Visitor Design pattern - Class Diagram

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Anatomy of the Visitor Class Diagram

The Visitor Design Pattern represents an operation to be performed on the elements of an object structure without changing the classes of the elements on which it operates. The UML class diagram breaks this relationship down into five fundamental participants:

• Visitor Interface: Declares a set of visiting methods (e.g., visitConcreteElementA(), visitConcreteElementB()), with each method corresponding to a specific concrete element class.
• ConcreteVisitor: Implements each visit method declared by the Visitor interface to execute a specific algorithm across elements.
• Element Interface: Declares an accept(Visitor visitor) method that takes a visitor interface as an argument.
• ConcreteElement: Implements the accept() method by invoking the corresponding visit method on the visitor, passing itself as a parameter (visitor.visitConcreteElement(this)).
• ObjectStructure: Holds a collection of elements and provides high-level interfaces allowing visitors to traverse the structure.

The Double Dispatch Mechanism

The core dynamic powering the Visitor class diagram is Double Dispatch, which determines the executed operation based on two receiver types:

1. First Dispatch: The client asks an element to accept a visitor: element.accept(visitor). Polymorphism resolves the specific ConcreteElement type at runtime.
2. Second Dispatch: Inside the element's accept() method, it calls back to the visitor: visitor.visit(this). Method overloading and polymorphism resolve the exact ConcreteVisitor and method implementation.

Architectural Benefits and Trade-offs

Understanding the structural design of the Visitor pattern highlights key advantages and considerations:

• Open/Closed Principle: Easily introduce new operations by writing new concrete visitor classes without touching existing element hierarchies.
• Single Responsibility Principle: Gathers related operations into a single class rather than scattering them across various element classes.
• Structure Stability Requirement: While adding new operations (visitors) is straightforward, adding a new element class requires updating the Visitor interface and every existing concrete visitor.

Related Tutorials from Ram N Java

• 👉 Visitor Design Pattern: Introduction and Basics
• 👉 Visitor Design Pattern Implementation: Products Example
• 👉 Visitor Design Pattern Implementation: Doctor's Visit Scenario

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Visitor Design pattern - Class Diagram