CSC/ECE 517 Fall 2007/wiki1b 8 sa

Note: Santthosh (sbselvad@ncsu.edu) and Agustin (agusvega@nc.rr.com) are editing this page.

Strategy Pattern

Strategy pattern is one of the several software design patterns, where in algorithms can be selected on-the-fly at runtime execution of the program. The Strategy pattern lets one build software as a loosely coupled collection of interchangeable parts, in contrast to a monolithic, tightly coupled system. That loose coupling makes the software much more extensible, maintainable, and reusable. The strategy pattern is useful for situations where it is necessary to dynamically swap the algorithms used in an application.

Concept to explore

Take a case of the Strategy pattern and implement it as succinctly as possible in Ruby and Java. Compare the two implementations in terms of clarity and succinctness. The example should be a "real-world" example. While it may be grossly oversimplified for the purpose of illustration, it should not be totally contrived (i.e., should not raise the question, Why would anyone ever want to do that?).

Definitions

Design Patterns

A design pattern is a general repeatable solution to a commonly occurring problem in software design. A design pattern is neither a finished design nor a solution that can be transformed directly into code. It is a template on how to solve a problem that can be used in many different situations. Design patterns are more often use in Object oriented designs as they typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved.

Note: Algorithms are not design patterns, algorithms solve computational problems and design patterns solve design problems.

Strategy Pattern

The Strategy Design Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeagle. Stratergy lets the algorithm vary independently from clients that use it.

Merits

1. A family of algorithms can be defined as a class hierarchy and can be used interchangeably to alter application behavior without changing its architecture.
2. By encapsulating the algorithm separately, new algorithms complying with the same interface can be easily introduced.
3. The application can switch strategies at run-time.
4. Strategy enables the clients to choose the required algorithm, without using a "switch" statement or a series of "if-else" statements.
5. Data structures used for implementing the algorithm is completely encapsulated in Strategy classes. Therefore, the implementation of an algorithm can be changed without affecting the Context class.
6. Strategy Pattern can be used instead of sub-classing the Context class. Inheritance hardwires the behavior with the Context and the behavior cannot be changed dynamically.
7. The same Strategy object can be strategically shared between different Context objects. However, the shared Strategy object should not maintain states across invocations.

Demerits

1. The application must be aware of all the strategies to select the right one for the right situation.
2. Strategy and Context classes may be tightly coupled. The Context must supply the relevant data to the Strategy for implementing the algorithm and sometimes, all the data passed by the Context may not be relevant to all the Concrete Strategies.
3. Context and the Strategy classes normally communicate through the interface specified by the abstract Strategy base class. Strategy base class must expose interface for all the required behaviors, which some concrete Strategy classes might not implement.
4. In most cases, the application configures the Context with the required Strategy object. Therefore, the application needs to create and maintain two objects in place of one.
5. Since, the Strategy object is created by the application in most cases; the Context has no control on lifetime of the Strategy object. However, the Context can make a local copy of the Strategy object. But, this increases the memory requirement and has a sure performance impact.

Related Patterns

Command and Bridge are two closely related patterns to Strategy.

• A Command pattern encapsulates a single action, therefore it tends to have a single method with a rather generic signature. It often is intended to be stored for a longer time and to be executed later. Strategy, in contrast, is used to customize an algorithm. A strategy might have a number of methods specific to the algorithm. Most often strategies will be instantiated immediately before executing the algorithm, and discarded afterwards.

• Bridge pattern is meant for structure, whereas the Strategy pattern is meant for behavior. The coupling between the context and the strategies is tighter than the coupling between the abstraction and the implementation in the Bridge pattern.

Alternatives

In some programming languages, such as those without polymorphism, the issues addressed by strategy pattern are handled through forms of reflection, such as the native function pointer or function delegate syntax.

Example & Illustration

Normally there are 3 types of classes and or objects that participate in a strategy pattern

• Strategy

declares an interface common to all supported algorithms. Context uses this interface to call the algorithm defined by a ConcreteStrategy

• ConcreteStrategy

implements the algorithm using the Strategy interface

• Context

is configured with a ConcreteStrategy object

maintains a reference to a Strategy object

may define an interface that lets Strategy access its data.

For example lets take the general case of sorting a list. One may wish to use different sort strategies based on the length of the list as performance of these sort algorithms vary depending on the size of the list. Here SortStrategy will be the common interface to all supported sort algorithms (like QuickSort, MergeSort, SyncSort etc.,) and SortedList will be the context that lets Strategy access the list and operate on it.

References

Books

1. Programming Ruby: The programmatic programmer’s guide
2. Head First Design Patterns

Links

1. Strategy Pattern - Data and Object Factory
2. Design Patterns - Wikipedia
3. Strategy for Success - Java World

See Also

Strategy Design Pattern and Open-Closed Principle