Add w6 folder with animal sound system

w6/Animal.java

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+public abstract class Animal {
+    public abstract void makeSound();
+}

w6/Cat.java

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+public class Cat extends Animal {
+    @Override
+    public void makeSound() {
+        System.out.println("Meow!");
+    }
+}

w6/Dog.java

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+public class Dog extends Animal implements Swimmable {
+    @Override
+    public void makeSound() {
+        System.out.println("Woof!");
+    }
+
+    @Override
+    public void swim() {
+        System.out.println("Dog is swimming");
+    }
+}

w6/Main.java

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+public class Main {
+    public static void main(String[] args) {
+        // Test polymorphism with Animal references
+        Animal dog = new Dog();
+        Animal cat = new Cat();
+
+        System.out.println("Testing makeSound():");
+        dog.makeSound(); // Should output "Woof!"
+        cat.makeSound(); // Should output "Meow!"
+
+        // Test swim() method for Dog
+        System.out.println("\nTesting swim():");
+        if (dog instanceof Swimmable) {
+            Swimmable swimmableDog = (Swimmable) dog;
+            swimmableDog.swim(); // Should output "Dog is swimming"
+        }
+
+        // Test that Cat doesn't implement Swimmable
+        if (!(cat instanceof Swimmable)) {
+            System.out.println("Cat cannot swim");
+        }
+    }
+}

w6/Swimmable.java

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+public interface Swimmable {
+    void swim();
+}