Visibility modifiers (public, private, internal, protected) in Kotlin

Introduction

Visibility modifiers control who can see and use parts of your code. They help keep your code safe and organized.

When you want to hide details inside a class so others can't change them by mistake.

When you want some code to be used only inside the same file or module.

When you want to share code only with related classes but not everyone.

When you want everything to be open and accessible everywhere.

Syntax

Kotlin

public class MyClass {
    private val secret = "hidden"
    internal fun insideModule() {}
    protected open fun forSubclasses() {}
    public fun everyoneCanUse() {}
}

public means anyone can use it.

private means only inside the class.

internal means only inside the same module.

protected means only inside the class and subclasses.

Examples

This shows all four modifiers in one class.

Kotlin

class Example {
    private val secret = 42  // only inside Example
    internal fun moduleFun() {} // inside same module
    protected fun subclassFun() {} // inside Example and subclasses
    public fun openFun() {} // anywhere
}

Private class is only visible inside the file it is declared in.

Kotlin

private class HiddenClass {
    fun doSomething() {}
}

// HiddenClass is only visible inside this file.

This function can be used anywhere inside the same module but not outside.

Kotlin

internal fun moduleFunction() {
    println("Only inside this module")
}

Sample Program

This program shows how each visibility modifier controls access from inside the class, subclass, and outside.

Kotlin

open class Parent {
    private val privateVal = "private"
    internal val internalVal = "internal"
    protected val protectedVal = "protected"
    public val publicVal = "public"

    fun showValues() {
        println("Inside Parent:")
        println(privateVal)
        println(internalVal)
        println(protectedVal)
        println(publicVal)
    }
}

class Child : Parent() {
    fun showChildValues() {
        println("Inside Child:")
        // println(privateVal) // Error: privateVal not visible
        println(internalVal)    // visible
        println(protectedVal)   // visible
        println(publicVal)      // visible
    }
}

fun main() {
    val parent = Parent()
    parent.showValues()

    val child = Child()
    child.showChildValues()

    println("Outside classes:")
    // println(parent.privateVal) // Error: private
    println(parent.internalVal)  // visible inside module
    // println(parent.protectedVal) // Error: protected
    println(parent.publicVal)    // visible everywhere
}

Important Notes

By default, Kotlin declarations are public if no modifier is given.

private at top-level means visible only inside the file.

internal is useful to hide code from other modules but share inside your app.

Summary

public: visible everywhere.

private: visible only inside the class or file.

internal: visible inside the same module.

protected: visible inside class and subclasses.

Practice

1. Which visibility modifier in Kotlin allows a class member to be accessed from anywhere in the project?

easy

A. internal

B. private

C. public

D. protected

Solution

1. Step 1: Understand the meaning of public

   The public modifier means the member is visible everywhere in the project.

2. Step 2: Compare with other modifiers

   private restricts to class/file, internal restricts to module, protected restricts to class and subclasses only.

3. Final Answer:

   public -> Option C

4. Quick Check:

   Visible everywhere = public [OK]

Hint: Public means visible everywhere in the project [OK]

Common Mistakes:

• Confusing private with public visibility
• Thinking internal is visible everywhere
• Assuming protected is public

2. Which of the following is the correct syntax to declare a private property named count in a Kotlin class?

easy

A. private val count: Int = 0

B. val private count: Int = 0

C. val count private: Int = 0

D. private count val: Int = 0

Solution

1. Step 1: Recall Kotlin property declaration syntax

   Visibility modifier comes before val or var, so private val count: Int = 0 is correct.

2. Step 2: Check other options for syntax errors

   Options B, C, and D have incorrect order of keywords, which is invalid in Kotlin.

3. Final Answer:

   private val count: Int = 0 -> Option A

4. Quick Check:

   Modifier before val/var = correct syntax [OK]

Hint: Put visibility modifier before val or var [OK]

Common Mistakes:

• Placing private after val
• Mixing order of keywords
• Omitting colon before type

3. Consider the following Kotlin code:

open class Parent {
    protected val secret = "hidden"
}

class Child : Parent() {
    fun reveal() = secret
}

fun main() {
    val child = Child()
    println(child.reveal())
    // println(child.secret) // Line X
}

What will happen if the commented Line X is uncommented?

medium

A. Compilation error: Cannot access 'secret' outside the class or subclass

B. Prints 'hidden' without error

C. Runtime error: Illegal access

D. Compilation error: 'secret' is private

Solution

1. Step 1: Understand protected visibility

   protected members are accessible inside the class and its subclasses, but not outside.

2. Step 2: Analyze access in main function

   Accessing child.secret from outside the class hierarchy causes a compilation error.

3. Final Answer:

   Compilation error: Cannot access 'secret' outside the class or subclass -> Option A

4. Quick Check:

   Protected blocks external access [OK]

Hint: Protected is accessible only inside class and subclasses [OK]

Common Mistakes:

• Assuming protected is public
• Expecting runtime error instead of compile error
• Confusing private and protected

4. Given this Kotlin code snippet:

class Example {
    private val secret = "top secret"
}

fun main() {
    val ex = Example()
    println(ex.secret)
}

What is the error and how to fix it?

medium

A. No error; code runs and prints 'top secret'

B. Error: Missing constructor; fix by adding constructor

C. Error: Syntax error; fix by adding semicolon

D. Error: Cannot access private property outside class; fix by changing to public

Solution

1. Step 1: Identify visibility of secret

   The property secret is declared private, so it cannot be accessed outside the class.

2. Step 2: Understand the error cause and fix

   Trying to print ex.secret from outside causes a compilation error. Changing private to public or providing a public method to access it fixes the error.

3. Final Answer:

   Error: Cannot access private property outside class; fix by changing to public -> Option D

4. Quick Check:

   Private blocks external access [OK]

Hint: Private hides members outside class; use public or accessor [OK]

Common Mistakes:

• Thinking private allows outside access
• Confusing syntax errors with visibility errors
• Ignoring need for accessor methods

5. You have two Kotlin files in the same module: FileA.kt and FileB.kt. In FileA.kt, there is an internal function fun greet() = "Hello". Which of the following statements is true about calling greet() from FileB.kt?

hard

A. greet() cannot be called because internal means private

B. greet() can be called from FileB.kt because internal allows access within the same module

C. greet() cannot be called because internal restricts access to the same file only

D. greet() can be called only if FileB.kt inherits from FileA.kt

Solution

1. Step 1: Understand internal visibility

   internal means visible anywhere inside the same module, regardless of file.

2. Step 2: Apply to given files

   Since FileA.kt and FileB.kt are in the same module, greet() is accessible from FileB.kt.

3. Final Answer:

   greet() can be called from FileB.kt because internal allows access within the same module -> Option B

4. Quick Check:

   Internal = visible in module [OK]

Hint: Internal means visible anywhere in the same module [OK]

Common Mistakes:

• Confusing internal with private or file-private
• Thinking internal restricts to one file
• Assuming inheritance needed for internal access