Singleton is a creational design pattern that ensures a class has only one instance and provides a global access point to that instance.

Problem

In software development, it’s common to need a single instance of a class to be shared across an application. Having multiple instances can lead to issues such as unexpected behaviors or inefficient resource usage.

Solution

The Singleton pattern addresses this problem by:

• Making the class constructor private to prevent direct instantiation from outside the class.
• Providing a static public method that acts as a constructor. Before creating a new instance, it checks if an instance already exists in a private static field. If the field is null, it creates a new instance and assigns it to the field; otherwise, it returns the existing instance.

Java example

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public class Singleton {
	private static Singleton instance;

	private Singleton() {
		// initialization code
	}

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

Applications

Use the Singleton pattern when you need a single instance of a class to be shared across the entire application.

Tips

• Be careful when using the Singleton pattern in multi-threaded scenarios, as it is not inherently thread-safe. Check this article for further details