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

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

• the logging class
• managing a connection to a database
• file manager

• java.lang.Runtime#getRuntime()
• java.awt.Desktop#getDesktop()
• java.lang.System#getSecurityManager()

• 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 that is difficult to test.
• Makes it almost impossible to subclass a Singleton.

• Design Patterns: Elements of Reusable Object-Oriented Software
• Effective Java (2nd Edition)