Strategy pattern
From Wikipedia, the free encyclopedia
In computer programming, the strategy pattern (also known as the policy pattern) is a particular software design pattern, whereby algorithms can be selected at runtime.
In some programming languages, such as those without polymorphism, the issues addressed by this pattern are handled through forms of reflection, such as the native function pointer or function delegate syntax.
This pattern is invisible in languages with first-class functions. See the Python, Scala or Perl implementations for examples.
The strategy pattern is useful for situations where it is necessary to dynamically swap the algorithms used in an application. The strategy pattern is intended to provide a means to define a family of algorithms, encapsulate each one as an object, and make them interchangeable. The strategy pattern lets the algorithms vary independently from clients that use them.
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[edit] Code Examples
[edit] Ruby
class Context def initialize(strategy) @strategy = strategy end def execute raise 'Strategy object does not respond to the execute method' unless @strategy.respond_to?(:execute) @strategy.execute end end class StrategyA def execute puts 'Doing the task the normal way' end end class StrategyB def execute puts 'Doing the task alternatively' end end class StrategyC def execute puts 'Doing the task even more alternatively' end end a = Context.new(StrategyA.new) a.execute #=> Doing the task the normal way b = Context.new(StrategyB.new) b.execute #=> Doing the task alternatively c = Context.new(StrategyC.new) c.execute #=> Doing the task even more alternatively
[edit] Ruby using blocks
The previous ruby example uses typical OO features, but the same effect can be accomplished with ruby's blocks in much less code.
class Context def initialize(&strategy) @strategy = strategy end def execute @strategy.call end end c = Context.new { puts "strategy A"} c.execute
[edit] C++
#include <iostream> using namespace std; class StrategyInterface { public: virtual void execute() const = 0; }; class ConcreteStrategyA: public StrategyInterface { public: virtual void execute() const { cout << "Called ConcreteStrategyA execute method" << endl; } }; class ConcreteStrategyB: public StrategyInterface { public: virtual void execute() const { cout << "Called ConcreteStrategyB execute method" << endl; } }; class ConcreteStrategyC: public StrategyInterface { public: virtual void execute() const { cout << "Called ConcreteStrategyC execute method" << endl; } }; class Context { private: StrategyInterface * strategy_; public: explicit Context(StrategyInterface *strategy):strategy_(strategy) { } void set_strategy(StrategyInterface *strategy) { strategy_ = strategy; } void execute() const { strategy_->execute(); } }; int main(int argc, char *argv[]) { ConcreteStrategyA concreteStrategyA; ConcreteStrategyB concreteStrategyB; ConcreteStrategyC concreteStrategyC; Context contextA(&concreteStrategyA); Context contextB(&concreteStrategyB); Context contextC(&concreteStrategyC); contextA.execute(); contextB.execute(); contextC.execute(); contextA.set_strategy(&concreteStrategyB); contextA.execute(); contextA.set_strategy(&concreteStrategyC); contextA.execute(); return 0; }
[edit] Java
//StrategyExample test application class StrategyExample { public static void main(String[] args) { Context context; // Three contexts following different strategies context = new Context(new ConcreteStrategyAdd()); int resultA = context.executeStrategy(3,4); context = new Context(new ConcreteStrategySubtract()); int resultB = context.executeStrategy(3,4); context = new Context(new ConcreteStrategyMultiply()); int resultC = context.executeStrategy(3,4); } } // The classes that implement a concrete strategy should implement this // The context class uses this to call the concrete strategy interface Strategy { int execute(int a, int b); } // Implements the algorithm using the strategy interface class ConcreteStrategyAdd implements Strategy { public int execute(int a, int b) { System.out.println("Called ConcreteStrategyA's execute()"); return a + b; // Do an addition with a and b } } class ConcreteStrategySubtract implements Strategy { public int execute(int a, int b) { System.out.println("Called ConcreteStrategyB's execute()"); return a - b; // Do a subtraction with a and b } } class ConcreteStrategyMultiply implements Strategy { public int execute(int a, int b) { System.out.println("Called ConcreteStrategyC's execute()"); return a * b; // Do a multiplication with a and b } } // Configured with a ConcreteStrategy object and maintains a reference to a Strategy object class Context { private Strategy strategy; // Constructor public Context(Strategy strategy) { this.strategy = strategy; } public int executeStrategy(int a, int b) { return strategy.execute(a, b); } }
[edit] Python
Python has first-class functions, so there's no need to implement this pattern explicitly. However one loses information because the interface of the strategy is not made explicit. Here's an example you might encounter in GUI programming, using a callback function:
class Button: """A very basic button widget.""" def __init__(self, submit_func, label): self.on_submit = submit_func # Set the strategy function directly self.label = label # Create two instances with different strategies button1 = Button(sum, "Add 'em") button2 = Button(lambda nums: " ".join(map(str, nums)), "Join 'em") # Test each button numbers = range(1, 10) # A list of numbers 1 through 9 print button1.on_submit(numbers) # displays "45" print button2.on_submit(numbers) # displays "1 2 3 4 5 6 7 8 9"
[edit] Scala
Like Python, Scala also supports first-class functions. The following implements the basic functionality shown in the Python example.
// A very basic button widget. class Button(val on_submit:Range => Any, val lbl:String) val button1 = new Button(_.reduceLeft(_ + _), "Add 'em") val button2 = new Button(_.mkString(" "), "Join 'em") // Test each button val numbers = 1 to 9 // A list of numbers 1 through 9 println(button1.on_submit(numbers)) // displays 45 println(button2.on_submit(numbers)) // displays 1 2 3 4 5 6 7 8 9
[edit] C
A struct in C can be used to define a class, and the strategy can be set using a function pointer. The following mirrors the Python example, and uses C99 features:
#include <stdio.h> void print_sum(int n, int *array) { int total = 0; for (int i=0; i<n; i++) total += array[i]; printf("%d", total); } void print_array(int n, int *array) { for (int i=0; i<n; i++) printf("%d ", array[i]); } int main(void) { typedef struct { void (*submit_func)(int n, int *array); char *label; } Button; // Create two instances with different strategies Button button1 = {print_sum, "Add 'em"}; Button button2 = {print_array, "List 'em"}; int n = 10, numbers[n]; for (int i=0; i<n; i++) numbers[i] = i; button1.submit_func(n, numbers); button2.submit_func(n, numbers); return 0; }
[edit] C# 3.5
Delegates in C# follow the strategy pattern, where the delegate definition defines the strategy interface and the delegate instance represents the concrete strategy. C# 3.5 defines the Func<,> delegate which can be used to quickly implement the strategy pattern as shown in the example below. Note the 3 different methods for defining a delegate instance.
using System; using System.Linq; class Program { static void Main(string[] args) { var context = new Context<int>(); // Delegate var concreteStrategy1 = new Func<int, int, int>(PerformLogicalBitwiseOr); // Anonymous Delegate var concreteStrategy2 = new Func<int, int, int>( delegate(int op1, int op2) { return op1 & op2; }); // Lambda Expressions var concreteStrategy3 = new Func<int, int, int>((op1, op2) => op1 >> op2); var concreteStrategy4 = new Func<int, int, int>((op1, op2) => op1 << op2); context.Strategy = concreteStrategy1; var result1 = context.Execute(8, 9); context.Strategy = concreteStrategy2; var result2 = context.Execute(8, 9); context.Strategy = concreteStrategy3; var result3 = context.Execute(8, 1); context.Strategy = concreteStrategy4; var result4 = context.Execute(8, 1); } static int PerformLogicalBitwiseOr(int op1, int op2) { return op1 | op2; } class Context<T> { public Func<T, T, T> Strategy { get; set; } public T Execute(T operand1, T operand2) { return this.Strategy != null ? this.Strategy(operand1, operand2) : default(T); } } }
[edit] C#
using System; namespace Wikipedia.Patterns.Strategy { // MainApp test application class MainApp { static void Main() { Context anObject; // Three contexts following different strategies anObject= new Context(new ConcreteStrategyA()); anObject.Execute(); anObject.UpdateContext(new ConcreteStrategyB()); anObject.Execute(); anObject.UpdateContext(new ConcreteStrategyC()); anObject.Execute(); } } // The classes that implement a concrete strategy must implement this Execute function. // The context class uses this to call the concrete strategy interface IStrategy { void Execute(); } // Implements the algorithm using the strategy interface class ConcreteStrategyA : IStrategy { public void Execute() { Console.WriteLine( "Called ConcreteStrategyA.Execute()" ); } } class ConcreteStrategyB : IStrategy { public void Execute() { Console.WriteLine( "Called ConcreteStrategyB.Execute()" ); } } class ConcreteStrategyC : IStrategy { public void Execute() { Console.WriteLine( "Called ConcreteStrategyC.Execute()" ); } } // Configured with a ConcreteStrategy object and maintains a reference to a Strategy object class Context { IStrategy strategy; // Constructor public Context(IStrategy strategy) { this.strategy = strategy; } public void UpdateContext(IStrategy strategy) { this.strategy = strategy; } public void Execute() { strategy.Execute(); } } }
[edit] ActionScript 3
//invoked from application.initialize private function init() : void { var context:Context; context = new Context( new ConcreteStrategyA() ); context.execute(); context = new Context( new ConcreteStrategyB() ); context.execute(); context = new Context( new ConcreteStrategyC() ); context.execute(); } package org.wikipedia.patterns.strategy { public interface IStrategy { function execute() : void ; } } package org.wikipedia.patterns.strategy { public final class ConcreteStrategyA implements IStrategy { public function execute():void { trace( "ConcreteStrategyA.execute(); invoked" ); } } } package org.wikipedia.patterns.strategy { public final class ConcreteStrategyB implements IStrategy { public function execute():void { trace( "ConcreteStrategyB.execute(); invoked" ); } } } package org.wikipedia.patterns.strategy { public final class ConcreteStrategyC implements IStrategy { public function execute():void { trace( "ConcreteStrategyC.execute(); invoked" ); } } } package org.wikipedia.patterns.strategy { public class Context { private var strategy:IStrategy; public function Context(strategy:IStrategy) { this.strategy = strategy; } public function execute() : void { strategy.execute(); } } }
[edit] PHP
<?php class StrategyExample { public function __construct() { $context = new Context(new ConcreteStrategyA()); $context->execute(); $context = new Context(new ConcreteStrategyB()); $context->execute(); $context = new Context(new ConcreteStrategyC()); $context->execute(); } } interface IStrategy { public function execute(); } class ConcreteStrategyA implements IStrategy { public function execute() { echo "Called ConcreteStrategyA execute method\n"; } } class ConcreteStrategyB implements IStrategy { public function execute() { echo "Called ConcreteStrategyB execute method\n"; } } class ConcreteStrategyC implements IStrategy { public function execute() { echo "Called ConcreteStrategyC execute method\n"; } } class Context { private $strategy; public function __construct(IStrategy $strategy) { $this->strategy = $strategy; } public function execute() { $this->strategy->execute(); } } new StrategyExample(); ?>
[edit] Perl
Perl has first-class functions, so it doesn't require this pattern to be coded explicitly:
sort { lc($a) cmp lc($b) } @items
The strategy pattern can be formally implemented with Moose:
package Strategy; use Moose::Role; requires 'execute'; package FirstStrategy; use Moose; with 'Strategy'; sub execute { print "Called FirstStrategy->execute()\n"; } package SecondStrategy; use Moose; with 'Strategy'; sub execute { print "Called SecondStrategy->execute()\n"; } package ThirdStrategy; use Moose; with 'Strategy'; sub execute { print "Called ThirdStrategy->execute()\n"; } package Context; use Moose; has 'strategy' => ( is => 'rw', does => 'Strategy', handles => [ 'execute' ], # automatic delegation ); package StrategyExample; use Moose; # Moose's constructor sub BUILD { my $context; $context = Context->new(strategy => 'FirstStrategy'); $context->execute; $context = Context->new(strategy => 'SecondStrategy'); $context->execute; $context = Context->new(strategy => 'ThirdStrategy'); $context->execute; } package main; StrategyExample->new;
[edit] Strategy versus Bridge
The UML class diagram for the Strategy pattern is the same as the diagram for the Bridge pattern. However, these two design patterns aren't the same in their intent. While the Strategy pattern is meant for behavior, the Bridge pattern is meant for structure.
The coupling between the context and the strategies is tighter than the coupling between the abstraction and the implementation in the Bridge pattern.
[edit] Strategy Pattern and Open Closed Principle
According to 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 behaviors of car are brake and accelerate.
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. This may not be a concern when there are only a small number of models, but 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 composition instead of inheritance. In the strategy pattern behaviors are defined as separate interfaces and specific classes that implement these interfaces. Specific classes encapsulate 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. For instance, a car object’s brake behavior can be changed from BrakeWithABS()
to Brake()
by changing the brakeBehavior
member to:
brakeBehavior = new Brake();
This gives greater flexibility in design and is in harmony with the Open/closed principle (OCP) that states classes should be open for extension but closed for modification.
[edit] See also
- Mixin
- Policy-based design
- Template method pattern
- Factory Pattern
- List of object-oriented programming terms
[edit] External links
- The Strategy Pattern from the Net Objectives Repository
- Strategy Pattern for Java article
- Strategy pattern in UML and in LePUS3 (a formal modelling notation)
- Data & object factory
- Refactoring: Replace Type Code with State/Strategy
- Jt J2EE Pattern Oriented Framework
- Strategy Pattern with a twist!
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