🧪🧠 TDD vs BDD & 🤔 Why Interviewers Ask “Which Design Pattern Are You Using?”

Two interview questions that look simple… but silently decide your fate. 😄☕

If you’re a Java / Spring Boot developer and you’ve attended even 2–3 interviews, you’ve definitely heard these questions:

• ❓ “Are you using TDD or BDD?”
• ❓ “What design pattern are you using in your project?”

And suddenly your brain goes like…

🧠 “Wait… we are just writing code da… what pattern??”

Don’t worry. This blog will spoon-feed you the answer 👶🥄 — slowly, clearly, and in an interview-safe way.

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🧪 Question 1: TDD vs BDD – What are you really doing?

🤯 Dumb Question:

“Both are testing… then why two names?”

💡 Brilliant Answer (Baby Explanation):

Think like this 👇

TDD	BDD
Developer talking to code 🧑‍💻➡️💻	Business talking to system 🧑‍💼➡️🖥️

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🔵 TDD – Test Driven Development 🧪

👶 One-line concept:
Write test first ❌, then write code ✔, then clean it 🔄

@Test

void shouldCalculateTotalAmount() {

    OrderService service = new OrderService();

    assertEquals(100, service.calculateTotal());

}

public int calculateTotal() {

    return 100;

}

✔ Developer focused
✔ Unit testing
✔ Clean design

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🟢 BDD – Behavior Driven Development 🎭

👶 One-line concept:
Describe behavior in human language, then automate it

Scenario: Successful order placement

Given user is logged in

When user places an order

Then order should be created

@When("user places an order")

public void placeOrder() {

    orderService.placeOrder();

}

✔ Business friendly
✔ Acceptance testing
✔ Common in Cucumber

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🤔 Question 2: Which Design Pattern are you using?

😨 Dumb Question:

“We didn’t sit and choose pattern… then what to answer?”

💡 One-line truth:
You are already using design patterns — Spring forces you to 😄

👑 Singleton Pattern

@Service

public class PaymentService { }

✔ One instance per container

🔌 Dependency Injection

@Service

public class OrderService {

  private final PaymentService paymentService;

  public OrderService(PaymentService paymentService) {

    this.paymentService = paymentService;

  }

}

🏭 Factory Pattern

🧠 One-line concept:
Factory means – you ask for an object, factory decides which object to give.

😵 Dumb Question:

“Why can’t I just use new keyword?”

👶 Baby Explanation:

Think like this 🍼👇

👶 Baby: “I want milk”
🏭 Factory (Mom): decides cow milk or formula

Baby doesn’t care HOW milk is made. Baby just drinks 😄

☕ Normal Java (Without Factory) ❌

PaymentService service = new CreditCardPaymentService();

❌ Tight coupling
❌ Code change needed everywhere

☕ Factory Pattern (Manual Way) ✅

public class PaymentFactory {

    public static PaymentService getPayment(String type) {

        if ("CARD".equals(type)) {

            return new CreditCardPaymentService();

        } else if ("UPI".equals(type)) {

            return new UpiPaymentService();

        }

        throw new IllegalArgumentException("Invalid type");

    }

}

PaymentService service = PaymentFactory.getPayment("CARD");

✔ Loose coupling
✔ Centralized object creation

🤯 How Spring does Factory internally

👉 ApplicationContext is a BIG factory

PaymentService service = context.getBean(PaymentService.class);

Spring decides:

• Which implementation
• When to create
• How many instances

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🧩 Strategy Pattern

🧠 One-line concept:
Same job, different strategies — choose at runtime.

👶 Baby Explanation:

🍼 Baby wants to go to school:

• Bus 🚌
• Auto 🛺
• Cycle 🚲

Destination same, travel strategy different.

☕ Java Example

interface PaymentStrategy {

    void pay(int amount);

}

class CardPayment implements PaymentStrategy {

    public void pay(int amount) { }

}

class UpiPayment implements PaymentStrategy {

    public void pay(int amount) { }

}

Spring usage: choose implementation using @Qualifier

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🧱 Template Method Pattern

🧠 One-line concept:
Fix the steps, allow subclasses to change details.

👶 Baby Explanation:

🍼 Making tea ☕:

1. Boil water
2. Add base ingredient
3. Add extras

Steps same, ingredients change.

☕ Java Example

abstract class DataProcessor {

    public final void process() {

        read();

        validate();

        save();

    }

    abstract void read();

}

Spring usage: JdbcTemplate, RestTemplate

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👀 Observer Pattern

🧠 One-line concept:
One change, many listeners notified.

👶 Baby Explanation:

🍼 Baby cries 😭 → Mom 👩, Dad 👨, Grandma 👵 all react.

☕ Java Example

@EventListener

public void handleOrderEvent(OrderCreatedEvent event) {

}

Spring usage: ApplicationEventPublisher, Kafka listeners

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🔄 Question 3: @Transactional – Class Level or Method Level?

🧠 One-line concept:
Always prefer @Transactional at method level.

@Service

@Transactional

public class OrderService { }

❌ All methods transactional
❌ Performance overhead

@Transactional

public void placeOrder() { }

✔ Fine-grained control
✔ Better performance

⚠️ Interview trap: Self-invocation bypasses Spring proxy

public void methodA() {

  methodB(); // Transaction won't work

}

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✨ Wrapping Up

❌ Interviews don’t test memory
✅ They test understanding

If this blog helped you, share it with someone who still fears interviews 😄💪

Happy coding & happy interviewing! 🚀👨‍💻