Companion object with interfaces in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a companion object inside the class.

Kotlin

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

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

Acompanion

BcompanionObject

CCompanionObject

DCompanion

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Companion' instead of lowercase 'companion'.

Trying to name the companion object explicitly without the keyword.

In Kotlin, the keyword companion is lowercase when declaring a companion object.

2fill in blank

medium

Complete the code to make the companion object implement the interface.

Kotlin

interface Greeter {
    fun greet(): String
}

class MyClass {
    companion object : [1] {
        override fun greet() = "Hi"
    }
}

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

AGreeting

BGreeter

CGreetable

DGreets

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong interface name that does not exist.

Forgetting to add the colon before the interface name.

The companion object implements the interface named Greeter.

3fill in blank

hard

Fix the error in the companion object declaration to correctly implement the interface.

Kotlin

interface Speaker {
    fun speak(): String
}

class Talk {
    companion object [1] Speaker {
        override fun speak() = "Hello"
    }
}

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

Aextends

Bimplements

C:

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'implements' like in Java.

Using 'extends' which is for class inheritance.

In Kotlin, to implement an interface, use a colon : before the interface name.

4fill in blank

hard

Fill both blanks to declare a companion object that implements two interfaces.

Kotlin

interface A {
    fun a()
}
interface B {
    fun b()
}

class Multi {
    companion object [1] [2] {
        override fun a() {}
        override fun b() {}
    }
}

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

A:

B,

C;

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using semicolons or ampersands to separate interfaces.

Forgetting the colon before the interface list.

Use a colon : to start the interface list and commas , to separate multiple interfaces.

5fill in blank

hard

Fill both blanks to create a companion object that implements an interface and has a function.

Kotlin

interface Logger {
    fun log(message: String)
}

class App {
    companion object : Logger {
        fun info(msg: String) = println(msg)
        override fun log(message: String) = info([1])
    }
}

fun main() {
    App.Companion.log([2])
}

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

A:

Bmessage

C"App started"

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong symbols instead of colon for interface implementation.

Passing wrong variable names inside the function call.

Not using quotes around the string message.

Use colon : to implement the interface, pass the parameter message to info, and call log with the string "App started".

Practice

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

easy

A. To create multiple instances of a class

B. To inherit from multiple classes

C. To define instance variables for each object

D. To hold static-like members that belong to the class, not instances

Solution

1. Step 1: Understand companion object role

   A companion object is a special object inside a class that holds members shared by all instances.

2. Step 2: Compare with instance members

   Unlike instance variables, companion object members belong to the class itself, acting like static members.

3. Final Answer:

   To hold static-like members that belong to the class, not instances -> Option D

4. Quick Check:

   Companion object = static-like members [OK]

Hint: Companion object = static members inside class [OK]

Common Mistakes:

• Thinking companion object creates instances
• Confusing companion object with instance variables
• Believing companion object can inherit multiple classes

2. Which of the following is the correct syntax to declare a companion object implementing an interface MyInterface inside a class MyClass?

easy

A. class MyClass { object companion : MyInterface { } }

B. class MyClass { companion object : MyInterface { } }

C. class MyClass { companion MyInterface object { } }

D. class MyClass { companion object implements MyInterface { } }

Solution

1. Step 1: Recall companion object syntax

   The correct syntax is companion object followed by a colon and the interface name.

2. Step 2: Check each option

   class MyClass { companion object : MyInterface { } } matches the correct syntax. The other options have incorrect keywords or order.

3. Final Answer:

   class MyClass { companion object : MyInterface { } } -> Option B

4. Quick Check:

   companion object : InterfaceName [OK]

Hint: Use 'companion object : Interface' syntax [OK]

Common Mistakes:

• Using 'implements' keyword instead of ':'
• Swapping 'object' and 'companion' keywords
• Omitting the colon before interface name

3. Given the code below, what will be printed when MyClass.printMessage() is called?

interface Printer {
    fun printMessage()
}

class MyClass {
    companion object : Printer {
        override fun printMessage() {
            println("Hello from companion")
        }
    }
}

fun main() {
    MyClass.printMessage()
}

medium

A. Hello from companion

B. No output

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand companion object implementing interface

   The companion object implements Printer and overrides printMessage to print a message.

2. Step 2: Calling printMessage on class

   Calling MyClass.printMessage() calls the companion object's method, printing the message.

3. Final Answer:

   Hello from companion -> Option A

4. Quick Check:

   Companion object method called = prints message [OK]

Hint: Companion object methods called on class directly [OK]

Common Mistakes:

• Expecting instance creation before method call
• Thinking printMessage is not accessible on class
• Assuming compilation error due to interface

4. Identify the error in the following Kotlin code snippet:

interface Logger {
    fun log(message: String)
}

class Service {
    companion object Logger {
        override fun log(message: String) {
            println("Log: $message")
        }
    }
}

medium

A. Companion object must use ':' to implement interface, not name after object

B. Missing override keyword for log function

C. Interface Logger cannot be implemented by companion object

D. Companion object cannot have functions

Solution

1. Step 1: Check companion object declaration

   The code uses companion object Logger, which names the object but does not implement the interface.

2. Step 2: Correct syntax for interface implementation

   To implement Logger, it should be companion object : Logger with a colon.

3. Final Answer:

   Companion object must use ':' to implement interface, not name after object -> Option A

4. Quick Check:

   Use ':' to implement interface in companion object [OK]

Hint: Use ':' not name to implement interface in companion object [OK]

Common Mistakes:

• Confusing object name with interface implementation
• Omitting colon before interface name
• Thinking companion object cannot implement interfaces

5. You want to create a Kotlin class Calculator with a companion object that implements an interface Operations having a method add(a: Int, b: Int): Int. How can you call add without creating an instance of Calculator?

interface Operations {
    fun add(a: Int, b: Int): Int
}

class Calculator {
    companion object : Operations {
        override fun add(a: Int, b: Int): Int = a + b
    }
}

hard

A. Call Operations.add(5, 3)

B. Create instance and call Calculator().add(5, 3)

C. Call Calculator.add(5, 3)

D. Call Calculator.companion.add(5, 3)

Solution

1. Step 1: Understand companion object access

   Companion object members can be called directly on the class name like Calculator.add() (syntactic sugar) or explicitly via Calculator.Companion.

2. Step 2: Check method call options

   Calculator.add(5, 3) is the standard direct call. B requires an instance which doesn't have the method. C calls the interface directly (wrong). D uses incorrect casing ("companion" instead of "Companion").

3. Final Answer:

   Call Calculator.add(5, 3) -> Option C

4. Quick Check:

   Class.method() = companion access [OK]

Hint: Companion object methods called on class directly [OK]

Common Mistakes:

• Using lowercase 'companion' instead of 'Companion'
• Creating instance unnecessarily
• Trying to call interface name directly