Visibility modifiers (public, private, internal, protected) in Kotlin - Time & Space Complexity

Time Complexity: Visibility modifiers (public, private, internal, protected)

O(n)

Understanding Time Complexity

Let's explore how the use of visibility modifiers affects the time it takes for Kotlin code to run.

We want to see if changing visibility changes how long the program takes as it grows.

Scenario Under Consideration

Analyze the time complexity of the following Kotlin class with different visibility modifiers.

class Example {
    private val secretData = List(1000) { it * 2 }
    fun getData(): List {
        return secretData.filter { it % 3 == 0 }
    }
}
    

This code creates a list and filters it using a public function, while the list itself is private.

Identify Repeating Operations

Look for loops or repeated actions in the code.

• Primary operation: Filtering the list with filter which checks each item.
• How many times: Once per call, it checks all 1000 items in the list.

How Execution Grows With Input

As the list size grows, the filtering takes longer because it looks at each item.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The work grows directly with the list size; double the list, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the size of the list being filtered.

Common Mistake

[X] Wrong: "Making a variable private makes the code run faster."

[OK] Correct: Visibility controls who can use the code, not how fast it runs. The time depends on what the code does, not who can see it.

Interview Connect

Understanding visibility modifiers helps you write clear and safe code. Knowing they don't affect speed shows you focus on what really matters when coding.

Self-Check

"What if the filter function was called multiple times instead of once? How would the time complexity change?"

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