Design Patterns Skill

Practical design patterns reference for Java with modern examples.

When to Use

• User asks to implement a specific pattern
• Designing extensible/flexible components
• Refactoring rigid code structures
• Code review suggests pattern usage

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Quick Reference: When to Use What

Problem	Pattern
Complex object construction	Builder
Create objects without specifying class	Factory
Multiple algorithms, swap at runtime	Strategy
Add behavior without changing class	Decorator
Notify multiple objects of changes	Observer
Ensure single instance	Singleton
Convert incompatible interfaces	Adapter
Define algorithm skeleton	Template Method

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Creational Patterns

Builder

Use when: Object has many parameters, some optional.

// ❌ Telescoping constructor antipattern
public class User {
    public User(String name) { }
    public User(String name, String email) { }
    public User(String name, String email, int age) { }
    public User(String name, String email, int age, String phone) { }
    // ... explosion of constructors
}

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

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

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

    public static class Builder {
        // Required
        private final String name;
        private final String email;
        // Optional with defaults
        private int age = 0;
        private String phone = "";
        private String address = "";

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

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

        public Builder phone(String phone) {
            this.phone = phone;
            return this;
        }

        public Builder address(String address) {
            this.address = address;
            return this;
        }

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

// Usage
User user = User.builder("John", "john@example.com")
    .age(30)
    .phone("+1234567890")
    .build();

With Lombok:

@Builder
@Getter
public class User {
    private final String name;
    private final String email;
    @Builder.Default private int age = 0;
    private String phone;
}

---

Factory Method

Use when: Need to create objects without specifying exact class.

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

public class EmailNotification implements Notification {
    @Override
    public void send(String message) {
        System.out.println("Email: " + message);
    }
}

public class SmsNotification implements Notification {
    @Override
    public void send(String message) {
        System.out.println("SMS: " + message);
    }
}

public class PushNotification implements Notification {
    @Override
    public void send(String message) {
        System.out.println("Push: " + message);
    }
}

// Factory
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: " + type);
        };
    }
}

// Usage
Notification notification = NotificationFactory.create("EMAIL");
notification.send("Hello!");

With Spring (preferred):

public interface NotificationSender {
    void send(String message);
    String getType();
}

@Component
public class EmailSender implements NotificationSender {
    @Override public void send(String message) { /* ... */ }
    @Override public String getType() { return "EMAIL"; }
}

@Component
public class SmsSender implements NotificationSender {
    @Override public void send(String message) { /* ... */ }
    @Override public String getType() { return "SMS"; }
}

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

---

Singleton

Use when: Exactly one instance needed (use sparingly!).

// ✅ Modern singleton (enum-based, thread-safe)
public enum DatabaseConnection {
    INSTANCE;

    private Connection connection;

    DatabaseConnection() {
        // Initialize connection
    }

    public Connection getConnection() {
        return connection;
    }
}

// Usage
Connection conn = DatabaseConnection.INSTANCE.getConnection();

With Spring (preferred):

@Component  // Default scope is singleton
public class DatabaseConnection {
    // Spring manages single instance
}

Warning: Singletons can be problematic:

• Hard to test (global state)
• Hidden dependencies
• Consider dependency injection instead

---

Behavioral Patterns

Strategy

Use when: Multiple algorithms for same operation, need to swap at runtime.

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

public class CreditCardPayment implements PaymentStrategy {
    private final String cardNumber;

    public CreditCardPayment(String cardNumber) {
        this.cardNumber = cardNumber;
    }

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

public class PayPalPayment implements PaymentStrategy {
    private final String email;

    public PayPalPayment(String email) {
        this.email = email;
    }

    @Override
    public void pay(BigDecimal amount) {
        System.out.println("Paid " + amount + " via PayPal: " + email);
    }
}

public class CryptoPayment implements PaymentStrategy {
    private final String walletAddress;

    public CryptoPayment(String walletAddress) {
        this.walletAddress = walletAddress;
    }

    @Override
    public void pay(BigDecimal amount) {
        System.out.println("Paid " + amount + " to wallet: " + walletAddress);
    }
}

// Context
public class ShoppingCart {
    private PaymentStrategy paymentStrategy;

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

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

// Usage
ShoppingCart cart = new ShoppingCart();
cart.setPaymentStrategy(new CreditCardPayment("4111-1111-1111-1111"));
cart.checkout(new BigDecimal("99.99"));

// Change strategy at runtime
cart.setPaymentStrategy(new PayPalPayment("user@example.com"));
cart.checkout(new BigDecimal("49.99"));

With Java 8+ (functional):

// Strategy as functional interface
@FunctionalInterface
public interface PaymentStrategy {
    void pay(BigDecimal amount);
}

// Usage with lambdas
PaymentStrategy creditCard = amount ->
    System.out.println("Card payment: " + amount);

PaymentStrategy paypal = amount ->
    System.out.println("PayPal payment: " + amount);

cart.setPaymentStrategy(creditCard);

---

Observer

Use when: Objects need to be notified of changes in another object.

// ✅ Observer pattern (modern Java)
public interface OrderObserver {
    void onOrderPlaced(Order order);
}

public class OrderService {
    private final List<OrderObserver> observers = new ArrayList<>();

    public void addObserver(OrderObserver observer) {
        observers.add(observer);
    }

    public void removeObserver(OrderObserver observer) {
        observers.remove(observer);
    }

    public void placeOrder(Order order) {
        // Process order
        saveOrder(order);

        // Notify all observers
        observers.forEach(observer -> observer.onOrderPlaced(order));
    }
}

// Observers
public class InventoryService implements OrderObserver {
    @Override
    public void onOrderPlaced(Order order) {
        // Reduce inventory
        order.getItems().forEach(item ->
            reduceStock(item.getProductId(), item.getQuantity())
        );
    }
}

public class EmailNotificationService implements OrderObserver {
    @Override
    public void onOrderPlaced(Order order) {
        sendConfirmationEmail(order.getCustomerEmail(), order);
    }
}

public class AnalyticsService implements OrderObserver {
    @Override
    public void onOrderPlaced(Order order) {
        trackOrderEvent(order);
    }
}

// Setup
OrderService orderService = new OrderService();
orderService.addObserver(new InventoryService());
orderService.addObserver(new EmailNotificationService());
orderService.addObserver(new AnalyticsService());

With Spring Events (preferred):

// Event
public record OrderPlacedEvent(Order order) {}

// Publisher
@Service
public class OrderService {
    private final ApplicationEventPublisher eventPublisher;

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

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

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

    @EventListener
    @Async  // Async processing
    public void handleOrderPlacedAsync(OrderPlacedEvent event) {
        // Send email asynchronously
    }
}

---

Template Method

Use when: Define algorithm skeleton, let subclasses fill in steps.

// ✅ Template Method pattern
public abstract class DataProcessor {

    // Template method - defines the algorithm
    public final void process() {
        readData();
        processData();
        writeData();
        if (shouldNotify()) {
            notifyCompletion();
        }
    }

    // Steps to be implemented by subclasses
    protected abstract void readData();
    protected abstract void processData();
    protected abstract void writeData();

    // Hook - optional override
    protected boolean shouldNotify() {
        return true;
    }

    protected void notifyCompletion() {
        System.out.println("Processing completed!");
    }
}

public class CsvDataProcessor extends DataProcessor {
    @Override
    protected void readData() {
        System.out.println("Reading CSV file...");
    }

    @Override
    protected void processData() {
        System.out.println("Processing CSV data...");
    }

    @Override
    protected void writeData() {
        System.out.println("Writing to database...");
    }
}

public class ApiDataProcessor extends DataProcessor {
    @Override
    protected void readData() {
        System.out.println("Fetching from API...");
    }

    @Override
    protected void processData() {
        System.out.println("Transforming API response...");
    }

    @Override
    protected void writeData() {
        System.out.println("Writing to cache...");
    }

    @Override
    protected boolean shouldNotify() {
        return false;  // Override hook
    }
}

// Usage
DataProcessor csvProcessor = new CsvDataProcessor();
csvProcessor.process();

DataProcessor apiProcessor = new ApiDataProcessor();
apiProcessor.process();

---

Structural Patterns

Decorator

Use when: Add behavior dynamically without modifying existing classes.

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

public class SimpleCoffee implements Coffee {
    @Override
    public String getDescription() {
        return "Coffee";
    }

    @Override
    public BigDecimal getCost() {
        return new BigDecimal("2.00");
    }
}

// Base decorator
public abstract class CoffeeDecorator implements Coffee {
    protected final Coffee coffee;

    public CoffeeDecorator(Coffee coffee) {
        this.coffee = coffee;
    }

    @Override
    public String getDescription() {
        return coffee.getDescription();
    }

    @Override
    public BigDecimal getCost() {
        return coffee.getCost();
    }
}

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

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

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

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

    @Override
    public String getDescription() {
        return coffee.getDescription() + ", Sugar";
    }

    @Override
    public BigDecimal getCost() {
        return coffee.getCost().add(new BigDecimal("0.20"));
    }
}

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

    @Override
    public String getDescription() {
        return coffee.getDescription() + ", Whipped Cream";
    }

    @Override
    public BigDecimal getCost() {
        return coffee.getCost().add(new BigDecimal("0.70"));
    }
}

// Usage - compose decorators
Coffee coffee = new SimpleCoffee();
coffee = new MilkDecorator(coffee);
coffee = new SugarDecorator(coffee);
coffee = new WhippedCreamDecorator(coffee);

System.out.println(coffee.getDescription());  // Coffee, Milk, Sugar, Whipped Cream
System.out.println(coffee.getCost());         // 3.40

Java I/O uses Decorator:

// Classic example from Java
BufferedReader reader = new BufferedReader(
    new InputStreamReader(
        new FileInputStream("file.txt")
    )
);

---

Adapter

Use when: Make incompatible interfaces work together.

// ✅ Adapter pattern

// Existing interface our code uses
public interface MediaPlayer {
    void play(String filename);
}

// Legacy/third-party interface
public class LegacyAudioPlayer {
    public void playMp3(String filename) {
        System.out.println("Playing MP3: " + filename);
    }
}

public class AdvancedVideoPlayer {
    public void playMp4(String filename) {
        System.out.println("Playing MP4: " + filename);
    }

    public void playAvi(String filename) {
        System.out.println("Playing AVI: " + filename);
    }
}

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

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

public class VideoPlayerAdapter implements MediaPlayer {
    private final AdvancedVideoPlayer videoPlayer = new AdvancedVideoPlayer();

    @Override
    public void play(String filename) {
        if (filename.endsWith(".mp4")) {
            videoPlayer.playMp4(filename);
        } else if (filename.endsWith(".avi")) {
            videoPlayer.playAvi(filename);
        }
    }
}

// Usage
MediaPlayer mp3Player = new Mp3PlayerAdapter();
mp3Player.play("song.mp3");

MediaPlayer videoPlayer = new VideoPlayerAdapter();
videoPlayer.play("movie.mp4");

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Pattern Selection Guide

Situation	Consider
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
Common algorithm, varying steps	Template Method
Need single instance	Singleton (use sparingly)

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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 is known
Deep decorator chains	Hard to debug	Keep chains short, consider composition
Observer with many events	Spaghetti notifications	Event bus, clear event hierarchy

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Related Skills

• solid-principles - Design principles that patterns help implement
• clean-code - Code-level best practices
• spring-boot-patterns - Spring-specific implementations