Strategy

• Encapsulate various algorithms to do more or less the same thing.
• Need one of several algorithms dynamically.
• The algorithms are exchangeable and vary independently
• Configure a class with one of many related classes (behaviors).
• Avoid exposing complex and algorithm-specific structures.
• Data is transparent to the clients.
• Reduce multiple conditional statements.
• Provide an alternative to subclassing.
• Related patterns include
  State, which can activate several states, whereas a strategy can only activate one of the algorithms.
  • Flyweight, which provides a shared object that can be used in multiple contexts simultaneously, whereas a strategy focuses on one context.
  • Decorator, which changes the skin of an object, whereas a strategy changes the guts of an object.
  • Composite, which is used to combine with a strategy to improve efficiency.

Example

Compress files using different algorithms or save files in different formats or draw graphic in different presentations. Here is a simple example. Just to show the concept of a strategy pattern.

interface FortuneCookies {
    public void print();
}
class Five implements FortuneCookies {
    public void print() {
        System.out.println("It is your turn to get it");
    }
}
class Two implements FortuneCookies {
    public void print() {
        System.out.println("It is never too late to start");
    }
}
class Null implements FortuneCookies {
    public void print() {
        System.out.println("You got nothing");    
    }
}
class Dice {
   public int throwIt() {
       return (int)(Math.random()*6)+1;
   }
}
//more class...
class Test {  
    static void goodFortune() {
        int luckyNum = new Dice().throwIt();
        FortuneCookies fc;
        switch (luckyNum) {
            case 2: fc = new Two();
                    break;
            case 5: fc = new Five();
                    break;
            //more
            default: fc = new Null();
        }
        fc.print();
    }
    public static void main(String[] args) {
        goodFortune();
    }
}

Return to top