data caching strategies. In the business arena, the Strategy pattern is sometimes used to represent different possible approaches to performing business actions. Placing an order for a workstation, for example, might be implemented as a Strategy if processing an order that had to be custom built was significantly different from processing one that was based on a standard product model.

Like the State pattern (see “ State ” on page 104), Strategy decouples part of a component into a separate group of classes. Part of a component’s behavior is delegated to a set of handlers.

Benefits and Drawbacks

Each behavior is defined in its own class, so the Strategy leads to more easily maintainable behaviors. It also becomes easier to extend a model to incorporate new behaviors without extensive recoding of the application.

The primary challenge in the Strategy pattern lies in deciding exactly how to represent the callable behavior. Each Strategy must have the same interface for the calling object. You must identify one that is generic enough to apply to a number of implementations, but at the same time specific enough for the various concrete Strategies to use.

Implementation

The Strategy class diagram is shown in Figure 2.14.

Figure 2.14. Strategy class diagram

To implement the Strategy pattern, use the following:

StrategyClient – This is the class that uses the different strategies for certain tasks. It keeps a reference to the Strategy instance that it uses and has a method to replace the current Strategy instance with another Strategy implementation.

Strategy – The interface that defines all the methods available for the StrategyClient to use.

ConcreteStrategy – A class that implements the Strategy interface using a specific set of rules for each of the methods in the interface.

Pattern Variants

None.

Related Patterns

Related patterns include the following:

Singleton (page 34) – Strategy implementations are sometimes represented as Singletons or static resources.

Flyweight (page 183) – Sometimes Strategy objects are designed as Flyweights to make them less expensive to create.

Factory Method (page 21) – The Strategy pattern is sometimes defined as a Factory so that the using class can use new Strategy implementations without having to recode other parts of the application.

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