🔍 State vs Strategy

✅ Purpose of Comparison

The State and Strategy patterns both aim to enable interchangeable behaviors, but they serve different design purposes. This comparison clarifies when to use each pattern by examining their intent, structure, and usage in practice.

✅ Comparison Overview

Aspect	State	Strategy
Primary Purpose	Switch behavior based on internal state	Switch behavior via external strategy
Structure	Delegates to state-specific classes	Delegates to strategy-specific classes
Trigger for Change	Internal state transitions	External configuration or injection
Switching Mechanism	Object manages its own state transition	Client selects and injects strategy
State Management	Maintains current state	Stateless, focuses on algorithms

✅ Similarities

• Both patterns support behavioral flexibility without complex conditionals.
• Behavior is delegated via shared interfaces, improving extensibility.
• Used to encapsulate variant logic and reduce code duplication.

✅ Key Differences

Perspective	State Pattern	Strategy Pattern
Ownership	Controlled by the object’s internal logic	Controlled by the external caller
Dynamic Switching	State transitions happen within the object	Strategy changes happen externally
Concept of State	Explicit stateful behavior with transitions	Stateless, context-independent behavior
Client Role	Unaware of internal state handling	Responsible for selecting strategy

✅ When to Choose Which

• ✅ Use State when behavior depends on the object's internal condition
• ✅ Use Strategy when you need to switch algorithms or policies externally
• ✅ Choose State when transitions happen as part of object workflow
• ✅ Choose Strategy when you need configurable or testable behaviors

✅ UML Class Diagram

State Pattern

Strategy Pattern

✅ Practical Design Notes

• ✅ Use State when implementing domain logic that changes over time (e.g., login status, order phases).
• ✅ Use Strategy when separating interchangeable policies or behaviors (e.g., notification methods, authentication).
• ▶️ These patterns are often confused—understand who initiates the change and where state is stored.

✅ Summary

• State encapsulates behavior changes based on internal transitions.
• Strategy encapsulates interchangeable behaviors selected externally.
• While structurally similar, they differ in control flow and behavioral ownership.
• Choosing the correct pattern depends on whether flexibility is required in internal evolution or external configuration.