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) { ... }
})

Last modified: 25 September 2024