Nested and Inner 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

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.

interface OuterInterface {
    class InnerClass
    interface InnerInterface
}
class OuterClass {
    class InnerClass
    interface InnerInterface
}

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:

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

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

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:

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