Kotlin Nested and Inner Class

In this article, you will learn to work with nested and inner classes with the help of examples.

Kotlin Nested Class

Similar like Java, Kotlin allows you to define a class within another class known as nested class.

class Outer {
    ... .. ...
    class Nested {
        ... .. ...
    }
}

Since Nested class is a member of its enclosing class Outer, you can use . notation to access Nested class and its members.


Example: Kotlin Nested Class

class Outer {

    val a = "Outside Nested class."

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

fun main(args: Array<String>) {
    // accessing member of Nested class
    println(Outer.Nested().b)

    // creating object of Nested class
    val nested = Outer.Nested()
    println(nested.callMe())
}

When you run the program, the output will be:

Inside Nested class.
Function call from inside Nested class.

For Java Users

The nested class in Kotlin is similar to static nested class in Java.

In Java, when you declare a class inside another class, it becomes an inner class by default. However in Kotlin, you need to use inner modifier to create an inner class which we will discuss next.


Kotlin Inner Class

The nested classes in Kotlin do not have access to the outer class instance. For example,

class Outer {
    val foo = "Outside Nested class."

    class Nested {
        // Error! cannot access member of outer class.
        fun callMe() = foo
    }
}

fun main(args: Array<String>) {

    val outer = Outer()
    println(outer.Nested().callMe())
}

The above code won't compile because we tried to access foo property of Outer class from inside Nested class.

In order to solve this issue, you need to mark the nested class with inner to create an inner class. Inner classes carry a reference to an outer class, and can access outer class members.


Example: Kotlin Inner Class

class Outer {

    val a = "Outside Nested class."

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

fun main(args: Array<String>) {

    val outer = Outer()
    println("Using outer object: ${outer.Inner().callMe()}")

    val inner = Outer().Inner()
    println("Using inner object: ${inner.callMe()}")
}

When you run the program, the output will be:

Using outer object: Outside Nested class.
Using inner object: Outside Nested class.

Recommended Reading: Anonymous Inner Class