Singleton pattern purpose, strategy, usage and example

Singleton Pattern is one of the Gangs of Four Design Patterns and comes in Creational Design Patterns that restricts the instantiation of a class and ensures only one object of the class exists in the JVM (java virtual machine)

Purpose of singleton pattern

There should be only one instance allowed for a class and we should allow global point of access to that single instance.

Singleton pattern implementation strategy

• We hide the constructor of the class and don’t allow even a single instance for the class. This is done by declaring all the constructors of the class to be private.
• Define public static method (getSingletonInstance()) that returns the reference to the only instance of the class.
• The singleton instance is usually stored as a private static variable and the singleton instance is created when the variable is initialized.

Singleton pattern example

package com.sneppets.designpattern.creational.singleton;

public final class Singleton {
	
	private static final Singleton singleInstance = new Singleton();
	
	private Singleton() {
		
	}
	
	public static Singleton getSingleInstance() {
		
		return singleInstance;
	}	
	
}

Where exactly the singleton design pattern is used in real application?

We use this pattern for logger classes, configuration classes, caching classes etc.,