Applied Design Patterns with Java

Creational :: Singleton (127) {C ch 6}

Collaborations

• Clients access a Singleton instance solely through Singleton's Instance operation.

Consequences

The Singleton pattern has several benefits:

1. Controlled access to sole instance. Because the Singleton class encapsulates its sole instance, it can have strict control over how and when clients access it.
2. Reduced name space. The Singleton pattern is an improvement over global variables. It avoids polluting the name space with global variables that store sole instances.
3. Permits refinement of operations and representation. The Singleton class may be subclassed, and it's easy to configure an application with an instance of this extended class. You can configure the application with an instance of the class you need at run-time.
4. Permits a variable number of instances. The pattern makes it easy to change a program and allow more than one instance of the Singleton class. Also, the same approach is used to control the number of instances that the application uses. Only the operation that grants access to the Singleton instance needs to change.
5. More flexible than class operations. Another way to package a Singleton's functionality is to use class operations (that is, static member functions in C++ or static class methods in Java). But both of these language techniques make it hard to change a design to allow more than one instance of a class. Note that static member functions are never virtual, so subclasses can't define them polymorphically.

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