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.
license: Apache-2.0
---

# 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

```java
// ✅ 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", "john@example.com")
    .age(30)
    .build();
```

### Factory
**Problem:** Create objects without knowing exact class upfront

```java
// ✅ 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

```java
// ✅ 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

```java
// ✅ 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

```java
// ✅ 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

```java
// ✅ 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 |