Kotlin Classes & Inheritance

In my previous article I talked about how Kotlin saves us writing boilerplate codes the one we have to write in Java and there I gave an example of a simple class showing the differences in class structures in both languages. Now let’s know more facts about Kotlin classes and functions in this article.

How to declare a class

If you want to declare a class, you just need to use the keyword class:

 class MainActivity { 
}

Classes have a unique default constructor. We’ll see that we can create extra constructors for some exceptional cases, but keep in mind that most situations only require a single constructor.

If the class has no body, curly braces can be omitted.

class Empty

Parameters are written just after the name. Brackets are not required if the class doesn’t have any content:

class Person(name: String, surname: String)

Constructors

A class in Kotlin can have a primary constructor and one or more secondary constructors.

The primary constructor

The primary constructor is part of the class header: it goes after the class name (and optional type parameters).

class Person constructor(firstName: String) {
}

If the primary constructor does not have any annotations or visibility modifiers, the constructor keyword can be omitted:

class Person(firstName: String) {
}

But if the constructor has annotations or visibility modifiers, the constructor keyword is required, and the modifiers go before it:

class Customer public @Inject constructor(name: String) { ... }

For more details, see Visibility Modifiers.

The primary constructor cannot contain any code. Initialization code can be placed in initializer blocks, which are prefixed with the init keyword.

During an instance initialization, the initializer blocks are executed in the same order as they appear in the class body, interleaved with the property initializers:

class InitOrderDemo(name: String) {
val firstProperty = "First property: $name".also(::println)
init {
println("First initializer block that prints ${name}")
}
val secondProperty = "Second property:${name.length}".also(::println)
init {
println("Second initializer block that prints ${name.length}")
}
}

Note that parameters of the primary constructor can be used in the initializer blocks. They can also be used in property initializers declared in the class body:

class Customer(name: String) {
    val customerKey = name.toUpperCase()
}

In fact, for declaring properties and initializing them from the primary constructor, Kotlin has a concise syntax:

class Person(val name: String, val surname: String) {
    // ...
}

For example the code above is equivalent to the code shown below in Java:

public class Person{
private String name;
private String surname;

public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSurname() {
return surname;
}
public void setSurname(String surname) {
this.name = surname;
}

You can access these properties (fields in kotlin are called properties) just using the names

val person = Person()
person.name = “shashank”
val name = person.name

Secondary Constructor

The class can also declare secondary constructors, which are prefixed with constructor:

class Person {
    constructor(parent: Person) {
        parent.children.add(this)
    }
}

If the class has a primary constructor, each secondary constructor needs to delegate to the primary constructor, either directly or indirectly through another secondary constructor(s). Delegation to another constructor of the same class is done using the this keyword:

class Person(val name: String) {
    constructor(name: String, parent: Person) : this(name) {
        parent.children.add(this)
    }
}

Note that code in initializer blocks effectively becomes part of the primary constructor. Delegation to the primary constructor happens as the first statement of a secondary constructor, so the code in all initializer blocks is executed before the secondary constructor body. Even if the class has no primary constructor, the delegation still happens implicitly, and the initializer blocks are still executed:

class Constructors {
init {
println("Init block")
}
constructor(i: Int) {
println("Constructor")
}
}

If a non-abstract class does not declare any constructors (primary or secondary), it will have a generated primary constructor with no arguments. The visibility of the constructor will be public. If you do not want your class to have a public constructor, you need to declare an empty primary constructor with non-default visibility:

class DontCreateMe private constructor () {
}

NOTE: On the JVM, if all of the parameters of the primary constructor have default values, the compiler will generate an additional parameterless constructor which will use the default values. This makes it easier to use Kotlin with libraries such as Jackson or JPA that create class instances through parameterless constructors.

class Customer(val customerName: String = "")

Creating instances of classes

To create an instance of a class, we call the constructor as if it were a regular function:

val invoice = Invoice()
val customer = Customer("Joe Smith")

Note that Kotlin does not have a new keyword.

Class Inheritance

By default, a class always extends from Any (similar to Java Object), but we can extend any other classes.

Note: Any is not java.lang.Object; in particular, it does not have any members other than equals(), hashCode() and toString().

Classes are closed by default (final), so we can only extend a class if it’s explicitly declared as open or abstract:

open class Animal(name: String) 
class Person(firstName: String, lastName: String) :Animal(firstName)

Note that when using the single constructor nomenclature, we need to specify the parameters we’re using for the parent constructor. That’s equivalent to calling super() in Java.

If the class has no primary constructor, then each secondary constructor has to initialize the base type using the super keyword, or to delegate to another constructor which does that. Note that in this case different secondary constructors can call different constructors of the base type:

class MyView : View {
    constructor(ctx: Context) : super(ctx)
    constructor(ctx: Context, attrs: AttributeSet) : super(ctx, attrs)
}

Overriding Methods

Unlike Java, Kotlin requires explicit annotations for overridable members (we call them open) and for overrides:

open class Base {
    open fun v() {}
    fun nv() {}
}

class Derived() : Base() {
    override fun v() {}
}

The override annotation is required for Derived.v(). If it were missing, the compiler would complain. If there is no open annotation on a function, like Base.nv(), declaring a method with the same signature in a subclass is illegal, either with override or without it. In a final class (e.g. a class with no open annotation), open members are prohibited.

A member marked override is itself open, i.e. it may be overridden in subclasses. If you want to prohibit re-overriding, use final:

open class AnotherDerived() : Base() {
    final override fun v() {}
}

Overriding Properties

Overriding properties works in a similar way to overriding methods; properties declared on a superclass that are then redeclared on a derived class must be prefaced with override, and they must have a compatible type. Each declared property can be overridden by a property with an initializer or by a property with a getter method.

open class Foo {
    open val x: Int get() { ... }
}

class Bar1 : Foo() {
    override val x: Int = ...
}

You can also override a val property with a var property, but not vice versa. This is allowed because a val property essentially declares a getter method, and overriding it as a var additionally declares a setter method in the derived class.

Note that you can use the override keyword as part of the property declaration in a primary constructor.

interface Foo {
    val count: Int
}

class Bar1(override val count: Int) : Foo

class Bar2 : Foo {
    override var count: Int = 0
}

Derived class initialization order

During construction of a new instance of a derived class, the base class initialization is done as the first step (preceded only by evaluation of the arguments for the base class constructor) and thus happens before the initialization logic of the derived class is run.

open class Base(val name: String) {
init { println(“Initializing Base”) }
open val size: Int = 
 name.length.also { println(“Initializing size in Base: $it”) }
}

class Derived(
 name: String,
 val lastName: String
) : Base(name.capitalize().also { println(“Argument for Base: $it”) }) {
init { println(“Initializing Derived”) }
override val size: Int =
 (super.size + lastName.length).also { println(“Initializing size in Derived: $it”) }
}

fun main(args: Array<String>) {
 println(“Constructing Derived(\”hello\”, \”world\”)”)
 val d = Derived(“hello”, “world”)
}

Output:

Constructing Derived(“hello”, “world”)
Argument for Base: Hello 
Initializing Base 
Initializing size in Base: 5 
Initializing Derived 
Initializing size in Derived: 10

It means that, by the time of the base class constructor execution, the properties declared or overridden in the derived class are not yet initialized. If any of those properties are used in the base class initialization logic (either directly or indirectly, through another overridden open member implementation), it may lead to incorrect behavior or a runtime failure. Designing a base class, you should therefore avoid using openmembers in the constructors, property initializers, and init blocks.

Calling the superclass implementation

Code in a derived class can call its superclass functions and property accessors implementations using the super keyword:

open class Foo {
    open fun f() { println("Foo.f()") }
    open val x: Int get() = 1
}

class Bar : Foo() {
    override fun f() { 
        super.f()
        println("Bar.f()") 
    }
    
    override val x: Int get() = super.x + 1
}

Inside an inner class, accessing the superclass of the outer class is done with the super keyword qualified with the outer class name: super@Outer:

class Bar : Foo() {
    override fun f() { /* ... */ }
    override val x: Int get() = 0
    
    inner class Baz {
        fun g() {
            super@Bar.f() // Calls Foo's implementation of f()
            println(super@Bar.x) // Uses Foo's implementation of x's getter
        }
    }
}

Overriding Rules

In Kotlin, implementation inheritance is regulated by the following rule: if a class inherits many implementations of the same member from its immediate superclasses, it must override this member and provide its own implementation (perhaps, using one of the inherited ones). To denote the supertype from which the inherited implementation is taken, we use super qualified by the supertype name in angle brackets, e.g. super:

open class A {
    open fun f() { print("A") }
    fun a() { print("a") }
}

interface B {
    fun f() { print("B") } // interface members are 'open' by default
    fun b() { print("b") }
}

class C() : A(), B {
    // The compiler requires f() to be overridden:
    override fun f() {
        super<\A>.f() // call to A.f()
        super<\B>.f() // call to B.f()
    }
}

It’s fine to inherit from both A and B, and we have no problems with a() and b() since C inherits only one implementation of each of these functions. But for f() we have two implementations inherited by C, and thus we have to override f() in C and provide our own implementation that eliminates the ambiguity.

Abstract Classes

A class and some of its members may be declared abstract. An abstract member does not have an implementation in its class. Note that we do not need to annotate an abstract class or function with open — it goes without saying.

We can override a non-abstract open member with an abstract one

open class Base {
    open fun f() {}
}

abstract class Derived : Base() {
    override abstract fun f()
}

This covered most of the concepts related to Classes and Inheritance in Kotlin. For more details visit Kotlin documentation.

Comment below if something needs to be corrected.

References:

1. Official Kotlin Documentation
2. Book — Kotlin for android developers by Antonio Levia