Kotlin Nested and Inner Classes

In this article, you will learn how to use nested and inner classes with examples.

Kotlin Nested Classes

Similar to Java, Kotlin allows you to define a class within another class called a nested class

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

Since the nested class is an external (Outer) member of the enclosing class, you can use the . symbol to access the nested class and its members.

Example: Using Kotlin nested classes example

class Outer {
　　　　val　a　=　"Outside the nested class."
　　　　class Nested {
　　　　　　　　val b = "Within the nested class."
　　　　　　　　fun callMe() = "Function call from within the nested class."
　　　　}
}
fun　main(args:　Array<String>)　{
　　　　//Access the members of the nested class
　　　　println(Outer.Nested().b)
　　　　//Create an object of the nested class
　　　　val nested = Outer.Nested()
　　　　println(nested.callMe())
}

When running the program, the output is:

Within the nested class.
Function calls from within the nested class.

For Java users

Nested classes in Kotlin are similar to static nested classes in Java.

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

Kotlin Inner Classes

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

class Outer {
　　　　val foo = "Outside the nested class."
　　　　class Nested {
　　　　　　　　//Error! Cannot access the members of the outer class.
　　　　　　　　fun callMe() = foo
　　　　}
}
fun　main(args:　Array<String>)　{
　　　　val　outer　=　Outer()
　　　　println(outer.Nested().callMe())
}

The above code cannot be compiled because we are trying to access the foo property of the outer class from within the nested class.

To solve this problem, you need to use the inner keyword to mark the nested class to create an inner class. An inner class carries a reference to the outer class and can access the members of the outer class.

Example: Using Kotlin inner classes

class Outer {
　　　　val　a　=　"Outside the nested class."
　　　　inner　class　Inner　{
　　　　　　　　fun　callMe()　=　a
　　　　}
}
fun　main(args:　Array<String>)　{
　　　　val　outer　=　Outer()
　　　　println("Use outer objects:　${outer.Inner().callMe()}")
　　　　val　inner　=　Outer().Inner()
　　　　println("Use inner objects:　${inner.callMe()}")
}

When running the program, the output is:

Use outer objects:　Outside the nested class.
Use inner objects:　Outside the nested class.