13. State Pattern: Order Lifecycle Management & Finite State Machines (FSM) • Low-Level Design (LLD) & Object-Oriented Machine Coding • Rishabh Ki Kaksha

Jab object ka behavior uske internal state ke sath badalta hai:
*   *Bad:* 50 nested `if-else` / `switch` statements.
*   *Good:* Har state ko ek independent Class bana do!

```
[ PlacedState ] ➔ [ PaidState ] ➔ [ ShippedState ] ➔ [ DeliveredState ]
       │                │                │
       ▼                ▼                ▼
[ CancelledState ] [ RefundedState ] [ ReturnedState ]
```

---

### 2. State Pattern Implementation for E-Commerce Order

```typescript
interface OrderState {
    payOrder(): void;
    shipOrder(): void;
    cancelOrder(): void;
}

class OrderContext {
    private state: OrderState;

constructor() { this.state = new PlacedState(this); }
    public setState(state: OrderState) { this.state = state; }

public pay() { this.state.payOrder(); }
    public ship() { this.state.shipOrder(); }
    public cancel() { this.state.cancelOrder(); }
}

class PlacedState implements OrderState {
    constructor(private context: OrderContext) {}

payOrder() {
        console.log("✅ Payment successful!");
        this.context.setState(new PaidState(this.context));
    }
    shipOrder() { console.error("❌ Cannot ship unpaid order!"); }
    cancelOrder() {
        console.log("🛑 Order cancelled.");
        this.context.setState(new CancelledState(this.context));
    }
}
```

---

### 3. Master Comparison Matrix

---

### 4. Interview Ke 3 Sawal Is Topic Se

1. **Finite State Machine (FSM) libraries (jaise XState) State pattern se kaise better hain?** -> Declarative JSON state transition schemas aur visual state charts provide karti hain.
2. **Invalid state transitions ko kaise prevent kiya jata hai?** -> Default interface methods me `throw new IllegalStateException("Cannot transition from X to Y")` define karke.
3. **State pattern me State objects ko Singleton kyu banaya ja sakta hai?** -> Agar state classes me koi internal instance state na ho (pure stateless methods), toh single instance reuse kiya ja sakta hai.

---
**Next Topic:** 14. Multi-Threaded Producer-Consumer Architecture with BlockingQueues

Loading lesson content...

Python 3.12

Full IDE

Live Execution Terminal

Click "Execute Live" to run code.
                

Lesson Mastery AI Quiz

AI Doubt Solver

CERTIFICATE OF COMPLETION

Rishabh Gehlot

PHP 8+ & Laravel 11 Master Track