🎬State Pattern

Allow an object to alter its behaviour when its internal state changes — object appears to change class.

📌 Key IdeaEncapsulate each state as a class implementing shared `State` interface. Context holds current state reference and delegates operations — transitions swap state objects. Replaces giant switch(state) methods.

🖼

Visual

🎬 Context delegates to State object

  TCPConnection.context.handle()
       │
       ├─ EstablishedState ── send()
       └─ ClosedState      ── error()

State transitions are explicit — each state class knows legal next moves.

✅

When to Use

Many modes with different behaviour (connection lifecycle, order workflow).
Large conditional chains on an enum / phase field.
States have entry/exit actions — classes clarify behaviour.

🚫

When NOT to Use

Two states total with trivial branches — YAGNI.

🚨

Anti-Pattern (What NOT to Do)

switch on phase in every methodBadjava

void ship(Order o) {
    switch (o.phase) {
        case NEW -> pay(o);
        case PAID -> warehouse.pack(o);
        default -> throw new IllegalStateException();
    }
}
// same switch repeated in cancel(), refund(), ...

Transitions and guards scattered; easy to miss a case.

✅

The Right Way

State object owns behaviourGoodjava

interface OrderState { void pay(Order o); void ship(Order o); }

class PaidState implements OrderState {
    public void ship(Order o) {
        o.setState(new ShippedState());
        warehouse.dispatch(o);
    }
}

Order delegates ship() to currentState; illegal ops become impossible or no-op.

⚖️

Pros & Cons

👍 Pros

Eliminates sprawling switch/if state machines.
Each state file stays small and testable.

👎 Cons

More classes — overhead for tiny machines.
Must keep transition table coherent.

🔀

Common Confusions Cleared Up

State vs Strategy

Strategy swaps algorithms deliberately from outside — usually stable choice. State transitions follow internal rules — object feels like it morphs as lifecycle progresses.

🎤

Interview Q&A

🔮

Likely Follow-up Questions

State pattern vs enum + methods in Java 21?

🧾

Quick Cheat Sheet

IntentBehaviour follows current state object.

vs StrategyLifecycle vs pluggable policy.

KillsGiant switch(phase).

🔗