Nested Classes

Classes can be nested in other classes

class Outer {
  private val bar: Int = 1
  class Nested {
    fun foo() = 2
  }
}

val demo = Outer.Nested().foo() // == 2

Inner classes

A class may be marked as inner{: .keyword } to be able to access members of outer class. Inner classes carry a reference to an object of an outer class:

class Outer {
  private val bar: Int = 1
  inner class Inner {
    fun foo() = bar
  }
}

val demo = Outer().Inner().foo() // == 1

See Qualified this{: .keyword } expressions to learn about disambiguation of this{: .keyword } in inner classes.

Anonymous inner classes

Anonymous inner class instances are created using an object expression:

window.addMouseListener(object: MouseAdapter() {
  override fun mouseClicked(e: MouseEvent) {
    // ...
  }

  override fun mouseEntered(e: MouseEvent) {
    // ...
  }
})

If the object is an instance of a functional Java interface (i.e. a Java interface with a single abstract method), you can create it using a lambda expression prefixed with the type of the interface:

val listener = ActionListener { println("clicked") }