In, the strategy pattern (also known as the policy pattern) is a that enables selecting an at runtime. The strategy pattern • defines a family of algorithms, • encapsulates each algorithm, and • makes the algorithms interchangeable within that family. Strategy lets the algorithm vary independently from clients that use it. Strategy is one of the patterns included in the influential book by Gamma et al.

That popularized the concept of using design patterns to describe how to design flexible and reusable object-oriented software. For instance, a class that performs validation on incoming data may use the Strategy pattern to select a validation algorithm depending on the type of data, the source of the data, user choice, or other discriminating factors. These factors are not known until run-time and may require radically different validation to be performed. The validation algorithms (strategies), encapsulated separately from the validating object, may be used by other validating objects in different areas of the system (or even different systems) without. The essential requirement to implement the Strategy pattern is the ability to store a reference to some code in a data structure and retrieve it. This can be achieved by mechanisms such as the native, the, classes or class instances in languages, or accessing the language implementation's internal storage of code via. Contents • • • • • • • • • • • Overview [ ] The Strategy design pattern is one of the twenty-three well-known that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

Design Patterns. In software engineering, a design pattern is a general repeatable solution to a commonly occurring problem in software design.

What problems can the Strategy design pattern solve? • A class should be configured with an algorithm instead of implementing an algorithm directly. • An algorithm should be selected and exchanged at run-time. What is an algorithm? An algorithm is usually defined as a procedure that takes some value as input, performs a finite number of steps, and produces some value as output. From a more general point of view, an algorithm is a piece of code that does something appropriate.

Implementing an algorithm directly within the class that uses the algorithm is inflexible because it commits the class to a particular algorithm at compile-time and makes it impossible to change the algorithm later independently from (without having to change) the class. This also stops the class from being reusable when another algorithm should be used. What solution does the Strategy design pattern describe? • Define a separate (strategy) object that encapsulates an algorithm. That is, define an interface (Strategy) for performing an algorithm, and define classes that implement the interface (algorithm) in different ways. • A class delegates an algorithm to a strategy object at run-time instead of implementing an algorithm directly (that is, instead of committing to an algorithm at compile-time).

This makes a class independent of a particular algorithm (how an algorithm is implemented). A class can be configured with a strategy object, which is used to perform an algorithm.

Moreover, the strategy object can be exchanged at run-time. See also the UML class and sequence diagram below. Structure [ ] UML class and sequence diagram [ ]. A sample UML class and sequence diagram for the Strategy design pattern. In the above, the Context class doesn't implement an algorithm directly. Instead, Context refers to the Strategy interface for performing an algorithm ( strategy.algorithm()), which makes Context independent of how an algorithm is implemented.

The Strategy1 and Strategy2 classes implement the Strategy interface, that is, implement (encapsulate) an algorithm. The shows the run-time interactions: The Context object delegates an algorithm to different Strategy objects. First, Context calls algorithm() on a Strategy1 object, which performs the algorithm and returns the result to Context.

Thereafter, Context changes its strategy and calls algorithm() on a Strategy2 object, which performs the algorithm and returns the result to Context. Class diagram [ ]. Accelerate and behaviors must be declared in each new. According to the strategy pattern, the behaviors of a class should not be inherited. Instead they should be encapsulated using interfaces. As an example, consider a car class. Two possible functionalities for car are brake and accelerate. Download Magellan Spring 2010 Map Upgrade V398.

Since accelerate and brake behaviors change frequently between models, a common approach is to implement these behaviors in subclasses. This approach has significant drawbacks: accelerate and brake behaviors must be declared in each new Car model.

The work of managing these behaviors increases greatly as the number of models increases, and requires code to be duplicated across models. Additionally, it is not easy to determine the exact nature of the behavior for each model without investigating the code in each. The strategy pattern uses. In the strategy pattern, behaviors are defined as separate interfaces and specific classes that implement these interfaces. This allows better decoupling between the behavior and the class that uses the behavior. The behavior can be changed without breaking the classes that use it, and the classes can switch between behaviors by changing the specific implementation used without requiring any significant code changes.

Behaviors can also be changed at run-time as well as at design-time. Turboprint 2 Serial Killers. For instance, a car object’s brake behavior can be changed from BrakeWithABS() to Brake() by changing the brakeBehavior member to. • Eric Freeman, Elisabeth Freeman, Kathy Sierra and Bert Bates, Head First Design Patterns, First Edition, Chapter 1, Page 24, O'Reilly Media, Inc, 2004. • Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994).

Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. CS1 maint: Multiple names: authors list () •. Retrieved 2017-08-12. Retrieved 2017-08-12. External links [ ] • The Wikibook has a page on the topic of: • • • • • (a formal modelling notation) • •.

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