pattern-visitor

To add operations to an existing object structure without modifying classes, use the Visitor pattern to decouple behaviors from the hierarchy. Declare visitor and element interfaces, then centralize subtype-specific logic in concrete visitor classes.

Double dispatch is a mechanism where an element accepts a visitor and passes itself to the visitor's method. This allows the visitor to execute subtype-specific logic, keeping the behavior centralized and separate from the original object structure.

Use the visitor pattern for domain entity hierarchies or UI component trees when you need to frequently add new operations like reporting, exporting, or analytics without changing the existing classes or sprinkling type checks throughout the codebase.

To generate different report formats from a stable class hierarchy, create distinct concrete visitors for each format. Each visitor visits the required entity types and emits the correct output without modifying the underlying domain classes.

A limitation of adding behaviors through the visitor pattern is that it requires a stable class hierarchy. Adding new element subtypes forces updates to all existing visitor interfaces and concrete implementations, making it rigid for frequently changing structures.