What is the role of dividing classes into inner and nested classes, and what is it used for?

In Kotlin, there are two types of nested classes: nested and inner classes.

• Nested class — a class declared inside another class without the inner keyword. Such a class does not have a reference to an instance of the outer class and cannot access its members directly.

• Inner class — declared with the inner keyword. It contains an implicit reference to an instance of the outer class and can access its properties and methods.

Role and application:

• Nested classes are used for logical grouping of classes that are closely related to the outer class but do not require access to its instance.
• Inner classes are used when the nested class needs access to the state of the outer object.

Example:

class Outer {
    private val outerData = "Outer class data"

    class Nested {
        fun nestedFunction() = "Nested class"
    }

    inner class Inner {
        fun innerFunction() = "Inner class and access to: $outerData"
    }
}

fun main() {
    val nested = Outer.Nested()
    println(nested.nestedFunction()) // Nested class

    val inner = Outer().Inner()
    println(inner.innerFunction()) // Inner class and access to: Outer class data
}

Thus, the distinction helps manage scope and the relationship between classes, improving code organization.