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()

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