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    By: Stephen Wong

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    "Provides an abstract means of performing particular operations for an object."

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Strategy Design Pattern

Module by: Stephen Wong, Dung Nguyen, Antonio Garcia Castañeda. E-mail the authors

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Summary: The Strategy Design Pattern encapsulates an abstract behavior to which the client delegates a specific task.

Strategy Design Pattern

The Strategy Design Pattern basically consists of decoupling an algorithm from its host, and encapsulating the algorithm into a separate class. More simply put, an object and its behavior are separated and put into two different classes. This allows you to switch the algorithm that you are using at any time.

There are several advantages to doing this. First, if you have several different behaviors that you want an object to perform, it is much simpler to keep track of them if each behavior is a separate class, and not buried in the body of some method. Should you ever want to add, remove, or change any of the behaviors, it is a much simpler task, since each one is its own class. Each such behavior or algorithm encapsulated into its own class is called a Strategy.

When you have several objects that are basically the same, and differ only in their behavior, it is a good idea to make use of the Strategy Pattern.. Using Strategies, you can reduce these several objects to one class that uses several Strategies. The use of strategies also provides a nice alternative to subclassing an object to achieve different behaviors. When you subclass an object to change its behavior, the behavior that it executes is static. If you wanted to change what it does, you'd have to create a new instance of a different subclass and replace that object with it. With Strategies, however, all you need to do is switch the object's strategy, and it will immediately change how it behaves. Using Strategies also eliminates the need for many conditional statements. When you have several behaviors together in one class, it is difficult to choose among them without resorting to conditional statements. If you use Strategies you won't need to check for anything, since whatever the current strategy is just executes unconditionally.

Figure 1: A Context has some strategy encapsulated in an AStrategy object, which could be of any subclass of AStrategy (ConcreteStrategyA, ConcreteStrategyB, ConcreteStrategyC, etc.)
Strategy Pattern
Strategy Pattern (Strategy.jpg)

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