VSkills Certified Kotlin Developer — Questions and Answers

Question 1: What is the default visibility modifier in Kotlin?

private
internal
protected
public (Correct answer)

Correct answer: public

In Kotlin, declarations are `public` by default, meaning they are visible everywhere.

Question 2: What will `println(10 / 3)` output in Kotlin?

3.333...
4
3.33
3 (Correct answer)

Correct answer: 3

Integer division in Kotlin truncates the decimal, so 10 divided by 3 equals 3.

Question 3: What does `run` scope function do when called on an object in Kotlin?

Starts a coroutine
Calls a lambda with the object as `this` and returns the lambda result (Correct answer)
Prints the object to console
Returns the object unchanged

Correct answer: Calls a lambda with the object as `this` and returns the lambda result

`run` calls the block with the receiver as `this` and returns the result of the block, useful for transforming objects.

Question 4: What is the difference between `List` and `MutableList` in Kotlin?

List allows nulls; MutableList does not
List is read-only; MutableList supports add/remove operations (Correct answer)
There is no difference
MutableList is faster than List

Correct answer: List is read-only; MutableList supports add/remove operations

`List` is a read-only interface in Kotlin, while `MutableList` extends it with mutation operations like `add()` and `remove()`.

Question 5: What keyword makes a Kotlin class open for inheritance?

abstract
inheritable
open (Correct answer)
extends

Correct answer: open

Kotlin classes are final by default; the `open` keyword must be added explicitly to allow a class to be subclassed.

Question 6: Which operator is used to perform a non-null assertion in Kotlin?

!! (Correct answer)
?:
&&
?.

Correct answer: !!

The `!!` operator asserts that a nullable expression is non-null, throwing a NullPointerException if it is null.

Question 7: What does `with` scope function do in Kotlin?

Executes code only if the object is non-null
Creates a new scope with imports
Calls a lambda with an object as `this` and returns the lambda result, without being an extension function (Correct answer)
Wraps an object in a nullable container

Correct answer: Calls a lambda with an object as `this` and returns the lambda result, without being an extension function

`with` is a non-extension function that takes an object as a parameter, calls the lambda with it as `this`, and returns the lambda result.

Question 8: What is a lambda expression in Kotlin?

A type alias
A class with a single method
An anonymous function literal that can be stored in a variable or passed as an argument (Correct answer)
A named function

Correct answer: An anonymous function literal that can be stored in a variable or passed as an argument

A lambda in Kotlin is an anonymous function literal enclosed in `{}` that can be passed as a value or stored in a variable.

Question 9: What is the `Set` interface in Kotlin?

An ordered collection that allows duplicates
A sorted list implementation
A key-value collection
An unordered collection that does not allow duplicate elements (Correct answer)

Correct answer: An unordered collection that does not allow duplicate elements

`Set` is a collection that holds unique elements; it has no defined order in the base interface and does not allow duplicates.

Question 10: What is a receiver in the context of Kotlin extension functions?

The parameter list of the function
The object on which the extension function is called, accessible as `this` (Correct answer)
The return value of the function
The class that defines the extension

Correct answer: The object on which the extension function is called, accessible as `this`

The receiver is the instance on which an extension function is invoked; inside the extension, it can be accessed implicitly as `this`.

Question 11: What is property delegation in Kotlin?

Delegating the implementation of a property's get/set to another object (Correct answer)
Making a property abstract
Creating a copy of a property
Passing a property to a function

Correct answer: Delegating the implementation of a property's get/set to another object

Property delegation lets you hand off the `get` and `set` implementations to a delegate object, enabling reusable property behavior.

Question 12: What does the `fold` function do in Kotlin?

Accumulates a value starting from an initial value (Correct answer)
Removes duplicate elements
Flattens a nested collection
Groups elements by a key

Correct answer: Accumulates a value starting from an initial value

`fold` accumulates a result by applying an operation to each element along with a running accumulator, starting from an initial value.

Question 13: What is the difference between `abstract class` and `interface` in Kotlin?

There is no difference
Abstract classes can have constructors and state; interfaces cannot have constructors or mutable state (Correct answer)
Interfaces can only have abstract methods
Interfaces support state; abstract classes do not

Correct answer: Abstract classes can have constructors and state; interfaces cannot have constructors or mutable state

Abstract classes can have constructors, state (fields), and initialization logic; interfaces can have abstract and default methods but no constructors or stored mutable state.

Question 14: Which keyword is used to define a class in Kotlin?

class (Correct answer)
object
struct
type

Correct answer: class

The `class` keyword is used to define a class in Kotlin, just as in Java.

Question 15: What is the Elvis operator `?:` used for in Kotlin?

Providing a default value when an expression is null (Correct answer)
Type checking at runtime
Declaring nullable types
Performing safe casts

Correct answer: Providing a default value when an expression is null

The Elvis operator `?:` returns the left-hand value if it's not null, or the right-hand value as a fallback when it is null.

Question 16: How do you write a single-expression function in Kotlin?

fun add(a: Int, b: Int) { return a + b }
fun add(a: Int, b: Int) -> a + b
def add(a: Int, b: Int) = a + b
fun add(a: Int, b: Int) = a + b (Correct answer)

Correct answer: fun add(a: Int, b: Int) = a + b

Kotlin allows single-expression functions using the `=` syntax, omitting the curly braces and `return` keyword.

Question 17: What is the key difference between `Any` and `Any?` in Kotlin?

There is no practical difference
Any can hold null while Any? cannot
Any? is the supertype of all types including nullable, while Any excludes null (Correct answer)
Any is for objects only while Any? includes primitives

Correct answer: Any? is the supertype of all types including nullable, while Any excludes null

`Any?` is the supertype of all Kotlin types including nullable types, while `Any` is the supertype only of non-nullable types and cannot hold null.

Question 18: What does `!!` do in Kotlin?

Non-null assertion that throws NullPointerException if null (Correct answer)
Double negation operator
Safe call on nullable
Checks if a value is not null

Correct answer: Non-null assertion that throws NullPointerException if null

The `!!` operator asserts that the value is non-null and throws `KotlinNullPointerException` if the value is null.

Question 19: What is the difference between `lazy` and `lateinit` in Kotlin?

`lazy` is for val properties with computed initialization; `lateinit` is for var properties initialized externally (Correct answer)
No difference
`lateinit` is thread-safe; `lazy` is not
`lazy` supports primitives; `lateinit` does not

Correct answer: `lazy` is for val properties with computed initialization; `lateinit` is for var properties initialized externally

`lazy` is used with `val` and initializes via a lambda on first access, while `lateinit` is for `var` properties that will be set later by external code.

Question 20: What is a sealed class in Kotlin used for?

Enabling multiple inheritance
Preventing any instantiation
Restricting class hierarchies to a fixed set of subclasses (Correct answer)
Creating singletons

Correct answer: Restricting class hierarchies to a fixed set of subclasses

A `sealed class` restricts its subclasses to be defined in the same file, enabling exhaustive `when` expressions over class hierarchies.

Question 21: What is a closure in Kotlin?

A function that closes a file resource
A function with no parameters
A lambda that captures variables from its enclosing scope (Correct answer)
A sealed class with no subclasses

Correct answer: A lambda that captures variables from its enclosing scope

A closure is a lambda or function literal that captures and can access variables from the surrounding scope.

Question 22: What is the correct way to define a nullable String in Kotlin?

String!
String? (Correct answer)
String
Nullable<String>

Correct answer: String?

Appending `?` to a type in Kotlin marks it as nullable, allowing it to hold null values.

Question 23: What is an abstract class in Kotlin?

A class with no constructor
A class that cannot be instantiated and may have abstract members (Correct answer)
A class with only one subclass
A class that cannot have properties

Correct answer: A class that cannot be instantiated and may have abstract members

An `abstract` class in Kotlin cannot be instantiated directly and may contain abstract members that subclasses must implement.

Question 24: What is a DSL in Kotlin and how are extension functions used to build them?

A database query language
A debugging syntax layer
A test framework
A Domain-Specific Language built using extension functions and lambdas with receivers for readable configuration code (Correct answer)

Correct answer: A Domain-Specific Language built using extension functions and lambdas with receivers for readable configuration code

Kotlin DSLs use extension functions, lambdas with receivers, and infix functions to create readable, type-safe configuration APIs like Gradle Kotlin DSL or HTML builders.

Question 25: Which keyword is used to define a constant at the top level in Kotlin?

static val
final val
immutable val
const val (Correct answer)

Correct answer: const val

`const val` declares a compile-time constant in Kotlin, which must be a primitive type or String at the top level or inside an object.

Question 26: What does `flatMap` do in Kotlin?

Flattens a nested list into a single list
Maps elements and flattens the resulting nested lists into one (Correct answer)
Filters and maps simultaneously
Groups elements into sublists

Correct answer: Maps elements and flattens the resulting nested lists into one

`flatMap` applies a transformation that returns a collection for each element, then flattens all results into a single list.

Question 27: Which of the following correctly declares a Kotlin function that returns an Int?

def add(a: Int, b: Int): Int
function add(a: Int, b: Int): Int
fun add(a: Int, b: Int) Int
fun add(a: Int, b: Int): Int (Correct answer)

Correct answer: fun add(a: Int, b: Int): Int

Kotlin functions use the `fun` keyword, with parameter types and return type separated by a colon.

Question 28: What is the purpose of generics in Kotlin?

To allow functions to accept any number of parameters
To avoid using nulls
To make classes serializable
To enable type-safe reusable code that works with different types (Correct answer)

Correct answer: To enable type-safe reusable code that works with different types

Generics allow writing type-safe classes and functions that work across different types without duplicating code.

Question 29: Which built-in higher-order function applies a transformation to each element of a collection?

map (Correct answer)
filter
reduce
fold

Correct answer: map

`map` applies a given transformation function to each element of a collection and returns a new list of results.

Question 30: What is the difference between `StateFlow` and `SharedFlow` in Kotlin?

SharedFlow holds state; StateFlow does not
StateFlow holds the latest value and replays it to new collectors; SharedFlow is configurable (Correct answer)
There is no difference
StateFlow is hot and holds state; SharedFlow is cold and stateless

Correct answer: StateFlow holds the latest value and replays it to new collectors; SharedFlow is configurable

`StateFlow` always holds and replays the latest value to new subscribers, while `SharedFlow` is configurable with custom replay and buffering.

Question 31: What is `Pair` in Kotlin?

A tuple with two comparable values
A data class holding two values of potentially different types (Correct answer)
A mutable two-element list
A map with two entries

Correct answer: A data class holding two values of potentially different types

`Pair<A, B>` is a simple data class that holds two values, accessible via `.first` and `.second`.

Question 32: What is the `Nothing` type in Kotlin primarily used for?

Representing void functions
Generic type wildcards
Storing null values
Functions that always throw or loop infinitely (Correct answer)

Correct answer: Functions that always throw or loop infinitely

`Nothing` represents a value that never exists; it's the return type of functions like `throw` expressions or infinite loops that never complete normally.

VSkills Certified Kotlin Developer

The VSkills Certified Kotlin Developer exam validates proficiency in Kotlin programming, covering language fundamentals, object-oriented and functional programming, collections, coroutines, and advanced language features like extensions and delegates.

Questions

35 min

Time Limit

50%

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