Named companion objects in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a named companion object called Factory.

Kotlin

class MyClass {
    companion object [1] {
        fun create(): MyClass = MyClass()
    }
}

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

AFactory

BCompanion

CInstance

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the companion object unnamed (empty).

Using keywords like 'Companion' as the name.

The named companion object is declared by giving a name after the companion object keyword. Here, Factory is the name.

2fill in blank

medium

Complete the code to call the create() function from the named companion object Factory.

Kotlin

val instance = MyClass.[1].create()

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

ACompanion

BInstance

CObject

DFactory

Attempts:

3 left

💡 Hint

Common Mistakes

Using the default name 'Companion' instead of the given name.

Trying to call create() directly on the class without the companion object.

To access a named companion object, use the class name followed by the companion object's name. Here, Factory is the name.

3fill in blank

hard

Fix the error in the code by completing the companion object declaration with the correct name.

Kotlin

class User {
    companion object [1] {
        fun newUser() = User()
    }
}

val u = User.[1].newUser()

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

ACreator

BCompanion

CFactory

DBuilder

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in declaration and usage.

Using the default 'Companion' when the object is named differently.

The companion object must have the same name in both declaration and usage. Here, Factory is the correct name to fix the error.

4fill in blank

hard

Fill both blanks to create a named companion object called Helper and call its greet() function.

Kotlin

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

val message = Greeter.[2].greet()

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

AHelper

BCompanion

CFactory

DAssistant

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in declaration and call.

Using the default 'Companion' instead of the given name.

The companion object is named Helper and accessed by the same name to call greet().

5fill in blank

hard

Fill all three blanks to define a named companion object Creator with a function build(), then call it.

Kotlin

class Product {
    companion object [1] {
        fun build() = Product()
    }
}

val p = Product.[2].[3]()

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

ACreator

Bbuild

CFactory

DCompanion

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the companion object name and function name.

Using the default 'Companion' when the object is named differently.

The companion object is named Creator. The function build() is called from it.

Practice

1. What is the main purpose of naming a companion object in Kotlin?

easy

A. To give a clear and specific name to the singleton object inside a class

B. To create multiple instances of the companion object

C. To make the companion object inherit from another class

D. To prevent the companion object from being accessed outside the class

Solution

1. Step 1: Understand companion objects

   Companion objects are singletons inside a class used to hold members related to the class itself.

2. Step 2: Purpose of naming companion objects

   Giving a name to a companion object helps to clearly identify and access its members, improving code readability.

3. Final Answer:

   To give a clear and specific name to the singleton object inside a class -> Option A

4. Quick Check:

   Named companion object = clear singleton name [OK]

Hint: Named companion objects clarify singleton usage [OK]

Common Mistakes:

• Thinking companion objects create multiple instances
• Confusing naming with inheritance
• Assuming naming restricts access

2. Which of the following is the correct syntax to declare a named companion object in Kotlin?

easy

A. class MyClass { companionobject Named { fun greet() = "Hi" } }

B. class MyClass { companion Named object { fun greet() = "Hi" } }

C. class MyClass { companion object Named { fun greet() = "Hi" } }

D. class MyClass { object companion Named { fun greet() = "Hi" } }

Solution

1. Step 1: Recall correct companion object syntax

   The correct syntax is 'companion object Name { ... }' inside the class.

2. Step 2: Check each option

   class MyClass { companion object Named { fun greet() = "Hi" } } matches the correct syntax exactly. Others have misplaced keywords or wrong order.

3. Final Answer:

   class MyClass { companion object Named { fun greet() = "Hi" } } -> Option C

4. Quick Check:

   companion object Name { } syntax [OK]

Hint: Remember: 'companion object Name' order is fixed [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Writing 'companionobject' as one word
• Placing the name before 'companion'

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

class Box {
    companion object Factory {
        fun create() = "Box created"
    }
}
fun main() {
    println(Box.Factory.create())
}

medium

A. Factory created

B. Box created

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand named companion object usage

   The companion object named 'Factory' holds the function 'create' which returns "Box created".

2. Step 2: Analyze the main function call

   Calling 'Box.Factory.create()' accesses the named companion object and calls 'create', printing "Box created".

3. Final Answer:

   Box created -> Option B

4. Quick Check:

   Box.Factory.create() prints "Box created" [OK]

Hint: Call named companion with ClassName.Name.method() [OK]

Common Mistakes:

• Trying to call create() without Factory prefix
• Expecting 'Factory created' output
• Confusing syntax causing compile errors

4. Identify the error in this Kotlin code snippet:

class Sample {
    companion object Companion {
        fun info() = "Info"
    }
}
fun main() {
    println(Sample.companion.info())
}

medium

A. Incorrect companion object name usage; should be 'Companion' not 'companion'

B. Missing 'object' keyword in companion declaration

C. Function 'info' must be static

D. No error; code runs fine

Solution

1. Step 1: Check companion object name and usage

   The companion object is named 'Companion' with uppercase C.

2. Step 2: Analyze the call in main()

   The code calls 'Sample.companion.info()' with lowercase 'companion', which does not match the name and causes a compile error.

3. Final Answer:

   Incorrect companion object name usage; should be 'Companion' not 'companion' -> Option A

4. Quick Check:

   Use exact companion object name when calling [OK]

Hint: Match companion object name exactly (case sensitive) [OK]

Common Mistakes:

• Using lowercase 'companion' instead of named object
• Forgetting 'object' keyword
• Assuming companion functions must be static

5. You want to create a Kotlin class Logger with a named companion object Factory that provides a method create() returning a new Logger instance. Which code correctly implements this?

hard

A. class Logger {
companion Factory object {
fun create() = Logger()
}
}

B. class Logger {
companion object {
fun create() = Logger()
}
}

C. class Logger {
object Factory companion {
fun create() = Logger()
}
}

D. class Logger {
companion object Factory {
fun create() = Logger()
}
}

Solution

1. Step 1: Recall correct syntax for named companion object

   The correct syntax is 'companion object Factory { ... }' inside the class.

2. Step 2: Check method implementation

   The method 'create()' returns a new instance of Logger using 'Logger()'. class Logger { companion object Factory { fun create() = Logger() } } matches both syntax and logic.

3. Final Answer:

   class Logger { companion object Factory { fun create() = Logger() } } -> Option D

4. Quick Check:

   Named companion object with factory method = class Logger { companion object Factory { fun create() = Logger() } } [OK]

Hint: Use 'companion object Name' and return new instance inside [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Omitting the companion object name
• Incorrect order of keywords