Singleton

The singleton creational pattern Abstract factory, Builder, Factory method, Prototype, Singleton restricts the instantiation of a class to one "single" instance so only one object is needed to coordinate actions across the system.

Overview

The singleton design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software with the aim of making it easier to implement, change, test, and reuse objects.

The singleton design pattern solves problems by allowing it to:
1. Ensure that a class only has one instance
2. Easily access the sole instance of a class
3. Control its instantiation
4. Restrict the number of instances
5. Access a global variable

The singleton design pattern describes how to solve such problems:
• Hide the constructors of the class.
• Define a public static operation (getInstance()) that returns the sole instance of the class.

In essence, the singleton pattern forces it to be responsible for ensuring that it is only instantiated once. A hidden constructor-declared private or protected- ensures that the class can never be instantiated from outside the class. The public static operation can be accessed by using the class name and operation name, e.g., Singleton.getInstance().