Visibility & Packages

Packages and imports

Each file can declare a package; imports bring in names from elsewhere:

package com.example.banking

import kotlin.math.max

The package should usually mirror the directory structure, though Kotlin does not strictly require it.

Visibility modifiers

Modifier	Visible to
public (default)	Everyone
internal	The same Gradle module
protected	The declaring class and subclasses
private	The declaring class (or file, for top-level declarations)

class Vault {
    private val secret = "🔑"          // only inside Vault
    internal val auditId = 42          // anywhere in this module
    protected open val policy = "std"   // subclasses can see it
    fun reveal() = secret              // public by default
}

For top-level declarations, private means “visible only within this file”:

private fun helper() { /* file-private */ }

There is no package-private like Java. The closest equivalent is internal, which is scoped to the whole module rather than a single package.

Why restrict visibility?

Keeping fields private and exposing behavior through public methods is encapsulation — it lets you change internals without breaking callers and keeps invariants (like a non-negative balance) enforced in one place.

Exercises

1. Take the BankAccount from the previous chapter and make its balance setter private so callers must use deposit/withdraw. (It already is — confirm why account.balance = -100 does not compile.)

2. Add an internal function auditLine(): String that returns a one-line summary, usable from elsewhere in the module but not from outside it.

---

Previous: Classes & Properties · Next: Inheritance & Interfaces