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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a companion object function named greet.

Kotlin

class Greeter {
    companion object {
        fun [1]() = "Hello from companion"
    }
}

Drag options to blanks, or click blank then click option'

Agreet

Bhello

CsayHello

Dwelcome

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than 'greet'.

The function name should be greet as specified.

2fill in blank

medium

Complete the code to define a top-level function named greet that returns a greeting string.

Kotlin

fun [1]() = "Hello from top-level function"

Drag options to blanks, or click blank then click option'

Agreet

BsayHello

Cwelcome

Dhello

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a function name that does not clearly indicate greeting.

The top-level function should be named greet to match the greeting purpose.

3fill in blank

hard

Fix the error in calling the companion object function greet from class Greeter.

Kotlin

fun main() {
    println(Greeter.[1]())
}

Drag options to blanks, or click blank then click option'

Ahello

BsayHello

Cwelcome

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong function name or forgetting to use the class name.

The companion object function is called using the class name followed by the function name greet.

4fill in blank

hard

Fill both blanks to create a companion object function that returns a greeting with a name parameter.

Kotlin

class Greeter {
    companion object {
        fun greet([1]: String): String = "Hello, [2]"
    }
}

Drag options to blanks, or click blank then click option'

Aname

Bperson

Cuser

Dguest

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and the string interpolation.

The parameter name must be the same in the function signature and the string interpolation to work correctly.

5fill in blank

hard

Fill all three blanks to create a top-level function that returns a greeting with a name parameter.

Kotlin

fun [1]([2]: String): String = "Hello, $[3]"

Drag options to blanks, or click blank then click option'

AsayHello

Bname

Dgreet

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatch between parameter name and interpolation variable.

The function name is greet, the parameter is name, and the string interpolation uses name.

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