Flyweight

Intent

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

Participants

• Flyweight - declares an interface through which flyweights (shared and unshared) can receive and act on extrinsic state.
• ConcreteFlyweight - implements the Flyweight interface with shared objects.
• UnsharedConcreteFlyweight - implements the Flyweight interface with unshared objects.
• FlyweightFactory - provides methods for creating instances of Flyweight subclasses. The instances may be shared (singletons).
• Client - stores or computes extrinsic state.

Additional Terminology

• intrinsic state
• extrinsic state

Examples

Borders for Java Swing Components

• Border is a Flyweight.
• simple raised and lowered BevelBorders may be ConcreteFlyweights since they have no extrinsic state beyond that provided by a JComponent.
• TitledBorder is an UnsharedConcreteFlyweight - the title is extrinsic state that cannot be found in a JComponent.
• BorderFactory is a FlyweightFactory.

Java Swing Component Delegates

• ComponentUI is a Flyweight. Each look-and-feel provides a ConcreteFlyweight for each subclass of JComponent. All extrinsic state is stored in JComponent objects.
• UIManager is a FlyweightFactory