Kotlin Programming: The Big Nerd Ranch Guide, 2nd Edition

Creating a Subclass

A subclass shares all properties with the class it inherits from, commonly known as the parent class or superclass.

For example, citizens of NyetHack will need a town square. A town square is a type of Room that has the features of a Room plus special features only town squares will have – like a welcome message that greets entering players. To create the TownSquare class, you will subclass Room, since they have common features, and then describe how TownSquare differs from Room.

But before defining a TownSquare class, you need to make a change to the Room class so that it can be subclassed.

Not every class you write is intended to be part of a hierarchy. By default, classes are closed for extension – meaning ...