Strategy Pattern

Behavioral Pattern

Strategy Pattern

Creational patterns Structural patterns Behavioral pattern J2EE patterns

Abstract Factory Adapter Chain of Responsibility
MVC

Builder Bridge Command Business Delegate

Factory method Composite Interpreter Composite Entity

Prototype Decorator Iterator Data Access Object

Singleton Façade
Mediator Front Controller

Flyweight Memento Intercepting Filter

Proxy Observer Service Locator

State Transfer Object

Strategy

Template Method

Visitor

Strategy Pattern

Define strategy pattern

Group several algorithms in a single module to provide alternatives. Also known as policy.

Where to use & benefits

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 of strategy pattern

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();
    }
}

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