Abstract Classes in Kotlin

abstract class Animal {
    abstract fun speak()   // no body — concrete
                           // subclasses must provide one
}
class Dog : Animal() {
    override fun speak() {
        println("Woof")
    }
}

Full lesson:

Sometimes you want to capture shared behavior across a family of classes without committing to one concrete implementation. For example, every Animal can speak(), but exactly how depends on whether it's a Dog or a Cat. That's what abstract classes are for.

Two superpowers

1. You can't write Animal() to create an object directly — the compiler stops you. You have to pick a concrete subclass.
2. Any subclass MUST implement every abstract method, or the compiler complains. The abstract class is a contract with teeth.

Mixing abstract and concrete

An abstract class can also have concrete methods (with bodies) and fields. Those are shared by all subclasses for free, just like normal inheritance. Only the methods marked abstract require an override.

Notes by language

• Kotlin — abstract on the class and on the method; a subclass writes override and calls the parent constructor, as in class Dog : Animal().
• C# / Java — keyword abstract on the class and on the method.
• Python — inherit from abc.ABC and decorate the method with @abstractmethod.
• JavaScript — no built-in support. The convention is a parent class whose method throws an error if not overridden.
• C++ — a method ending with = 0 is "pure virtual"; a class with any pure virtual method is abstract.

Your task

The abstract Animal class is already defined. Create a concrete Dog subclass and implement the speak method so it prints "Woof". The harness builds a Dog and calls speak().

Expected output:

Woof