Strategy Pattern

• Strategy lets the algorithm vary independently from the clients that use it.
• For e.g. let's say we have a base class that has defined a method called sort.
• Now many subclasses inherit from it. Probably not all the subclasses are sortable, so it may not make sense to add sort() in the base class.
• Alternatively an interface called sortable can be defined, and only sortable subclasses can implement the interface.
• The disadvantage with this is every time sort algorithm needs to change, all the base classes that are sortable need to change - maintenance headache.
• A solution is to encapsulate the sort algorithm into a separate interface and concrete classes.

interface SortAlgo {
   public void sort();
}

class QuickSort implements SortAlgo {
   public void sort() {
      //....
   }
}

class BubbleSort implements SortAlgo {
   public void sort() {
      //....
   }
}

class Base {
  SortAlgo sortAlgo;

  public void doSort() {
      sortAlgo.sort();
  }

}

class Sub extends Base {
   public Sub() {
       sortAlgo = new QuickSort();
   }

}

• Another example from Wikipedia:

http://upload.wikimedia.org/wikipedia/commons/4/4b/StrategyPattern_IBrakeBehavior.svg