Nested and Inner Classes

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

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

// Output:
2

All combinations of classes and interfaces are possible, You can nest

interfaces in classes
classes in interfaces
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()

// Output:
1

Anonymous inner classes

Anonymous inner class instances are created using an object expression:

window.addMouseListener(
    // Creates an instance of an anonymous inner class that extends `MouseAdapter`
    object : MouseAdapter() {
        override fun mouseClicked(e: MouseEvent) { ... }
        override fun mouseEntered(e: MouseEvent) { ... }
    }
)

Last modified: 04 March 2024