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

Realworld example

Real-world example

In plain words

Then in order to use:

var enumIvoryTower1=EnumIvoryTower.INSTANCE;

var enumIvoryTower2=EnumIvoryTower.INSTANCE;

assertEquals(enumIvoryTower1, enumIvoryTower2);// true

var enumIvoryTower1=EnumIvoryTower.INSTANCE;

var enumIvoryTower2=EnumIvoryTower.INSTANCE;

LOGGER.info("enumIvoryTower1={}", enumIvoryTower1);

LOGGER.info("enumIvoryTower2={}", enumIvoryTower2);

The console output

* 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

* The logging class

* Managing a connection to a database

* File manager

*[java.lang.Runtime#getRuntime()](http://docs.oracle.com/javase/8/docs/api/java/lang/Runtime.html#getRuntime%28%29)

*[java.awt.Desktop#getDesktop()](http://docs.oracle.com/javase/8/docs/api/java/awt/Desktop.html#getDesktop--)

* Violates Single Responsibility Principle (SRP) by controlling theirowncreation and lifecycle.

* Encourages using aglobal shared instance which prevents an object and resources used by this object from being deallocated.

* Violates Single Responsibility Principle (SRP) by controlling their creation and lifecycle.

* Encourages using aglobally 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.