17. An Introduction to Kotlin Inheritance and Subclassing

In “The Basics of Object Oriented Programming in Kotlin” we covered the basic concepts of object-oriented programming and worked through an example of creating and working with a new class using Kotlin. In that example, our new class was not specifically derived from a base class (though in practice, all Kotlin classes are ultimately derived from the Any class). In this chapter we will provide an introduction to the concepts of subclassing, inheritance and extensions in Kotlin.

17.1 Inheritance, Classes and Subclasses

The concept of inheritance brings something of a real-world view to programming. It allows a class to be defined that has a certain set of characteristics (such as methods and properties) and then other classes to be created which are derived from that class. The derived class inherits all of the features of the parent class and typically then adds some features of its own. In fact, all classes in Kotlin are ultimately subclasses of the Any superclass which provides the basic foundation on which all classes are based.

By deriving classes we create what is often referred to as a class hierarchy. The class at the top of the hierarchy is known as the base class or root class and the derived classes as subclasses or child classes. Any number of subclasses may be derived from a class. The class from which a subclass is derived is called the parent class or superclass.

Classes need not only be derived from a root class. For example, a subclass...

17.2 Subclassing Syntax

As a safety measure designed to make Kotlin code less prone to error, before a subclass can be derived from a parent class, the parent class must be declared as open. This is achieved by placing the open keyword within the class header:

open class MyParentClass {

    var myProperty: Int = 0

}

With a simple class of this type, the subclass can be created as follows:

class MySubClass : MyParentClass() {

 

}

For classes containing primary or secondary constructors, the rules for creating a subclass are slightly more complicated. Consider the following parent class which contains a primary constructor:

open class MyParentClass(var myProperty: Int) {

             

}

In order to create a subclass of this class, the subclass declaration references any base class parameters while...

17.3 A Kotlin Inheritance Example

As with most programming concepts, the subject of inheritance in Kotlin is perhaps best illustrated with an example. In “The Basics of Object Oriented Programming in Kotlin” we created a class named BankAccount designed to hold a bank account number and corresponding current balance. The BankAccount class contained both properties and methods. A simplified declaration for this class is reproduced below and will be used for the basis of the subclassing example in this chapter:

class BankAccount {

 

    var accountNumber = 0

    var accountBalance = 0.0

    

    constructor(number: Int, balance: Double) {

        accountNumber = number

        accountBalance = balance

    }

    

    ...

17.4 Extending the Functionality of a Subclass

So far we have been able to create a subclass that contains all the functionality of the parent class. In order for this exercise to make sense, however, we now need to extend the subclass so that it has the features we need to make it useful for storing savings account information. To do this, we simply add the properties and methods that provide the new functionality, just as we would for any other class we might wish to create:

class SavingsAccount : BankAccount {

         var interestRate: Double = 0.0

 

    constructor(accountNumber: Int, accountBalance: Double) :

                           super(accountNumber, accountBalance)

 

    fun calculateInterest(): Double

   ...

17.5 Overriding Inherited Methods

When using inheritance it is not unusual to find a method in the parent class that almost does what you need, but requires modification to provide the precise functionality you require. That being said, it is also possible you’ll inherit a method with a name that describes exactly what you want to do, but it actually does not come close to doing what you need. One option in this scenario would be to ignore the inherited method and write a new method with an entirely new name. A better option is to override the inherited method and write a new version of it in the subclass.

Before proceeding with an example, there are three rules that must be obeyed when overriding a method. First, the overriding method in the subclass must take exactly the same number and type of parameters as the overridden method in the parent class. Second, the new method must have the same return type as the parent method. Finally, the original method in the parent class...

17.6 Adding a Custom Secondary Constructor

As the SavingsAccount class currently stands, it makes a call to the secondary constructor from the parent BankAccount class which was implemented as follows:

constructor(accountNumber: Int, accountBalance: Double) :

super(accountNumber, accountBalance)

Clearly this constructor takes the necessary steps to initialize both the account number and balance properties of the class. The SavingsAccount class, however, contains an additional property in the form of the interest rate variable. The SavingsAccount class, therefore, needs its own constructor to ensure that the interestRate property is initialized when instances of the class are created. Modify the SavingsAccount class one last time to add an additional secondary constructor allowing the interest rate to also be specified when class instances are initialized:

class SavingsAccount : BankAccount {

 

    var interestRate: Double = 0.0

 ...

17.7 Using the SavingsAccount Class

Now that we have completed work on our SavingsAccount class, the class can be used in some example code in much the same way as the parent BankAccount class:

val savings1 = SavingsAccount(12311, 600.00, 0.07)

 

println(savings1.calculateInterest())

savings1.displayBalance()

17.8 Summary

Inheritance extends the concept of object re-use in object oriented programming by allowing new classes to be derived from existing classes, with those new classes subsequently extended to add new functionality. When an existing class provides some, but not all, of the functionality required by the programmer, inheritance allows that class to be used as the basis for a new subclass. The new subclass will inherit all the capabilities of the parent class, but may then be extended to add the missing functionality.