design-patterns

name	design-patterns
description	Common design patterns with Java examples (Factory, Builder, Strategy, Observer, Decorator, etc.). Use when user asks "implement pattern", "use factory", "strategy pattern", or when designing extensible components.

Design Patterns Skill

Quick reference for common design patterns in Java.

When to Use

• User asks to implement a specific pattern
• Designing extensible/flexible components
• Refactoring rigid code

Quick Reference: When to Use What

Problem	Pattern	Use When
Complex object construction	Builder	Many parameters, some optional
Create objects without specifying class	Factory	Type determined at runtime
Multiple algorithms, swap at runtime	Strategy	Behavior varies by context
Add behavior without changing class	Decorator	Dynamic composition needed
Notify multiple objects of changes	Observer	One-to-many dependency
Convert incompatible interfaces	Adapter	Integrate legacy/3rd party code

---

Creational Patterns

Builder

Problem: Telescoping constructors, many optional parameters

// ✅ Builder pattern
public class User {
    private final String name;  // required
    private final String email; // required
    private final int age;      // optional

    private User(Builder builder) {
        this.name = builder.name;
        this.email = builder.email;
        this.age = builder.age;
    }

    public static Builder builder(String name, String email) {
        return new Builder(name, email);
    }

    public static class Builder {
        private final String name;
        private final String email;
        private int age = 0;

        private Builder(String name, String email) {
            this.name = name;
            this.email = email;
        }

        public Builder age(int age) {
            this.age = age;
            return this;
        }

        public User build() {
            return new User(this);
        }
    }
}

// Usage
User user = User.builder("John", "[e-mail]")
    .age(30)
    .build();

Factory

Problem: Create objects without knowing exact class upfront

// ✅ Factory pattern
public interface Notification {
    void send(String message);
}

public class NotificationFactory {
    public static Notification create(String type) {
        return switch (type.toUpperCase()) {
            case "EMAIL" -> new EmailNotification();
            case "SMS" -> new SmsNotification();
            case "PUSH" -> new PushNotification();
            default -> throw new IllegalArgumentException("Unknown: " + type);
        };
    }
}

// Spring version - preferred
@Component
public class NotificationFactory {
    private final Map<String, NotificationSender> senders;

    public NotificationFactory(List<NotificationSender> senderList) {
        this.senders = senderList.stream()
            .collect(Collectors.toMap(
                NotificationSender::getType,
                Function.identity()
            ));
    }

    public NotificationSender get(String type) {
        return Optional.ofNullable(senders.get(type))
            .orElseThrow(() -> new IllegalArgumentException("Unknown: " + type));
    }
}

---

Behavioral Patterns

Strategy

Problem: Multiple algorithms for same operation, choose at runtime

// ✅ Strategy pattern
public interface PaymentStrategy {
    void pay(BigDecimal amount);
}

public class CreditCardPayment implements PaymentStrategy {
    private final String cardNumber;

    @Override
    public void pay(BigDecimal amount) {
        System.out.println("Paid " + amount + " with card");
    }
}

public class ShoppingCart {
    private PaymentStrategy paymentStrategy;

    public void setPaymentStrategy(PaymentStrategy strategy) {
        this.paymentStrategy = strategy;
    }

    public void checkout(BigDecimal total) {
        paymentStrategy.pay(total);
    }
}

// Usage
cart.setPaymentStrategy(new CreditCardPayment("4111..."));
cart.checkout(new BigDecimal("99.99"));

// Functional variant (Java 8+)
@FunctionalInterface
public interface PaymentStrategy {
    void pay(BigDecimal amount);
}

PaymentStrategy creditCard = amount -> System.out.println("Card: " + amount);
cart.setPaymentStrategy(creditCard);

Observer

Problem: Notify multiple objects when state changes

// ✅ Spring Events (preferred)
public record OrderPlacedEvent(Order order) {}

@Service
public class OrderService {
    private final ApplicationEventPublisher eventPublisher;

    public void placeOrder(Order order) {
        saveOrder(order);
        eventPublisher.publishEvent(new OrderPlacedEvent(order));
    }
}

@Component
public class InventoryListener {
    @EventListener
    public void handleOrderPlaced(OrderPlacedEvent event) {
        // Reduce inventory
    }
}

@Component
public class EmailListener {
    @EventListener
    @Async
    public void handleOrderPlaced(OrderPlacedEvent event) {
        // Send email
    }
}

---

Structural Patterns

Decorator

Problem: Add behavior dynamically without modifying class

// ✅ Decorator pattern
public interface Coffee {
    String getDescription();
    BigDecimal getCost();
}

public class SimpleCoffee implements Coffee {
    public String getDescription() { return "Coffee"; }
    public BigDecimal getCost() { return new BigDecimal("2.00"); }
}

public abstract class CoffeeDecorator implements Coffee {
    protected final Coffee coffee;
    public CoffeeDecorator(Coffee coffee) { this.coffee = coffee; }
}

public class MilkDecorator extends CoffeeDecorator {
    public MilkDecorator(Coffee coffee) { super(coffee); }

    public String getDescription() {
        return coffee.getDescription() + ", Milk";
    }

    public BigDecimal getCost() {
        return coffee.getCost().add(new BigDecimal("0.50"));
    }
}

// Usage
Coffee coffee = new SimpleCoffee();
coffee = new MilkDecorator(coffee);
coffee = new SugarDecorator(coffee);

Adapter

Problem: Make incompatible interfaces work together

// ✅ Adapter pattern
public interface MediaPlayer {
    void play(String filename);
}

// Legacy code
public class LegacyAudioPlayer {
    public void playMp3(String filename) { /* ... */ }
}

// Adapter
public class Mp3PlayerAdapter implements MediaPlayer {
    private final LegacyAudioPlayer legacyPlayer = new LegacyAudioPlayer();

    @Override
    public void play(String filename) {
        legacyPlayer.playMp3(filename);
    }
}

// Usage
MediaPlayer player = new Mp3PlayerAdapter();
player.play("song.mp3");

---

Pattern Selection Guide

Situation	Pattern
Object creation is complex	Builder, Factory
Need to add features dynamically	Decorator
Multiple implementations of algorithm	Strategy
React to state changes	Observer
Integrate with legacy code	Adapter

Anti-Patterns to Avoid

Anti-Pattern	Problem	Better Approach
Singleton abuse	Global state, hard to test	Dependency Injection
Factory everywhere	Over-engineering	Simple new if type known
Deep decorator chains	Hard to debug	Composition, keep chains short