kotlin constructor properties function nested class inner class

Classes can contain:

  1. Constructors
  2. Constructor initializer blocks
  3. Functions
  4. Properties
  5. Nested and Inner Classes (That we will talk about here)
  6. Object Declarations

Nested and inner classes

Nested class

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



That gives in repl:

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



Inner class

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



Gives in repl:

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

foo 2

You can see the difference between the syntax when you use a nested class versus when you use 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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