Visibility modifiers

Classes, objects, interfaces, constructors, and functions, as well as properties and their setters, can have visibility modifiers. Getters always have the same visibility as their properties.

There are four visibility modifiers in Kotlin: private, protected, internal, and public. The default visibility is public.

On this page, you'll learn how the modifiers apply to different types of declaring scopes.

Packages

Functions, properties, classes, objects, and interfaces can be declared at the "top-level" directly inside a package:

// file name: example.kt
package foo

fun baz() { ... }
class Bar { ... }

If you don't use a visibility modifier, public is used by default, which means that your declarations will be visible everywhere.
If you mark a declaration as private, it will only be visible inside the file that contains the declaration.
If you mark it as internal, it will be visible everywhere in the same module.
The protected modifier is not available for top-level declarations.

Examples:

// file name: example.kt
package foo

private fun foo() { ... } // visible inside example.kt

public var bar: Int = 5 // property is visible everywhere
    private set         // setter is visible only in example.kt
    
internal val baz = 6    // visible inside the same module

Class members

For members declared inside a class:

private means that the member is visible inside this class only (including all its members).
protected means that the member has the same visibility as one marked as private, but that it is also visible in subclasses.
internal means that any client inside this module who sees the declaring class sees its internal members.
public means that any client who sees the declaring class sees its public members.

If you override a protected or an internal member and do not specify the visibility explicitly, the overriding member will also have the same visibility as the original.

Examples:

open class Outer {
    private val a = 1
    protected open val b = 2
    internal open val c = 3
    val d = 4  // public by default
    
    protected class Nested {
        public val e: Int = 5
    }
}

class Subclass : Outer() {
    // a is not visible
    // b, c and d are visible
    // Nested and e are visible

    override val b = 5   // 'b' is protected
    override val c = 7   // 'c' is internal
}

class Unrelated(o: Outer) {
    // o.a, o.b are not visible
    // o.c and o.d are visible (same module)
    // Outer.Nested is not visible, and Nested::e is not visible either 
}

Constructors

Use the following syntax to specify the visibility of the primary constructor of a class:

class C private constructor(a: Int) { ... }

Here the constructor is private. By default, all constructors are public, which effectively amounts to them being visible everywhere the class is visible (this means that a constructor of an internal class is only visible within the same module).

For sealed classes, constructors are protected by default. For more information, see Sealed classes.

Local declarations

Local variables, functions, and classes can't have visibility modifiers.

Modules

The internal visibility modifier means that the member is visible within the same module. More specifically, a module is a set of Kotlin files compiled together, for example:

An IntelliJ IDEA module.
A Maven project.
A Gradle source set (with the exception that the test source set can access the internal declarations of main).
A set of files compiled with one invocation of the <kotlinc> Ant task.

25 September 2024