Kotlin Programming Cookbook

How to do it…

A class derived from another class is called a subclass, whereas the class from which a subclass is derived is called a superclass. In this example, we will create a superclass A and a subclass B. To extend class B, we need to use : in the class declaration, and then add the superclass name with its primary constructor. Let's take a look at the following steps:

1. A key thing to remember is that classes in Kotlin are closed for extension by default, so we need to open them by adding the open keyword before the class declaration. So our superclass A looks like this:

open class A

2. Then, we can extend our class B, as follows:

class B:A()

3. Now, suppose our class A has a primary constructor that takes in a String variable, such as ...