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Singleton

Difficulty-Beginner Gang Of Four Java Creational

Intent

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

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

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