Strategy Design Pattern Overview

• Defines a family of algorithms, encapsulates each one, and makes them interchangeable.
• Identify the aspects of your application that vary and separate them from what stays the same.
• The varying aspect is pulled out so it can be reused (avoid code duplication).
• No modification on Dog classes required when adding new or modifying existing bark behavior.
• Dynamically change bark behavior during runtime.

Before

public class Dog {

    public void bark() {
        System.out.println("grr");
    };

    public void intro() {
        System.out.println("Dog is a domesticated carnivorous mammal.");
    }
}

public class GermanShepherd extends Dog {

    @Override
    public void bark() {
        System.out.println("woof");
    }

    @Override
    public void intro() {
        System.out.println("German Shepherds are working dogs developed originally for herding sheep.");
    }
}

public class GoldenRetriever extends Dog {

    @Override
    public void bark() {
        System.out.println("woof");
    }

    @Override
    public void intro() {
        System.out.println("Golden Retriever are gun dogs trained to retrieve shot waterfowl such as ducks.");
    }
}

public class RubberDog extends Dog {

    @Override
    public void bark() {
        System.out.println("I can't bark.");
    }

    @Override
    public void intro() {
        System.out.println("A toy used in bath.");
    }
}

public class Simulator {
    public static void main(String[] args) {
        Dog dog = new RubberDog();
        dog.bark(); // I can't bark.
    }
}

After

interface BarkBehavior {
    void bark();
}

public class BarkGrr implements BarkBehavior {
    @Override
    void bark() {
        System.out.println("grr");
    }
}

public class BarkMute implements BarkBehavior {
    @Override
    void bark() {
        System.out.println("I can't bark.");
    }
}

public class BarkWoof implements BarkBehavior {
    @Override
    void bark() {
        System.out.println("woof");
    }
}

public class Dog {

    BarkBehavior barkBehavior;

    public void setBarkBehavior(BarkBehavior bb) {
        barkBehavior = bb;
    }

    public Dog() {
        barkBehavior = new BarkGrr();
    }

    public void bark() {
        barkBehavior.bark();
    };

    public void intro() {
        System.out.println("Dog is a domesticated carnivorous mammal.");
    }
}

public class GermanShepherd extends Dog {

    public GermanShepherd() {
        barkBehavior = new BarkWoof();
    }

    @Override
    public void intro() {
        System.out.println("German Shepherds are working dogs developed originally for herding sheep.");
    }
}

public class GoldenRetriever extends Dog {

    public GoldenRetriever() {
        barkBehavior = new BarkWoof();
    }

    @Override
    public void intro() {
        System.out.println("Golden Retriever are gun dogs trained to retrieve shot waterfowl such as ducks.");
    }
}

public class RubberDog extends Dog {

    public RubberDog() {
        barkBehavior = new BarkMute();
    }

    @Override
    public void intro() {
        System.out.println("A toy used in bath.");
    }
}

public class Simulator {
    public static void main(String[] args) {
        Dog dog = new RubberDog();
        dog.bark(); //I can't bark.
        dog.setBarkBehavior(new BarkWoof());
        dog.bark(); //woof
    }
}