Intent

Ensure a class only has one instance, and provide a global point of access to it.

Explanation

Real world example

There can only be one ivory tower where the wizards study their magic. The same enchanted ivory tower is always used by the wizards. Ivory tower here is singleton.

In plain words

Ensures that only one object of a particular class is ever created.

Wikipedia says

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.

Programmatic Example

Joshua Bloch, Effective Java 2nd Edition p.18

A single-element enum type is the best way to implement a singleton

public enum EnumIvoryTower {
  INSTANCE;
}

Then in order to use

EnumIvoryTower enumIvoryTower1 = EnumIvoryTower.INSTANCE;
EnumIvoryTower enumIvoryTower2 = EnumIvoryTower.INSTANCE;
assertEquals(enumIvoryTower1, enumIvoryTower2); // true

Applicability

Use the Singleton pattern when

there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code

Typical Use Case

the logging class
managing a connection to a database
file manager

Real world examples

Consequences

Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.
Creates tightly coupled code. The clients of the Singleton become difficult to test.
Makes it almost impossible to subclass a Singleton.

Credits