13 — Behavioral Patterns I

Driving question: How should responsibility move when behavior varies independently from data?

Learning objectives

• Use Strategy, Template Method, State, and Command appropriately.
• Compare composition-based and inheritance-based variation.
• Model state transitions explicitly.
• Recognize conditional complexity that can—but need not—motivate a pattern.

Variation axes

Strategy isolates a family of interchangeable algorithms. Template Method fixes an algorithm skeleton and allows selected steps to vary. State moves state-dependent behavior into state objects. Command reifies an action as an object to enable queuing, undo, logging, retry, or composition.

Conditional logic is not automatically a smell

A small switch over a closed stable enum may be clearer than a hierarchy of tiny classes. Pattern use is justified when the axis of variation, extension frequency, testing needs, or behavioral complexity warrants the structure.

Design / research exercise

Take a conditional with at least four branches. Implement both a straightforward conditional version and a Strategy/State version. Define a change scenario under which each version is superior.

Suggested reading

• GoF: Strategy, Template Method, State, Command.