Lesson 18 - Abstract class, comparing and operators in Kotlin

In the previous lesson, Type casting and object hierarchy in Kotlin, we learned about type casting and created an object hierarchy with a common ancestor in Kotlin. In today's tutorial, we're going to continue with that project.

Abstract class

An abstract class is a class whose instance doesn't make sense. Mainly because it's too vague, e.g. our Animal class. The point is to call on specific Animal descendants, e.g. a dog, so we won't actually instantiate the Animal class directly ever. In other words, it's better to prevent any instantiation of the class overall. To do that, we simply add the abstract modifier before the Animal class:

abstract class Animal(var weight: Int) {
    // . . .

The program works the same as before, but if we try to create an instance of the Animal class, we'll get an error:

// this code won't work
animals += Animal(10)

Other than preventing instantiation, abstract classes can do several other things

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