Why use a singleton instead of static methods?

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Often, singletons are used to introduce some kind of global state to an application. (More often than really necessary, to be honest, but that’s a topic for another time.)

However, there are a few corner cases where even a stateless singleton can be useful:

  • You expect to extend it with state in the foreseeable future.
  • You need an object instance for some particular technical reason.
    Example: Synchonization objects for the C# lock or the Java synchronized statement.
  • You need inheritance, i.e., you want to be able to easily replace your singleton with another one using the same interface but a different implementation.
    Example: The Toolkit.getDefaultToolkit() method in Java will return a singleton whose exact type is system dependent.
  • You want reference equality for a sentinel value.
    Example: DBNull.Value in C#.

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