Skip to main content

Checkpoint: The Zoo Census

beginner45 min readLesson 39 of 180

An abstract base, an extra interface capability, and an instanceof census — identity and capability side by side.

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.