add polymorphism code

oop_practice/src/main/java/Polymorphism/Main.java

@@ -0,0 +1,31 @@
+package Polymorphism;
+
+class Animal {
+    public void animalSound() {
+        System.out.println("An animal can make a sound.");
+    }
+}
+
+class Cow extends Animal {
+    public void animalSound() {
+        System.out.println("A cow says: Moooo");
+    }
+}
+
+class Cat extends Animal {
+    public void animalSound() {
+        System.out.println("A cat says: Meeooww");
+    }
+}
+
+class Main {
+    public static void main(String[] args) {
+        Animal myAnimal = new Animal();
+        Animal myCow = new Cow();
+        Animal myCat = new Cat();
+
+        myAnimal.animalSound();
+        myCow.animalSound();
+        myCat.animalSound();
+    }
+}