Visibility Modifiers

Visibility modifiers control which code can see a class, property, or function. Hiding implementation details lets you change them later without breaking other code — this is encapsulation.

The four levels

Example

class PasswordValidator(private val minimumLength: Int = 8) {
    fun isStrong(password: String) = isLongEnough(password) && hasDigit(password)

    private fun isLongEnough(password: String) = password.length >= minimumLength

    private fun hasDigit(password: String) = password.any { it.isDigit() }
}

fun main() {
    val validator = PasswordValidator()
    println(validator.isStrong("abc12345"))
    println(validator.isStrong("password"))
}

true
false

isStrong is the class's interface. The private helpers can be renamed or rewritten freely — nothing outside can call them. Trying validator.hasDigit("x") fails to compile: Cannot access 'fun hasDigit(password: String): Boolean': it is private in 'PasswordValidator'.

Constructor properties can be private too — private val minimumLength is stored in the object but hidden from outside.

Read-only from outside

A common pattern exposes a read-only view of mutable internal state:

class Playlist {
    private val _songs = mutableListOf<String>()
    val songs: List<String> get() = _songs

    fun add(song: String) {
        _songs += song
    }
}

fun main() {
    val p = Playlist()
    p.add("Intro")
    println(p.songs)
}

[Intro]

Outside code sees a read-only List and can't add songs directly.

In apps and libraries

In an app, public (the default) and private cover most needs. In a library, mark helpers internal or private, since everything public is part of what you promise to keep working.