Strategy Pattern

Definition-defines a family of algorithms, encapsulate each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Key points: separate different parts/behaviors and encapsulate each part/behavior

Advantage: not only does it let you encapsulate a family of algorithms into their own set of classes, but it also lets you change behavior at runtime .

Example1 (quote from First Head In Design Pattern) :

//inteface code

 

//FlyBehavior.java
public interface FlyBehavior ...{
    public void Fly();
}
//QuakeBehavior.java
public interface QuakeBehavior ...{
    public void Quake();
}
//Duck.java
public abstract class Duck ...{
    FlyBehavior flyBehavior;
    QuakeBehavior quakeBehavior;
    public void performFly()
    ...{
        flyBehavior.Fly();
    }
    public void performQuake()
    ...{
        quakeBehavior.Quake();
    }
    public abstract void Display();
    public void SetFlyBehavior(FlyBehavior flyb)
    ...{
        this.flyBehavior = flyb;
    }
    public void SetQuakeBehavior(QuakeBehavior quakeb)
    ...{
        this.quakeBehavior = quakeb;
    }
}

 

public class FlyWithWings implements FlyBehavior ...{

    public void Fly() ...{
        System.out.println("Fly with wings");
    }
}
public class FlyWithNoWings implements FlyBehavior ...{
    public void Fly() ...{
        System.out.println("Fly without wings");
    }

}
public class NormalQuake implements QuakeBehavior ...{

    @Override
    public void Quake() ...{
        System.out.println("Normal Quake");
    }

}
public class MuteQuake implements QuakeBehavior ...{

    @Override
    public void Quake() ...{
        // TODO Auto-generated method stub
        System.out.println("Mute Quake");
    }

}
public class MallardDuck extends Duck ...{

    @Override
    public void Display() ...{
        // TODO Auto-generated method stub
        System.out.println("I'm Mallard Duck");
    }

}
public class MiniDuckSimulator extends Duck ...{

    @Override
    public void Display() ...{
        // TODO Auto-generated method stub
        System.out.println("I'm mini-duck");
    }

}

//display result

 

public class Main ...{

    public static void main(String[] args) ...{
        // TODO Auto-generated method stub
        Duck mallardDuck=new MallardDuck();
        Duck miniDuck=new MiniDuckSimulator();
        
        FlyBehavior flyWithWings=new FlyWithWings();
        FlyBehavior flyWithNoWings=new FlyWithNoWings();
        QuakeBehavior normalQuake=new NormalQuake();
        QuakeBehavior muteQuake=new MuteQuake();
        
        //Customize
        mallardDuck.SetFlyBehavior(flyWithWings);
        mallardDuck.SetQuakeBehavior(normalQuake);
        
        miniDuck.SetFlyBehavior(flyWithNoWings);
        miniDuck.SetQuakeBehavior(muteQuake);
        
        //perfomance and display
        mallardDuck.Display();
        mallardDuck.performFly();
        mallardDuck.performQuake();
        miniDuck.Display();
        miniDuck.performFly();
        miniDuck.performQuake();
    }

}

Example2 (talk about PetShop4.0 developed by MS):

When inserting Order, PetShop System enable user customize which strategy he want to use, synchronization or asynchronization. i.e. the insert-order strategy is changeable at runtime, determined by user himself.