Companion vs top-level functions decision in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of companion object function call

What is the output of this Kotlin code when calling MyClass.greet()?

Kotlin

class MyClass {
    companion object {
        fun greet() = "Hello from companion"
    }
}

fun main() {
    println(MyClass.greet())
}

A Hello from companion

B Compilation error: greet is not a member of MyClass

C Hello from top-level function

D Runtime error: Unresolved reference greet

💡 Hint

Companion object functions can be called using the class name.

The function greet() is inside the companion object of MyClass, so it can be called as MyClass.greet(). This prints the string returned by greet().

🧠 Conceptual

intermediate

Choosing between companion and top-level functions

Which situation is best suited for using a companion object function instead of a top-level function in Kotlin?

A When you want to define a function that can be called without importing

B When the function is a utility unrelated to any class

C When you want to avoid creating any class in your code

D When the function logically belongs to a class and needs access to its private members

💡 Hint

Think about encapsulation and access to class internals.

Companion object functions can access private members of their class, so they are suitable when the function logically belongs to the class and needs such access. Top-level functions are better for unrelated utilities.

❓ Predict Output

advanced

Output of top-level vs companion function with same name

What is the output of this Kotlin code?

Kotlin

fun greet() = "Hello from top-level"

class MyClass {
    companion object {
        fun greet() = "Hello from companion"
    }
}

fun main() {
    println(greet())
    println(MyClass.greet())
}

A Compilation error: greet is ambiguous

B

Hello from top-level
Hello from companion

C

Hello from companion
Hello from top-level

D Runtime error: greet not found

💡 Hint

Top-level and companion functions with the same name are called differently.

The top-level greet() is called directly, printing "Hello from top-level". The companion greet() is called with MyClass.greet(), printing "Hello from companion".

🔧 Debug

advanced

Why does this companion object function cause a compilation error?

Consider this Kotlin code snippet: class MyClass { companion object { fun printMessage() { println(message) } } private val message = "Hello" } Why does this code cause a compilation error?

A Companion object cannot access instance properties like 'message'

B The 'message' property is not initialized

C The function printMessage must be marked as static

D The companion object must be named explicitly

💡 Hint

Think about what companion objects can access inside a class.

Companion objects are like static members and cannot access instance properties directly. 'message' is an instance property, so the companion object function cannot access it.

🧠 Conceptual

expert

Best practice for organizing utility functions in Kotlin

You have several utility functions that do not require access to any class members. What is the best practice for organizing these functions in Kotlin?

A Place them inside an object declaration to simulate static methods

B Place them inside a companion object of a class

C Place them as top-level functions in a Kotlin file

D Place them as member functions inside a class instance

💡 Hint

Consider simplicity and idiomatic Kotlin style.

Top-level functions are the idiomatic way in Kotlin to define utility functions that do not need class context. Companion objects or object declarations are used when grouping related functions or needing static-like behavior.

Practice

1. Which statement best describes when to use a companion object function in Kotlin?

easy

A. When the function logically belongs to a class and should be called with the class name.

B. When the function is a general helper unrelated to any class.

C. When you want to create an instance method that requires an object.

D. When you want to define a function inside a function.

Solution

1. Step 1: Understand companion object purpose

   Companion object functions belong to a class and are called using the class name without needing an instance.

2. Step 2: Compare with other options

   Top-level functions are for general helpers, instance methods require an object, and functions inside functions are local functions.

3. Final Answer:

   When the function logically belongs to a class and should be called with the class name. -> Option A

4. Quick Check:

   Companion functions = class-related [OK]

Hint: Use companion for class-related static-like functions [OK]

Common Mistakes:

• Confusing companion functions with instance methods
• Using companion for unrelated helper functions
• Thinking companion functions need an object instance

2. Which of the following is the correct syntax to declare a companion function named create inside a Kotlin class MyClass?

easy

A. class MyClass { fun companion create() { } }

B. class MyClass { companion object { fun create() { } } }

C. class MyClass { companion fun create() { } }

D. companion object MyClass { fun create() { } }

Solution

1. Step 1: Recall companion object syntax

   In Kotlin, companion functions are declared inside a companion object block within the class.

2. Step 2: Check each option

   class MyClass { companion object { fun create() { } } } correctly declares a companion object with a function. Syntaxes like fun companion create() { } and companion fun create() { } are invalid. companion object MyClass { fun create() { } } incorrectly places companion object outside the class.

3. Final Answer:

   class MyClass { companion object { fun create() { } } } -> Option B

4. Quick Check:

   Companion object syntax = class MyClass { companion object { fun create() { } } } [OK]

Hint: Companion functions go inside companion object block [OK]

Common Mistakes:

• Writing companion fun instead of inside companion object
• Placing companion object outside the class
• Using fun companion instead of companion object

3. What will be the output of the following Kotlin code?

class Example {
    companion object {
        fun greet() = "Hello from companion"
    }
}

fun greet() = "Hello from top-level"

fun main() {
    println(Example.greet())
    println(greet())
}

medium

A. Compilation error due to function name conflict

B. "Hello from top-level" followed by "Hello from companion"

C. "Hello from companion" followed by "Hello from top-level"

D. Runtime error due to ambiguous call

Solution

1. Step 1: Identify function calls

   Example.greet() calls the companion object function returning "Hello from companion". The top-level greet() returns "Hello from top-level".

2. Step 2: Understand call order in main()

   First println prints Example.greet() output, second println prints top-level greet() output.

3. Final Answer:

   "Hello from companion" followed by "Hello from top-level" -> Option C

4. Quick Check:

   Companion vs top-level calls = "Hello from companion" followed by "Hello from top-level" [OK]

Hint: ClassName.func() calls companion; direct call is top-level [OK]

Common Mistakes:

• Assuming name conflict causes error
• Mixing up which greet() is called
• Expecting runtime ambiguity error

4. Consider this Kotlin code snippet:

class Data {
    companion object {
        fun info() = "Data info"
    }
}

fun main() {
    println(info())
}

What is the error and how to fix it?

medium

A. Error: Unresolved reference 'info'. Fix: Call as Data.info()

B. Error: Companion object missing. Fix: Add companion object

C. Error: info() must be top-level. Fix: Move info() outside class

D. No error, prints "Data info"

Solution

1. Step 1: Identify the call to info()

   The main function calls info() directly, but info() is inside Data's companion object, so it must be called with Data.info().

2. Step 2: Fix the call

   Change println(info()) to println(Data.info()) to access the companion function correctly.

3. Final Answer:

   Error: Unresolved reference 'info'. Fix: Call as Data.info() -> Option A

4. Quick Check:

   Companion functions need class name prefix [OK]

Hint: Call companion functions with ClassName.function() [OK]

Common Mistakes:

• Calling companion function without class name
• Thinking companion functions are top-level
• Trying to move companion function outside class

5. You want to create a utility function formatDate() used by multiple unrelated classes in your Kotlin project. Which approach is best and why?

hard

A. Put formatDate() inside a companion object of one class and call it via that class.

B. Define formatDate() inside a nested object inside each class.

C. Make formatDate() an instance method in each class that needs it.

D. Define formatDate() as a top-level function in a utility file for easy reuse.

Solution

1. Step 1: Analyze function usage

   The function is a utility used by many unrelated classes, so it should be accessible without class dependency.

2. Step 2: Choose best location

   Top-level functions are ideal for general helpers to avoid unnecessary class coupling. Companion objects tie functions to one class, which is not suitable here.

3. Final Answer:

   Define formatDate() as a top-level function in a utility file for easy reuse. -> Option D

4. Quick Check:

   General helpers = top-level functions [OK]

Hint: Use top-level for shared helpers, companion for class-specific [OK]

Common Mistakes:

• Placing utility in companion object of unrelated class
• Duplicating instance methods in multiple classes
• Using nested objects unnecessarily