Strategy Pattern

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

Strategy Pattern Diagram

Implementation

Creating a Duck interface:

public abstract class Duck {
	/* The behavior variables are declared as the behavior INTERFACE type	
	Instance veriables hold a reference to a specific behavior at runtime */
	FlyBehavior flyBehavior;
	QuackBehavior quackBehavior;
 
	/* Constructor.	
	The constructor name is equivalent to the class name.
	Even if you don't define this, Java defines it by default. */
	public Duck() {}
	
	public abstract void display();
 
	/* Replacing fly and quack methods.	
	Rather than handling the behavior itself, the Duck object delegates
	that behavior to the object referened by flyBehavior or quackBehavior */
	public void performFly() {
		flyBehavior.fly();
	}
	
	public void performQuack() {
		quackBehavior.quack();
	}
	
	public void swim() {
		System.out.println("All ducks float even decoys!");
	}
	
	/* Seeter methods
	With these setters, you can dynamically set behavior of duck at runtime */
	public void setFlyBehavior(FlyBehavior fb) {
		flyBehavior = fb;
	}
	
	public void setQuackBehavior(QuackBehavior qb) {
		quackBehavior = qb;
	}
}

Creating a Concrete instance:

public class MallardDuck extends Duck {
	public MallardDuck() {	
		/* Uses the Quack class to handle its quack, so when performQuack
		is called, the responsbility for the quack is delegated to the
		Quack object and we get a real quack. */
		quackBehavior = new Quack();
		flyBehavior = new FlyWithWings();
	}
	
	public void display() {
		System.out.println("I'm a real Mallard duck.");
	}
}

public class ModelDuck extends Duck {
	public ModelDuck() {
		flyBehavior = new FlyNoWay();
		quackBehavior = new Quack();
	}
	
	public void display() {
		System.out.println("I'm a model duck");	
	}
}

Creating Behavior interfaces:

public interface QuackBehavior {
	public void quack();
}

public interface FlyBehavior {
	public void fly();
}

Creating Concrete Behaviors:

public class FlyWithWings implements FlyBehavior {
	public void fly() {
		System.out.println("I'm flying");
	}
}
 
public class FlyNoWay implements FlyBehavior {
	public void fly() {
		System.out.println("I can't fly");
	}
}

public class Quack implements QuackBehavior {
	public void quack() {
		System.out.println("Quack");
	}
}
 
public class Squeak implements QuackBehavior {
	public void quack() {
		System.out.println("Squeak");
	}
}

Testing:

public class MiniDuckSimulator {
	public static void main(String[] args) {		
		Duck mallard = new MallardDuck();
		mallard.performQuack(); // Quack
		mallard.performFly(); // I'm flying!
		
		Duck model = new ModelDuck();
		model.performFly(); //
		model.setFlyBehavior(new FlyRocketPowered());
		model.performFly();
	}
}