Chapter 5: Behavioral Design Patterns

Behavioral Design Patterns patterns are categorized into those relying on inheritance, such as the Template Method, which fixes the overall structure or skeleton of an algorithm while allowing subclasses to define specific steps through abstract or primitive operations; and the Interpreter pattern, which uses a class hierarchy to represent the rules of a simple grammar, defining an operation on instances of these classes to interpret sentences in that language. Object behavioral patterns, which rely on composition, introduce several concepts for managing cooperation and coupling: Mediator centralizes the interactions between peer objects (colleagues) to promote loose coupling by ensuring colleagues only communicate through the mediator object; Observer (or publish-subscribe) defines a critical one-to-many relationship where a subject automatically notifies all dependent observers when its state changes; and Chain of Responsibility allows a request to be passed along an open-ended sequence of handler objects until one assumes responsibility, decoupling the request's sender from its implicit receiver. Other patterns focus on encapsulating behavior: Strategy (Policy) defines a family of algorithms, encapsulating each one separately so that clients can dynamically select and interchange the algorithm used; State externalizes state-dependent behavior into separate state objects, allowing a context object to alter its behavior at runtime as its internal state changes; Command encapsulates a request as an object, which supports parameterization, logging, transaction management, and the crucial implementation of undoable operations; the Memento pattern captures and externalizes an object's precise internal state without exposing its private implementation details, allowing for later restoration, often supporting the undo functionality of Command objects; Iterator abstracts the process of accessing and traversing the elements of an aggregate object sequentially, providing a uniform interface that hides the aggregate’s internal representation and supports multiple pending traversals; and Visitor represents an operation to be performed on the elements of an object structure, making it easy to define new operations that depend on concrete classes without having to change those stable structure classes.