Understanding the Singleton Pattern

The Singleton Pattern is a way to make sure that a class has only one instance. This means that only one version of that class can be used throughout the application.

This is really helpful when you need to share something, like settings or a logging tool, across an entire app.

While it uses some concepts like inheritance and polymorphism, it doesn’t work the same way as other design patterns, like Strategy or Template Method.

Singleton and Inheritance

The main job of the Singleton Pattern is to control how instances are created.

The main class can have a special protected constructor. This stops other classes from creating a new instance of it directly. Instead, it provides a method that gives back the only instance:

public class Singleton {
    private static Singleton instance;

    protected Singleton() {
        // protected constructor
    }

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

Here, inheritance lets subclasses change how things behave without creating many instances.

For example, a subclass can add specific features to the Singleton class. But you need to be careful! If subclasses are created too freely, it can lead to having more than one instance, which breaks the Singleton rule.

To avoid this, some Singleton setups might make the constructor final or protected, limiting how subclasses can be made.

Singleton and Polymorphism

Polymorphism comes in when using the Singleton instance in a setup that expects subclasses.

This means that the program can choose which method to run based on the instance it is looking at, making the design more flexible.

For instance, if a Singleton class follows a certain guideline (interface), any subclass can still follow those guidelines while changing what it does:

public interface Logger {
    void log(String message);
}

public class FileLogger extends Singleton implements Logger {
    public void log(String message) {
        // log to file
    }
}

public class ConsoleLogger extends Singleton implements Logger {
    public void log(String message) {
        // log to console
    }
}

Logger logger = FileLogger.getInstance(); // uses polymorphism
logger.log("Hello World");

In this example, no matter what kind of logger you are using—whether it's for files, the console, or any other type—the log method can be used in a flexible way.

Conclusion

In short, the Singleton Pattern focuses on having just one instance but also includes aspects of inheritance and polymorphism.

It allows subclasses to change how things work while keeping one instance alive in the app.

But developers have to be careful to stop multiple instances from being created when using inheritance with Singletons.

The Singleton Pattern is a helpful tool for managing polymorphism while keeping things simple, but there are rules to follow to make it work right!