Nested and Inner Classes

Classes can be nested in other classes:

1. class Outer {
2. private val bar: Int = 1
3. class Nested {
4. fun foo() = 2
5. }
6. }

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

You can also use interfaces with nesting. All combinations of classes and interfaces are possible: You can nest interfaces in classes, classes in interfaces, and interfaces in interfaces.

1. interface OuterInterface {
2. class InnerClass
3. interface InnerInterface
4. }

6. class OuterClass {
7. class InnerClass
8. interface InnerInterface
9. }

Inner classes

A nested class marked as inner can access the members of its outer class. Inner classes carry a reference to an object of an outer class:

1. class Outer {
2. private val bar: Int = 1
3. inner class Inner {
4. fun foo() = bar
5. }
6. }

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

See Qualified this expressions to learn about disambiguation of this in inner classes.

Anonymous inner classes

Anonymous inner class instances are created using an object expression:

1. window.addMouseListener(object : MouseAdapter() {

3. override fun mouseClicked(e: MouseEvent) { ... }

5. override fun mouseEntered(e: MouseEvent) { ... }
6. })

Note: on the JVM, 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:

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