Kotlin - Inner & Nested Class Tutorial

In this article we are going to more about kotlin class and object with Inner and Nested class.

Nested Class:
Class which is created inside another class, is called as a nested class. Nested Class in kotlin, can be accessed without creating any object of that class, because it is by default static.
Note: Kotlin nested Class is similar to static nested class in Java.

Example:

class outerClass {

    val a = "Outside Of Nested class."

    class nestedClass {
        val b = "Inside Nested class."
        fun callMe() = "Function, inside Nested class."
    }
}

fun main(args: Array<String>) {
     // accessing member of Outer Class 
    println(outerClass().a)
    // accessing member of Nested Class
    println(outerClass.nestedClass().b)
    // creating object of Nested Class 
    val nested = outerClass.nestedClass()

    // accessing function of Nested Class
    println(nested.callMe())
}

Output:
Outside Of Nested class.
Inside Nested class.
Function, inside Nested class.

Inner Class:
Inner class is a nested class which is marked with an "Inner" keyword. Once you marked a nested class with an "Inner", you will not have to create an objet object for Nested class.

Example.

class outerClass {

    val a = "Outside Of Nested class."

  inner class nestedClass {
        fun callMe() = a
    }
}

fun main(args: Array<String>) {
    val obj = outerClass()
    println(obj.nestedClass().callMe())
    
}

 Above example is similar to the first example. Only difference is that we declared nestedClass() with an "Inner" keyword. If we will not use that keyword, we will get an error.

Output:
Outside Of Nested class."