State Pattern Trade-offs and When to Use
When to Use State Pattern
State Pattern is appropriate when your object exhibits these characteristics:
First, the object's behavior changes significantly based on its state. Simple property changes do not justify this pattern. Second, you have multiple states (typically 3 or more) with different behavior for the same operations. Third, you find yourself writing large conditional blocks (if-else, switch-case) based on state fields scattered across multiple methods.
Fourth, state transitions follow business logic rules that states themselves understand best. Fifth, you expect to add new states in the future without modifying existing code.
When State Pattern Is Overkill
Avoid State Pattern when:
- Only 2 states exist: A simple boolean flag with if-else is clearer than separate state classes.
- States have identical behavior: If all states handle operations the same way, you just need a status field, not the pattern.
- No behavior variation: If state only affects data (not behavior), use a simple enum or status field.
- Transitions are purely data-driven: If a state machine diagram with just data conditions suffices, use a FSM (Finite State Machine) library instead.
State Pattern vs Strategy Pattern
Who decides: States trigger transitions themselves
Client aware: No, client just calls context methods
Relationship: States know about each other for transitions
Who decides: Client or context chooses strategy explicitly
Client aware: Yes, client picks which strategy to use
Relationship: Strategies are independent, unaware of each other
State Pattern vs Simple Enum
Use Enum when: You only need to track status for queries or display. Example: Order status (PENDING, SHIPPED, DELIVERED) where each status just stores data but does not change how cancel() or refund() behave.
Use State Pattern when: Different states execute operations differently. Example: A vending machine's insertMoney() behaves completely differently in IdleState versus DispensingState. The behavior change is substantial, not just a label change.
Advantages
- Single Responsibility Principle (SRP): Each state class handles one state's behavior
- Open/Closed Principle (OCP): Add new states without modifying existing state classes
- Eliminates conditional complexity: No nested if-else blocks based on state
- Explicit state transitions: Transitions are clear method calls, not buried in conditions
Disadvantages
- Increased class count: Each state requires a separate class, increasing codebase size
- Indirection: Following execution flow requires jumping between context and state classes
- Overkill for simple cases: A two-state system with minimal behavior difference is clearer with if-else
- Shared state complexity: If states need to share complex data, you must carefully design context's API