State

Intent

Let an object change behavior when its internal state changes by delegating state-specific behavior to state objects.

Use When

• You have a state machine with state-dependent behavior and complex transition rules.
• Conditionals over state are spreading across methods and becoming hard to maintain.
• You want transitions and per-state behavior to be explicit and testable.

Prefer Something Else When

• State logic is tiny and stable (an enum + switch may be simpler).
• You’re choosing among algorithms independent of internal state (Strategy).

Minimal Structure

• Context holds state: State
• State interface defines operations
• ConcreteStateX implements behavior and may initiate transitions

Implementation Steps

1. List states and valid transitions; define invariants per state.
2. Extract behavior per state into state classes or functions.
3. Decide where transitions live:
4. inside states (states decide next)
5. inside context (central transition rules)
6. Ensure context delegates behavior to the current state.

Pitfalls

• State explosion: too many micro-states; consider grouping or using data-driven tables.
• Hidden transitions: keep transition rules discoverable; log/trace transitions.
• Shared state instances: only safe if states are immutable or hold no per-context data.

Testing Checklist

• Per-state behavior tests.
• Transition tests (valid and invalid transitions).
• Invariant tests: context remains consistent after sequences of actions.