (1) Part objects and (2) Whole objects that act as containers for Part objects, clients must treat them separately, which complicates client code.
What solution does the Composite design pattern describe?
- Define a unified Component interface for both part (Leaf) objects and whole (Composite) objects.
- Individual Leaf objects implement the Component interface directly, and Composite objects forward requests to their child components.
This enables clients to work through the Component interface to treat Leaf and Composite objects uniformly: Leaf objects perform a request directly, and Composite objects forward the request to their child components recursively downwards the tree structure. This makes client classes easier to implement, change, test, and reuse.
The following diagram shows the Composite Pattern in UML notation. It is based on the corresponding chapter in the book “Head First Design Patterns“:
@startuml class Client class Component class Leaf class Composite Component : operation() Component : add(Component) Component : remove(Component) Component : getChild(int) Leaf : operation() Composite : operation() Composite : add(Component) Composite : remove(Component) Composite : getChild(int) Client -> Component Component <|-- Leaf Component <|-- Composite Component "0..*" <--o "1" Composite @enduml
Other Design Patterns
In another article you find information about how to put together a single-side web application using PlantUML.