31. Visibility Modifiers

Kotlin has four visibility modifiers: public (the default), private, protected, and internal (visible within the same module). Use them to expose a small public API while hiding internal state.

class BankAccount(private var balance: Int) {
    fun deposit(amount: Int) {       // public by default
        balance += amount
    }
    fun balance(): Int = balance
}

internal fun helper() = "module-internal"

fun main() {
    val account = BankAccount(100)
    account.deposit(50)
    // account.balance is private: this would not compile:
    // println(account.balance)
    println(account.balance())
    println(helper())
}

The balance field is private, so the only way in is through the public deposit() and balance(). protected works like private but is also visible to subclasses, and internal limits visibility to the current module.

Running it:

$ kotlin run
150
module-internal

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