CSC/ECE 517 Fall 2007/wiki1b 8 ktrk

Introduction

What is the Strategy Pattern?

The strategy pattern is a proven object oriented design pattern that can be used to solve a common coupling problem often found in software development. Often during the development of compositional classes behaviors and algorithms can become mixed into the class during development without realizing that these behaviors or algorithms may change, become more complex, or additional behaviors may need to be added. These changes could be implemented by extending the class's behavior through inheritance, but over time this could result in an overly complex class hierarchy. The strategy pattern is a design template that can be used to decouple the behavior and algorithms from object being acted upon.

When to Use the Strategy Pattern

In the seminal book on design patterns by the Gang-of-Four (Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides), situations (also labeled "code smells" by Martin Fowler in Refactoring) are described where the strategy design pattern could be used to architect a better solution. These include:

1. A class that exhibits many behaviors

2. A class that uses many variations of an algorithm

3. A class that uses data structures related to a behavior but not the class

4. Many similar classes that differ only in a type of behavior

Advantages of the Strategy Pattern

By decoupling through the use of composition rather than inheritance, several advantages emerge that improve the maintainability and readability of the code.

1. Allows classes to change behavior at runtime or design time

2. Decreases code duplication among classes using variations of the same behavior [1]

3. Behavior is better encapsulated by not being buried in its context [2]

4. A well known design pattern communicates the intent of the code more readily

Similar Patterns

State, Command, and Bridge

Use the state pattern when the state of the object changes with a change in behavior.

Use the bridge pattern when a structural design is needed.

For a discussion of the strategy pattern compared to the state and bridge patterns see Strategy Pattern vs. Bridge Pattern.

Use the command pattern when the invoking object (the context object in the state pattern) does not have knowlege of the recipient object (the behavior in the state pattern). The intermediary command object takes care of dispatching the request to the appropriate object.

Strategy Pattern Implementation in Java and Ruby

For our example, we chose to model a video game where there could be different types of Fighters pitted against each other, as you might see in the UFC. In our case, the strategy is the Attack. Each Fighter has a different type of Attack according to his training. The Fighter is therefore an "abstract" class. In statically-typed Java, the class itself is declared as abstract, and in dynamic Ruby it must be extended (or else the Fighter will not know any techniques). To keep things simple, our fighters are not very smart or well-trained, so they each only get one Attack.

Class Diagram

Java

Fighter.java:

package csc517.wiki1b;
public abstract class Fighter {
   private Attack attack;
   public Fighter(){
      setAttack();
   }
   public Attack getAttack() {
      return attack;
   }
   public void setAttack(Attack attack) {
      this.attack = attack;
   }
   protected abstract void setAttack();
   public void fight() {
      getAttack().execute();
   }
}

JudoPlayer.java:

package csc517.wiki1b;
public class JudoPlayer extends Fighter {
   public void setAttack() {
      setAttack(new LegSweep());
   }
}

KickBoxer.java:

package csc517.wiki1b;
public class KickBoxer extends Fighter {
   public void setAttack() {
      setAttack(new RoundhouseKick());
   }
}

Boxer.java:

package csc517.wiki1b;
public class Boxer extends Fighter {
   public void setAttack() {
      setAttack(new Jab());
   }
}

Wrestler.java:

package csc517.wiki1b;
public class Wrestler extends Fighter {
   public void setAttack() {
      setAttack(new ShootIn());
   }
}

Attack.java:

package csc517.wiki1b;
public interface Attack {
   public void execute();
}

RoundhouseKick.java:

package csc517.wiki1b;
public class RoundhouseKick implements Attack {
   public void execute() {
      System.out.println("Roundhouse Kick!");
   }
}

ShootIn.java:

package csc517.wiki1b;
public class ShootIn implements Attack {
   public void execute() {
      System.out.println("Shoot In!");
   }
}

LegSweep.java:

package csc517.wiki1b;
public class LegSweep implements Attack {
   public void execute() {
      System.out.println("Leg Sweep!");
   }
}

Jab.java:

package csc517.wiki1b;
public class Jab implements Attack {
   public void execute() {
      System.out.println("Jab!");
   }
}

Fight.java:

package csc517.wiki1b;
public class Fight {
   public static void main(String[] args) {
      Fighter[] fighters = { new Boxer(), new Wrestler() };
      for (int round = 0; round < 3; round++) {
         for (int fighter = 0; fighter < fighters.length; fighter++) {
            fighters[fighter].fight();
         }
      }

   }
}

Ruby

Fighter.rb:

class Fighter
  attr_accessor :attack
  def initialize(&technique)
    @attack = technique
  end
  def fight
    @attack.call
  end
  def method_missing(method_name)
    puts "I don't know how to #{method_name}"
  end
end

JudoPlayer.rb:

require 'Fighter'
require 'LegSweep'
class JudoPlayer < Fighter
  include LegSweep
  def initialize
    super { leg_sweep }
  end
end

KickBoxer.rb:

require 'Fighter'
require 'RoundhouseKick'
class KickBoxer < Fighter
  include RoundhouseKick
  def initialize
    super { roundhouse_kick }
  end
end

Boxer.rb:

require 'Fighter'
require 'Jab'
class Boxer < Fighter
  include Jab
  def initialize
    super { jab }
  end
end

Wrestler.rb:

require 'Fighter'
require 'ShootIn'
class Wrestler < Fighter
  include ShootIn
  def initialize
    super { shoot_in }
  end
end

RoundhouseKick.rb:

module RoundhouseKick
  def roundhouse_kick
    puts "Roundhouse Kick!"
  end
end

ShootIn.rb:

module ShootIn
  def shoot_in
    puts "Shoot In!"
  end
end

LegSweep.rb:

module LegSweep
  def leg_sweep
    puts "Leg Sweep!"
  end
end

Jab.rb:

module Jab
  def jab
    puts "Jab!"
  end
end

Fight.rb:

require 'Boxer'
require 'Wrestler'
require 'KickBoxer'
require 'JudoPlayer'
rounds = 3
fighters = [ Boxer.new, Wrestler.new ]
rounds.times do
  fighters.each do |fighter|
    fighter.fight
  end
end

Comparison of Implementations

It would be nice to be able to say that one implementation was always clearer or more succinct that the other, but as with most things, it is a compromise. To get a little more clarity in Java, you sacrifice some succinctness, and in Ruby you can get more succinctness for the price of some clarity.

Clarity

The Java implementation relies less on language idioms, making it somewhat more clear (but not always). For example, to anyone familiar with O-O languages (but not necessarily Java), it is clear what each of the concrete Fighter types does, as well as the Attacks. However, the abstract Fighter class might not be as obvious (but probably still should be). In Ruby, the strategy is implemented as a Proc object, so code to "call" an attack may not be as self-commenting as to execute an Attack. Also, it is obvious that setAttack(new LegSweep()) sets the attack to be a leg sweep, but the call to super { leg_sweep } may not be.

Succinctness

In general, the Ruby implementation is more concise. Ruby's built-in iterators make it easier to work with collections, like arrays of Fighters, or rounds in a Fight. Also, the Ruby version contains 10 fewer lines of code than the Java implementation (13 fewer if you don't count the method_missing method added to Fighter that is called if you ask him to execute an attack he hasn't learned). However, if you ignore packaging (Java package statements), the difference is much less. Considering that, since Ruby does not need Interfaces there is one less file in the Ruby implementation, there is hardly any difference at all. The real advantage to using Ruby becomes more apparent when you extend the design to model more real-world situations. Then you can take advantage of its dynamic nature. For example, let's consider when a Fighter wants to learn new techniques, and be able to choose which one to use. In Java, you need to create new Attack implementations before you can use them in a Fight. Then you need to create a mechanism to choose which attack to use. In Ruby, you can create a new attack simply by passing a Proc object to the Fighter's attack= method at run time. That object can be either a method from a new Module, or created on the fly using Proc.new followed by a block. Also, if you wanted to combine several attacks into an arsenal from which the fighter could choose any attack, this could be easily accomplished using Ruby's arrays and built-in iterators.

References

Design Patterns, Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, Addison-Wesley Pulishing, 1995

Refactoring: Improving the Design of Existing Code, Martin Fowler, Addison-Wesley Publishing, 1999

Head First Design Patterns, Eric Freeman and Elisabeth Freeman with Kathy Sierra and Bert Bates, O'Reilly Media, 2004

Programming Ruby, Dave Thomas with Chad Fowler and Andy Hunt, The Pragmatic Programmers, 2005

CSC/ECE517 Lecture Notes (lecture 10), Edward F. Gehringer, 2007

Strategy Pattern - Wikipedia

Command vs Strategy Pattern? (OO, Patterns, UML and Refactoring forum at JavaRanch)

Rubynoob Strategy Design Pattern in Ruby

Re: Head First Design Patterns - Strategy Pattern [was: Java/C# "interface" in Ruby ?]