Mediator Design pattern - Class Diagram

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Anatomy of the Mediator Class Diagram

A UML class diagram illustrates how the Mediator Design Pattern reorganizes complex relationships between multiple interacting components. Instead of a messy web of peer-to-peer references, the structure centers around distinct roles:

• Mediator Interface: Defines the communication contract or operations that colleague objects invoke to coordinate interactions.
• ConcreteMediator: Implements the mediator interface, coordinates multiple colleague instances, and maintains references to them to manage communication flow.
• Colleague (Abstract/Interface): Defines the common base or contract for participating components, holding a reference to the mediator.
• ConcreteColleague: Implements specific business functionality and communicates strictly with its mediator whenever an event occurs.

Structural Relationships in UML

When examining the class diagram for this pattern, notice how connections are structured:

• Association with Mediator: Each ConcreteColleague associates only with the Mediator interface, completely decoupling it from other colleague classes.
• Hub-and-Spoke Aggregation: The ConcreteMediator maintains references to each concrete colleague it directs, centralizing routing rules.
• Replacing Mesh with Star Topology: By funneling all interactions through the mediator, the diagram simplifies an N×(N-1) communication mesh into an easily manageable hub-and-spoke model.

Architectural Advantages for Developers

Understanding this class layout helps you build cleaner systems:

• Independent Subclassing: You can reuse and extend individual colleague classes without having to modify other colleagues.
• Centralized Interaction Policy: Changes to interaction logic and communication behavior are localized entirely within the concrete mediator.
• Improved Unit Testing: Isolating colleagues becomes trivial because they rely only on the mediator interface, which can easily be mocked.

Related Tutorials from Ram N Java

• 👉 Mediator Design Pattern: Key Points
• 👉 Mediator Design Pattern: A Comprehensive Introduction
• 👉 Mediator Design Pattern in Action: Developing a Chat Room