Basic design patterns in Java

 Here are some of the most commonly used design patterns in Java, especially in Spring Boot projects, along with real-world examples:

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✅ 1. Singleton Pattern

Purpose: Ensure only one instance of a class exists.

🧠 Use case: Spring-managed beans (@Component, @Service) by default are Singletons.

@Component
public class AppConfig {
    // only one instance in the Spring context
}

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✅ 2. Factory Pattern

Purpose: Create objects without exposing creation logic.

🧠 Use case: SessionFactory in Hibernate, DocumentBuilderFactory, or creating different NotificationService instances.

public class NotificationFactory {
    public Notification getNotification(String type) {
        if ("email".equals(type)) return new EmailNotification();
        if ("sms".equals(type)) return new SMSNotification();
        return null;
    }
}

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✅ 3. Builder Pattern

Purpose: Build complex objects step by step.

🧠 Use case: Creating ResponseEntity, or building custom DTOs in layers.

User user = User.builder()
               .name("Alice")
               .age(30)
               .email("a@x.com")
               .build();

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✅ 4. Prototype Pattern

Purpose: Create a new object by copying an existing one.

🧠 Use case: Cloning a template configuration, or in Spring when you define @Scope("prototype").

@Component
@Scope("prototype")
public class TemporaryFileHandler { ... }

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✅ 5. Adapter Pattern

Purpose: Allows incompatible interfaces to work together.

🧠 Use case: Integrating third-party APIs in Spring Boot using RestTemplate or WebClient.

public class LegacySystemAdapter implements NewSystemInterface {
    private LegacySystem legacy;

    public String fetchData() {
        return legacy.getDataLegacy(); // adapts method name
    }
}

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✅ 6. Decorator Pattern

Purpose: Add new behavior to objects dynamically.

🧠 Use case: Spring’s OncePerRequestFilter or interceptors decorating HTTP requests/responses.

@Component
public class LoggingFilter extends OncePerRequestFilter {
    protected void doFilterInternal(...) {
        // log request details
        filterChain.doFilter(request, response);
    }
}

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✅ 7. Observer Pattern

Purpose: One object notifies many others of changes.

🧠 Use case: Spring’s event system (ApplicationEventPublisher)

@Component
public class OrderService {
    @Autowired ApplicationEventPublisher publisher;

    public void placeOrder(Order order) {
        // logic...
        publisher.publishEvent(new OrderPlacedEvent(order));
    }
}

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✅ 8. Strategy Pattern

Purpose: Select algorithm at runtime.

🧠 Use case: Payment methods (card, PayPal), sorting algorithms, Spring Security password encoders.

public interface PaymentStrategy {
    void pay(int amount);
}

public class CardPayment implements PaymentStrategy { ... }
public class PayPalPayment implements PaymentStrategy { ... }

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✅ 9. Template Method Pattern

Purpose: Define a skeleton of an algorithm, subclasses fill in the steps.

🧠 Use case: JdbcTemplate, RestTemplate, and AbstractController

public abstract class ReportGenerator {
    public void generateReport() {
        fetchData();
        process();
        export();
    }

    abstract void fetchData();
    abstract void process();
    abstract void export();
}

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✅ 10. Proxy Pattern

Purpose: Control access to another object (add extra functionality like logging, security)

🧠 Use case: Spring AOP, Hibernate lazy loading, transaction proxy

@Around("execution(* com.service.*.*(..))")
public Object log(ProceedingJoinPoint joinPoint) throws Throwable {
    System.out.println("Before method: " + joinPoint.getSignature());
    Object result = joinPoint.proceed();
    System.out.println("After method");
    return result;
}

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🧠 Summary Table

Pattern	Used For	Real-Time Example in Spring
Singleton	One shared instance	@Component, @Service
Factory	Centralized object creation	SessionFactory
Builder	Step-by-step object creation	Lombok @Builder, DTOs
Prototype	New instance every time	@Scope("prototype")
Adapter	Integrating incompatible APIs	REST API wrappers
Decorator	Add extra behavior	Servlet filters, logging
Observer	Publish/subscribe	Spring Events
Strategy	Dynamic behavior selection	Payment services
Template Method	Reusable skeleton methods	JdbcTemplate, RestTemplate
Proxy	Intercept method calls	AOP, Logging, Transactions

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Let me know if you want diagrams or code implementations for any of these!