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Classes and Inheritance

Classes

Classes in Kotlin are declared using the keyword class:

class Invoice {
}

The class declaration consists of the class name, the class header (specifying its type parameters, the primary constructor etc.) and the class body, surrounded by curly braces. Both the header and the body are optional; if the class has no body, curly braces can be omitted.

class Empty

Constructors

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

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

class Customer(name: String) {
    init {
        logger.info("Customer initialized with value ${name}")
    }
}

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 firstName: String, val lastName: String, var age: Int) {
    // ...
}

Much the same way as regular properties, the properties declared in the primary constructor can be mutable (var) or read-only (val).

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.

Secondary Constructors

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)
    }
}

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.

Creating instances of nested, inner and anonymous inner classes is described in Nested classes.

Class Members

Classes can contain:

Inheritance

All classes in Kotlin have a common superclass Any, that is a default super for a class with no supertypes declared:

class Example // Implicitly inherits from Any

Any is not java.lang.Object; in particular, it does not have any members other than equals(), hashCode() and toString(). Please consult the Java interoperability section for more details.

To declare an explicit supertype, we place the type after a colon in the class header:

open class Base(p: Int)

class Derived(p: Int) : Base(p)

If the class has a primary constructor, the base type can (and must) be initialized right there, using the parameters of the primary constructor.

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)
}

The open annotation on a class is the opposite of Java's final: it allows others to inherit from this class. By default, all classes in Kotlin are final, which corresponds to Effective Java, Item 17: Design and document for inheritance or else prohibit it.

Overriding Methods

As we mentioned before, we stick to making things explicit in Kotlin. And 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
}

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<Base>:

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()
}

Companion Objects

In Kotlin, unlike Java or C#, classes do not have static methods. In most cases, it's recommended to simply use package-level functions instead.

If you need to write a function that can be called without having a class instance but needs access to the internals of a class (for example, a factory method), you can write it as a member of an object declaration inside that class.

Even more specifically, if you declare a companion object inside your class, you'll be able to call its members with the same syntax as calling static methods in Java/C#, using only the class name as a qualifier.