Not all Flyweights have to be shared, nor do the implementing classes need to be shared. This pattern allows object sharing, but does not require it.

Use Flyweight only when it’s easy to identify and extract the external data from the objects, and when the number of different states is limited.

Implementation

The Flyweight class diagram is shown in Figure 3.8.

Figure 3.8. Flyweight class diagram

To implement the Flyweight you need:

Flyweight – The interface defines the methods clients can use to pass external state into the flyweight objects.

ConcreteFlyweight – This implements the Flyweight interface, and implements the ability to store internal data. The internal data has to be representative for all the instances where you need the Flyweight.

FlyweightFactory (see “ Abstract Factory ” on page 6) – This factory is responsible for creating and managing the Flyweights. Providing access to Flyweight creation through the factory ensures proper sharing. The factory can create all the flyweights at the start of the application, or wait until they are needed.

Client (page 183) – The client is responsible for creating and providing the context for the flyweights. The only way to get a reference to a flyweight is through FlyweightFactory.

Benefits and Drawbacks

The obvious benefit of this pattern is the reduced number of objects to handle. This can save a lot of space, both in memory and on storage devices, if the objects are persisted.

The most space will be saved when the context information for the flyweight is computed instead of stored. However, this also leads to the drawback of this pattern: runtime costs.

Instead of storing many objects, clients now have to calculate the context and provide this to the flyweight. The flyweight then uses this information to compute/provide functions. Handling fewer objects should increase runtime performance if implemented correctly. Note that if the context information is small, and the flyweight is large, the savings will be significant.

Pattern Variants

None.

Related Patterns

Related patterns include the following:

Abstract Factory (page 6) – The Abstract Factory pattern is used to provide access to flyweights so that these factories ensure proper sharing of the Flyweight instances.

Composite (page 157) – The Composite is often used to provide structure.

State (page 104) – The State pattern is often implemented using the Flyweight pattern.

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