What are nested and inner classes in Kotlin?

A nested class does not have access to the members of the outer class. It is simply a class declared inside another.

class Outer {
    private val outerParameter = 1
    class Nested {
        fun printSomething() {
            // Compilation error: Cannot access 'outerParameter': it is private in 'Outer'
            // println(outerParameter)
            println("Hello from nested class")
        }
    }
}

An inner class has access to the members of the outer class, including private ones. This is achieved using the inner keyword. An instance of an inner class is always associated with an instance of the outer class.

class Outer {
    private val outerParameter = 1
    inner class Inner {
        fun printSomething() {
            // Access to outer class member
            println("Outer parameter is $outerParameter")
            println("Hello from inner class")
        }
    }
}

Main differences:

Feature	Nested Class	Inner Class
Access to outer	No	Yes (through an instance of the outer class)
Relationship with instance	Not linked to an instance of the outer class	Linked to an instance of the outer class
Declaration	Does not require a keyword (default is nested)	Requires the inner keyword
Usage	Can be used as a static equivalent (in Java context)	Cannot be static
Instance creation	Outer.Nested()	Outer().Inner()