Example of Use With The Factory Method Pattern
The singleton pattern is often used in conjunction with the factory method pattern to create a system-wide resource whose specific type is not known to the code that uses it. An example of using these two patterns together is the Java Abstract Window Toolkit (AWT).
java.awt.Toolkit
is an abstract class that binds the various AWT components to particular native toolkit implementations. The Toolkit
class has a Toolkit.getDefaultToolkit
factory method that returns the platform-specific subclass of Toolkit
. The Toolkit
object is a singleton because the AWT needs only a single object to perform the binding and the object is relatively expensive to create. The toolkit methods must be implemented in an object and not as static methods of a class because the specific implementation is not known by the platform-independent components. The name of the specific Toolkit
subclass used is specified by the "awt.toolkit" environment property accessed through System.getProperties
.
The binding performed by the toolkit allows, for example, the backing implementation of a java.awt.Window
to bind to the platform-specific java.awt.peer.WindowPeer
implementation. Neither the Window
class nor the application using the window needs to be aware of which platform-specific subclass of the peer is used.
Read more about this topic: Singleton Pattern
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