Class Members

Classes can contain:

  • Constructors
  • Constructor initializer blocks
  • Functions
  • Properties
  • Nested and Inner Classes
  • Object Declarations


Basic constructor syntax:

class Person

class Person (name: String, age: Int) {
    // class code here ...

The constructor keyword is only needed if the class has annotations or visibility modifiers:

class Person private constructor (name: String) { ... }
class Person public @Inject constructor (name: String) { ... }
class Person @JvmOverloads constructor (name: String, age: Int) { ... }

Primary constructor initialization code goes in an init block:

class Person (name: String) {
    init {
        println("Person instance created")

Initializer blocks

Classes can have initializer blocks that are called when the class is constructed, and those blocks can access the constructor parameters:

// a network socket
class Socket(var timeout: Int, var linger: Int) {
    init {
        println("Entered 'init' ...")
        println("timeout = ${timeout}, linger = ${linger}")

Here’s what it looks like when a Socket is created:

> val s = Socket(2000, 3000)
Entered 'init' ...
timeout = 2000, linger = 3000

You can have multiple initializer blocks, and the code in those blocks is effectively part of the primary constructor.

Methods (functions)

Just like Java and other languages, classes can have methods (which are referred to as functions):

class Person(val firstName: String, val lastName: String) {
    fun fullName() = "$firstName $lastName"

Here’s a REPL example:

> val p = Person("Hala", "Cina")
> p.fullName()
Hala Cina

Properties (fields)

Just like Java and other languages, classes can have properties (fields):

class Pizza () {
    val toppings = mutableListOf<String>()
    fun addTopping(t: String) { toppings.add(t) }
    fun showToppings() { println(toppings) }
    // more code ...

Here’s an example in the REPL:

> val p = Pizza()
> p.addTopping("Cheese")
> p.addTopping("Pepperoni")
> p.showToppings()
[Cheese, Pepperoni]

Nested and inner classes

Classes can be nested in other classes. Note that a nested class can’t access a parameter in the outer class:

class Outer {
    private val x = 1
    class Nested {
        //fun foo() = 2 * x   //this won’t compile
        fun foo() = 2

In the REPL:

> val foo = Outer.Nested().foo()
> foo

Inner classes

Mark a class as inner if you need to access members of the outer class:

class Outer {
    private val x = 1
    inner class Inner {
        fun foo() = x * 2

In the REPL:

> val foo = Outer().Inner().foo()
> foo


Notice the difference between the syntax when using a nested class versus an inner class:

val foo = Outer.Nested().foo()
val foo = Outer().Inner().foo()

If you try to create an inner class without first creating an instance of the outer class you’ll see this error:

> val foo = Outer.Inner().foo()
error: constructor of inner class Inner can be called only with receiver of containing class
val foo = Outer.Inner().foo()

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