Clean Architecture & Design Patterns in Practice · Lesson

Singleton and Factory Method

Implement the Singleton pattern for unique instances and the Factory Method for flexible object creation.

Singleton and Factory Method is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro to Creational Patterns

Welcome to our lesson on Creational Design Patterns! These patterns are all about how objects are created.

They help us manage object instantiation in a way that makes our systems more flexible and robust. Instead of directly creating objects everywhere, we use patterns to control this process.

Singleton: One Instance Only

The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance.

It's useful for resources that should be unique, like a configuration manager or a logger.
To achieve this, the class typically has a private constructor and a static method that returns the single instance.

Building a Basic Singleton

Here's a simple example of a Singleton. Notice the private constructor and the static getInstance() method.

The getInstance() method checks if an instance already exists; if not, it creates one. Otherwise, it returns the existing one.

public class SimpleLogger {
  private static SimpleLogger instance;

  // Private constructor to prevent direct instantiation
  private SimpleLogger() {
    System.out.println("SimpleLogger instance created!");
  }

  public static SimpleLogger getInstance() {
    if (instance == null) {
      instance = new SimpleLogger();
    }
    return instance;
  }

  public void log(String message) {
    System.out.println("LOG: " + message);
  }
}

public class Main {
  public static void main(String[] args) {
    SimpleLogger logger1 = SimpleLogger.getInstance();
    logger1.log("Application starting up.");

    SimpleLogger logger2 = SimpleLogger.getInstance();
    logger2.log("Processing user request.");

    if (logger1 == logger2) {
      System.out.println("Both logger references point to the same instance!");
    }
  }
}

Singleton: Thread-Safe Access

The basic Singleton can have issues in multi-threaded environments. If multiple threads call getInstance() at the same time when instance is null, more than one instance could be created.

We can make it thread-safe by using the synchronized keyword on the getInstance() method. This ensures only one thread can execute it at a time.

public class ThreadSafeLogger {
  private static ThreadSafeLogger instance;

  private ThreadSafeLogger() {
    System.out.println("ThreadSafeLogger instance created!");
  }

  public static synchronized ThreadSafeLogger getInstance() {
    if (instance == null) {
      instance = new ThreadSafeLogger();
    }
    return instance;
  }

  public void log(String message) {
    System.out.println("TS_LOG: " + message);
  }
}

public class Main {
  public static void main(String[] args) {
    // In a real app, threads would access this concurrently.
    // Here, we just show it works correctly sequentially.
    ThreadSafeLogger tsLogger1 = ThreadSafeLogger.getInstance();
    tsLogger1.log("Task A completed.");

    ThreadSafeLogger tsLogger2 = ThreadSafeLogger.getInstance();
    tsLogger2.log("Task B started.");

    if (tsLogger1 == tsLogger2) {
      System.out.println("Thread-safe instances are the same!");
    }
  }
}

Singleton Use Cases

When should you consider using the Singleton pattern?

Configuration Manager: To hold application settings, ensuring all parts of the app use the same settings.
Logger: For a single logging service to write application events.
Database Connection Pool: To manage a limited set of database connections efficiently across the application.

The Problem: Hardcoded Creation

Now, let's look at the Factory Method pattern. Imagine you have code that creates objects directly, like new Car() or new Truck().

What happens if you need to add a new vehicle type, or change how a Car is created? You'd have to find and update every place in your code that creates these objects. This makes your code rigid and hard to maintain!

Factory Method: Abstracting Creation

The Factory Method pattern solves this by defining an interface or abstract class for creating an object, but letting subclasses decide which class to instantiate.

It delegates object creation to specialized 'factory' methods.
This promotes loose coupling by allowing client code to work with interfaces instead of concrete classes.

Building with Factory Method

Let's create a system for different types of transport. We define a Transport interface, concrete transport classes, and then a TransportFactory with a factory method.

Each specific factory (e.g., CarFactory) knows how to create its specific product.

// Product Interface
interface Transport {
  void deliver();
}

// Concrete Products
class Truck implements Transport {
  @Override
  public void deliver() {
    System.out.println("Deliver by land in a truck.");
  }
}

class Ship implements Transport {
  @Override
  public void deliver() {
    System.out.println("Deliver by sea in a ship.");
  }
}

// Creator Interface (with Factory Method)
abstract class Logistics {
  public void planDelivery() {
    Transport t = createTransport();
    t.deliver();
  }

  // The Factory Method
  public abstract Transport createTransport();
}

// Concrete Creators
class RoadLogistics extends Logistics {
  @Override
  public Transport createTransport() {
    return new Truck();
  }
}

class SeaLogistics extends Logistics {
  @Override
  public Transport createTransport() {
    return new Ship();
  }
}

public class Main {
  public static void main(String[] args) {
    Logistics roadLogistics = new RoadLogistics();
    roadLogistics.planDelivery(); // Outputs: Deliver by land in a truck.

    Logistics seaLogistics = new SeaLogistics();
    seaLogistics.planDelivery(); // Outputs: Deliver by sea in a ship.
  }
}

Advantages of Factory Method

The Factory Method pattern brings several key benefits to your software design:

Loose Coupling: Your client code interacts only with the Logistics interface, not specific Truck or Ship classes.
Extensibility: You can easily add new transport types (e.g., AirLogistics with Plane) without changing existing Logistics or client code.
Single Responsibility: The responsibility of creating objects is moved to dedicated factory classes/methods.

Check Your Knowledge

Let's test your understanding of the Singleton and Factory Method patterns.

Recap: Singleton & Factory

Great job! In this lesson, we explored two powerful creational design patterns:

Singleton: Ensures a class has only one instance and provides a global access point. Remember to consider thread-safety!
Factory Method: Delegates object creation to subclasses, promoting loose coupling and making your code more extensible.

These patterns are fundamental for building flexible and maintainable software. Next, we'll dive into more creational patterns like Abstract Factory and Builder!

Frequently asked questions

Is the “Singleton and Factory Method” lesson free?

Yes — the full text of “Singleton and Factory Method” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.

What will I learn in “Singleton and Factory Method”?

Implement the Singleton pattern for unique instances and the Factory Method for flexible object creation. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Clean Architecture & Design Patterns in Practice?

No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Singleton and Factory Method” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?

Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Singleton and Factory Method
Abstract Factory and Builder
Prototype and Object Pool
Dependency Injection as a Creational Technique

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