Scala Interview Questions for Developers

Scala

Developed to address the weaknesses of Java, Scala maintains compatibility with the Java Virtual Machine (JVM) while having more concise syntax, embracing functional programming (in addition to object-oriented programming), and having more powerful type inference.

Drawing inspiration from multiple programming languages such as Java, Haskell, and ML, Scala combines the object-oriented and functional programming paradigms in an effort to offer the most advantageous aspects of each.

We have developed practical coding assignments and interview questions tailored to evaluate developers’ Scala skills during coding interviews. Furthermore, we have compiled a set of best practices to ensure that your interview questions accurately gauge the candidates’ proficiency in Scala.

Scala example question

Help us design a parking lot

Hey candidate! Welcome to your interview. Boilerplate is provided. Feel free to change the code as you see fit. To run the code at any time, please hit the run button located in the top left corner.

Goals: Design a parking lot using object-oriented principles

Here are a few methods that you should be able to run:

  • Tell us how many spots are remaining
  • Tell us how many total spots are in the parking lot
  • Tell us when the parking lot is full
  • Tell us when the parking lot is empty
  • Tell us when certain spots are full e.g. when all motorcycle spots are taken
  • Tell us how many spots vans are taking up

Assumptions:

  • The parking lot can hold motorcycles, cars and vans
  • The parking lot has motorcycle spots, car spots and large spots
  • A motorcycle can park in any spot
  • A car can park in a single compact spot, or a regular spot
  • A van can park, but it will take up 3 regular spots
  • These are just a few assumptions. Feel free to ask your interviewer about more assumptions as needed

Junior Scala interview questions

Question: Explain the concept of immutability in Scala and why it is important in functional programming.

Answer: In Scala, immutability refers to the property of an object or data structure that once created, cannot be modified. Immutability is fundamental in functional programming because it helps avoid side effects and makes programs easier to reason about and understand.

Example:

val numbers = List(1, 2, 3)
val doubledNumbers = numbers.map(_ * 2)

Question: The following Scala code is intended to calculate the factorial of a given number using recursion. However, it contains a syntax error and doesn’t compile. Identify the error and fix the code.

def factorial(n: Int): Int = {
  if (n == 0) {
    1
  } else {
    n * factorial(n - 1)
  }
}

Answer: The syntax error in the code is the missing closing brace } for the factorial function.

Question: Explain what Option[T] is in Scala, and how it helps in handling null or missing values.

Answer: In Scala, Option[T] is a container type that represents an optional value. It can either hold a value of type T (Some[T]) or be empty (None). This type is commonly used to handle situations where a value may be present or absent, such as when dealing with the possibility of null or missing values.

Example:

val maybeName: Option[String] = Some("John")
// Safe access to the value using pattern matching
val name = maybeName match {
  case Some(value) => value
  case None => "Unknown"
}

Question: The following Scala code is intended to filter out even numbers from a list. However, it contains a logical error and does not produce the correct result. Identify the error and fix the code.

def filterEvenNumbers(numbers: List[Int]): List[Int] = {
  var result: List[Int] = List()
  for (num <- numbers) {
    if (num % 2 == 0)
      result = num :: result
  }
  result.reverse
}

Answer: The logical error in the code is that the filtered even numbers are not collected into a new list. The correct code has been provided.

Intermediate Scala interview questions

Question: Explain the concept of immutability in Scala and why it is important in functional programming.

Answer: Immutability ensures that data does not change unexpectedly, which is crucial for achieving referential transparency and making functions pure.

Question: Explain the concept of higher-order functions in Scala and provide an example.

Answer: Higher-order functions are functions that can take other functions as parameters or return them as results.

def applyFunctionTwice(func: Int => Int, x: Int): Int = {
  func(func(x))
}
val increment: Int => Int = (x: Int) => x + 1
val result = applyFunctionTwice(increment, 5)

Senior Scala interview questions

Question: Explain the concept of Futures in Scala and their role in asynchronous programming.

Example:

import scala.concurrent.{Future, ExecutionContext}
import scala.concurrent.ExecutionContext.Implicits.global

def performTask(): Future[String] = Future {
  Thread.sleep(2000)
  "Task completed successfully!"
}

performTask().onComplete {
  case Success(result) => println(s"Result: $result")
  case Failure(ex) => println(s"Failed with: ${ex.getMessage}")
}

Question: Explain the concept of implicits in Scala and their role in automatic type conversions.

Example:

case class Meter(value: Double)
implicit def meterToCentimeter(m: Meter): Centimeter = Centimeter(m.value * 100)

val distance: Centimeter = Meter(5.5)

Best interview practices for Scala roles

To conduct successful Scala interviews, consider the following best practices:

  • Develop technical questions that reflect real-world scenarios.
  • Create a cooperative environment by encouraging questions.
  • Ensure understanding of concurrent and parallel programming.

Moreover, adjust the complexity of questions, promptly inform candidates about their application status, and provide opportunities for them to inquire about the evaluation process and collaboration with your team.