Checkpoint: The Zoo Census

The Zoo: one abstract base, three species, and a census. Inside Solution: 1. static abstract class Animal — abstract String sound(); and a public String toString() override returning sound() (so printing shows the sound). 2. static class Dog extends Animal — sound "woof". 3. static class Cat extends Animal — sound "meow". 4. static class Robot extends Animal — sound "beep", but Robot is NOT really an animal, so it must also implements Describable where interface Describable { String kind(); } returns "machine". 5. static int countRealAnimals(Animal[] zoo) — the number of animals in the array that implement Describable... **is zero logic**: instead count the ones whose sound() is a real animal sound. Simpler and honest: count entries that are NOT instanceof Describable (that is exactly the interface check the lesson allows here). So for { new Dog(), new Cat(), new Robot() }, countRealAnimals returns 2. The grader also checks polymorphism: every sound() must dispatch to the object's own class.

Difficulty: beginner