Visibility Workbook

Practice problems for Kotlin Has a Visibility Level Java Doesn’t: internal. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

defaults and the basics

1. Declare without a modifier

Declare a top-level class and a top-level function with no visibility modifier, and note (as comments) what visibility they get.

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class Service      // public by default
fun helper() { }   // public by default

In Kotlin the default is public, so you annotate to restrict, not to expose.

2. Hide a member

Give Account a balance property that is visible only inside the class.

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class Account {
    private var balance = 0
}

3. Read-public, write-private

Declare Counter whose value can be read anywhere but written only inside the class.

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class Counter {
    var value: Int = 0
        private set
}

internal and protected

4. Module-only

Declare a class Engine that is usable anywhere in the same module but invisible to code that depends on the module.

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internal class Engine

internal is the level Java has no equivalent for.

5. For subclasses only

Give an abstract class Repository a connect() method visible to subclasses but not to outside callers.

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abstract class Repository {
    protected abstract fun connect(): Connection
}

Unlike Java, Kotlin’s protected does not also leak to the package.

6. File-private

Declare a top-level helper function that’s visible only within its own file.

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private fun audit() { }

For a top-level declaration, private means file scope.

constructors and structure

7. A private constructor

Give class Email a private constructor and a companion of(raw: String): Email? factory that returns null for input without an @.

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class Email private constructor(val address: String) {
    companion object {
        fun of(raw: String): Email? =
            if ("@" in raw) Email(raw) else null
    }
}

8. Many declarations, one file

Write a single file Geometry.kt holding two classes and a top-level function — something Java’s one-class-per-file rule forbids.

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// Geometry.kt
class Point(val x: Int, val y: Int)
class Line(val from: Point, val to: Point)
fun distance(a: Point, b: Point): Double = 0.0

9. Package and import

Write the top two lines of a file: declare it in package geometry, then import kotlin.math.sqrt.

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package geometry
import kotlin.math.sqrt

The package need not mirror the directory path (matching them is just convention).

10. Three levels in one type

Declare class Token(val id: String) with a public id, an internal fun validate(), and a private secret field.

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class Token(val id: String) {
    private val secret = id.hashCode()
    internal fun validate() = secret != 0
}

Each member exposes the smallest surface that still works.

Back to the lesson, Kotlin Has a Visibility Level Java Doesn’t, or on to the next one: delegation.