Visibility modifiers are keywords that set the visibility (accessibility) of classes, objects, interface, constructors, functions, properties and their setters. (You cannot set visibility modifier of getters as they always take the same visibility as that of the property.)

In Kotlin Class and Objects article, you learned about visibility modifiers public and private in brief. You will learn about two more visibility modifiers protected and internal (as well as public and private) in detail.

Visibility Modifiers Inside Package

A package organizes a set of related functions, properties and classes, objects, and interfaces. Recommended reading: Kotlin Packages

Note: If visibility modifier is not specified, it is public by default.

Let’s take an example:

// file name: hello.kt

package test

fun function1() {}   // public by default and visible everywhere

private fun function2() {}   // visible inside hello.kt

internal fun function3() {}   // visible inside the same module

var name = "Foo"   // visible everywhere
    get() = field   // visible inside hello.kt (same as its property)
    private set(value) {   // visible inside hello.kt
        field = value
    }

private class class1 {}   // visible inside hello.kt

Visibility Modifiers Inside Classes and Interfaces

Here’s how visibility modifiers works for members (functions, properties) declared inside a class:

Note: If you override a protected member in the derived class without specifying its visibility, its visibility will also be protected.

Let’s take an example:

open class Base() {
    var a = 1                 // public by default
    private var b = 2         // private to Base class
    protected open val c = 3  // visible to the Base and the Derived class
    internal val d = 4        // visible inside the same module

    protected fun e() { }     // visible to the Base and the Derived class
}

class Derived: Base() {

    // a, c, d, and e() of the Base class are visible
    // b is not visible

    override val c = 9        // c is protected
}

fun main(args: Array<String>) {
    val base = Base()

    // base.a and base.d are visible
    // base.b, base.c and base.e() are not visible

    val derived = Derived()
    // derived.c is not visible
}

Changing Visibility of a Constructor

By default, the visibility of a constructor is public. However, you can change it. For that, you need to explicitly add constructor keyword.

The constructor is public by default in the example below:

class Test(val a: Int) {
    // code
}

Here’s how you can change its visibility.

class Test private constructor(val a: Int) {
    // code
}

Here the constructor is private.

Note: In Kotlin, local functions, variables and classes cannot have visibility modifiers.