Core Java Design Pattern : Structural Pattern : Flyweight Design Pattern

• Flyweight Design Pattern
• Examples

Use sharing to support large numbers of fine-grained objects efficiently.

Use the Flyweight pattern when

The Flyweight pattern’s effectiveness depends heavily on how and where it’s used.

Apply the Flyweight pattern when all of the following are true:

• An application uses a large number of objects.
• Storage costs are high because of the sheer quantity of objects.
• Most object state can be made extrinsic.
• Many groups of objects may be replaced by relatively few shared objects once extrinsic state is removed.
• The application doesn’t depend on object identity. Since flyweight objects may be shared, identity tests will return true for conceptually distinct objects.

For example, most document editor implementations have text formatting and editing facilities that are modularized to some extent. Object-oriented document editors typically use objects to represent embedded elements like tables and figures. However, they usually stop short of using an object for each character in the document, even though doing so would promote flexibility at the finest levels in the application. Characters and embedded elements could then be treated uniformly with respect to how they are drawn and formatted. The application could be extended to support new character sets without disturbing other functionality. The application’s object structure could mimic the document’s physical structure. The following diagram shows how a document editor can use objects to represent characters.