28 Scala Interview Questions & Answers

28 Scala Interview Questions & Answers

Most of the banks, Governments, Telecom and Social Networking companies, etc uses Scala because it supports both Object-Oriented Concepts and Functional Programming Concepts. Scala is a Framework like Apache Kafka. It is popular among big data professionals. Any developer at a big data world should be smart enough to understand and apply the complexity. Scala is hard to master but if you have experience in a spark programming language then it will be easy to learn Scala and its concepts like Tuples, Macros, Functions and make a better coding experience.

The designer and developer of the Scala is Martin Odersky started to work on Scala in 2001. Scala was publicly released in early 2004 on Java Platform and on .NET in June 2004.

Now, we are going to discuss the simple, intermediate and advanced questions that are mostly asked in the interview. And if you are going for the interview of Apache Spark then questions related to the Scala will be asked because Spark in developed in Scala. Scala can interact or it is language interoperability with Java Programming language and libraries of Scala and Java can be used in Java or Scala code. And Scala is derived from the words Scalable and Language.

Let’s see the questions that can be asked in the interview.

What is Scala? Is it a platform or language?

Scala is a general purpose programming language which supports Object-Oriented Programming Language concepts and Functional Programming Language concepts. Scala runs on JVM (Java Virtual Machine).

All the types are checked in compile-time.

Is Scala is pure Object Oriented Programming (OOP) Language?

Object-Oriented means everything is in an object.

Scala is a pure Object Oriented Programming Language because in Scala everything is an Object and values. Scala does not have primitive data types and also does not have static members.

What is Scala Map?

It is a collection of key-value pairs. We can bring back the value using the keys. In Scala Map keys are unique and values are not unique.

There are two types of Maps: mutable and immutable

In Scala Map by default immutable is available and you can use a mutable map by importing a package: scala.collection.mutable is the name of a package that has to be imported. If we want to use both mutable and immutable in the same program then we can access a mutable map using mutable. map and immutable map by using the name of the map.

Reasons to use Scala

1) The name itself says Scalable Language but it has many features like scalable, maintainability and testability etc which makes it more advantageous than  Functional Programming Language.

2) Singleton and Companion Objects provide clean solutions.

3) There is no need for the ternary operator.

What is the primary constructor?

The constructor is defined with the class definition itself. Each class must have one primary constructor either parameter or parameterless constructor.

Difference between Array and ArrayBuffer?

Array: It is similar to Java’s Primitive arrays. The size of an array cannot be changed once it is created. If you append an element in Array then a new array is created.

ArrayBuffer: It is similar to Java’s ArrayList. The size of ArrayBuffer is variable it can be increased or decreased dynamically. If you append element then the size of ArrayBuffer gets larger.

What is tail recursion?

This mechanism is used to optimize the recursive functions so that it will not create the new stack space instead of it will use current stack space. @annotation.tailrec is used before defining the function and a recursive call has to be the last statement then only it will run without giving any error.

What is Case class and give its advantages?

It is a class that is defined using the case keyword. It is beneficial to use the case keyword because it avoids boilerplate code.

Advantages are:

1) We can avoid writing bolierplate code because Scala by default adds toString, hashCode, and few more methods.

2) By default, there is a copy method.

3) We can use case class in Pattern Matching.

4) The Case Class is Serializable.

5) The companion object with apply and unapply methods is added so no need to use new keyword and create an instance of the case class.

What is “Type interference” in Scala?

The types that can be determined by the Scala in compile-time is called Type interference. The types are Data types or Result types. We use many types many times in the program in many places in the program like Variable types, Object types, Method/Function Parameters types, Function/Method Return types, etc.

What is Null in Scala?

The Null in Scala is a final class. The package for the Null is scale. Null and it has one and only one instance called null.

Difference between Val and var?

These two are used to define the variables. The difference between val and var is:

var:  var is variable. Variables can be changed. Variables are used to declare mutable variables so we can reassign the values.

Val: val is value. Values cannot be changed. Values are used to declare immutable variables so cannot reassign the values.

What is REPL in Scala?

REPL (Read Evaluate Print Loop) acts as an interpreter to execute the scala code from a command prompt. It is pronounced as the ripple. The REPL is also known as Scala CLI (Command Line Interface) or scala command line shell.

The REPL is used to test and develop the small snippets of scala for the practice.

Types of Evalutions and Explain each

There are two types of program evalutions:

1) Lazy Evalutions

2) Eager Evalutions

Eager Evalutions: It means that evaluating programs at compile time and the program can be evaluated anytime whether there is a client or not it is irrespective to the time.

Lazy Evaluations: It means evaluating programs at run time it means on demand the program is evaluated only when there is a client.

What is the Diamond Problem in Scala?

The Diamond Problem in Scala means the problem of Multiple Inheritance. It is also called as the Deadly  Diamond Problem. The Scala sloves the diamond problem automatically using some rules in the language and that rules are called as Class Linearization.

Why Scala does not have a static keyword?

This decision is taken by the designers of Scala Language. The reason not to have the static keyword in the language is we can access other class members without creating an object or without using object and this is completely against Object Oriented Programming (OOP) Language principles and this decision makes Scala pure Object Oriented Programming Language. If a language supports the static keyword then it is not pure Object Oriented Programming Language same as Java (it is not pure OOP language).

Uses of object keyword in Scala?

Purpose to use object keyword are:

1) Used to create Singleton object in Scala

eg: object MyFirstSingletonObject

It becomes the Singleton object automatically.

2) Used to define the scala executable programs

eg:

object MyExeProgram {
def main(args: Array[String]) {
println(“Hello World”)
                     }
          }

When we define the main method in an object then it automatically executable scala program.

3) We can define static members like static methods and static variables without using static keyword.

4)It is used to define the Factory Methods.

What is Scala Application?

An app is a trait defined in scala package like scala.app. When Object or Class extends this trait then it will become Scala Executable programs automatically.

The advantage of using the app is that there is no need to use the main method.

The disadvantage is we should same name args to refer command line argument.

Features supported by the Scala compared to Java

1) Scala supports Operator Overloading but does not support Operator Overloading.

2) Scala does not support ++ and –, but Java support.

3) Scala has only Unchecked Exceptions, but Java has both the Checked and Unchecked Exceptions.

4) Scala does not support break, continue statements, but Java supports and also uses them.

5) Scala does not have explicit Type Casting feature, but Java has explicit Type Casting feature.

6) Scala supports Pattern Matching feature, but Java does not support.

7) Scala does not uses Primitive data types, but Java uses Primitive data types.

8) Scala does not support static members, but Java has static member feature.

9) Scala supports the Implicit and Traits, but Java does not support Implicit and Traits.

These are important and comparable features between Scala and Java.

Difference between Functions and Methods in Scala?

There is a set of instructions and it is used many times in programs it is called as functions in Functional Programming Languages and methods in Object Oriented Programming Languages.

Scala supports both Functions and Methods there is no syntax difference. The difference is minor

Method is associated with an object and when we define a method in Scala class or trait than to call the method we use an instance of a class. We cannot use Scala method directly without using an object.

The functions are not associated with the Scala class or trait. It is defined in the Scala package. We can access functions without using objects.

What are the default imports in the Scala Language?

There are few default packages in the Scala Language:

1) java.lang.package

2) scala package

3) scala.PreDef

This is the packages by default in the Scala Programs.

Define apply methods and unapply methods in Scala?

The apply methods and unapply methods are very important in Scala. They play important role in mapping and unmapping data between Form data and Model data.

Apply method: It is used to compose or assemble the objects using its components. Combining the two or more components and forming the object is to apply a method in Scala.

Unapply method: It is used to decompose or disassembles the objects using its components. It is the reverse process of the apply method. If there is one object by decomposing we can form its two or more components.

What is Range in Scala?

The range is Lazy Collection in Scala. It is a class that is available in the package called as a scala. Range. It represents the sequence of the integer values and it is in the proper order.

What is a pure function?

To declare a function in Scala there is no need to create the instance object. The pure functions means which does not have any side effects. Means it doesn’t matter how many times we call same inputs it gives the same output without a change in the result.

What is auxiliary Constructor in Scala?

It is a secondary constructor in Scala declared using the keywords like def or this. Using the auxiliary constructor we can overload the constructors. In Scala, every auxiliary constructor should differ in the number of parameters and data types.

Difference in List and Array

1) List are immutable recursive data structure and an array is a mutable data structure.

2) List are covariants and Arrays are invariants.

3) List has variable sized data that is when we perform the operations the size of the list may increase or decrease but an array is fixed sized data that is we cannot change the size once the array is declared.

Which are the popluar Scala based Framework?

The popular frameworks based on the scala are:

1) Play Framework: REST API URL’s are called as routes and all the routes are placed at once in the same place. It is a stateless web framework.

2) Scalatra Framework: It is simple and easy to use and REST API can be easily developed.

3) Spray Framework: It is concise and built on top of Akka framework and it is simple to develop REST API.

4) Lift Framework: This framework allows the routing and we can use routing by the Parent Matching concept.

Considerations should have while using Scala Streams

In Scala, streams are of lazy collections which are created using the starting elements. Streams are like the list we can add elements only when they are accessed so they are called as the lazy. As they are lazy in adding elements they are unbounded and once they are added they become cached.

How yield works in Scala?

Yield is specified before the expressions and value returned from the expression will return as a collection. When you want to return the value of expression then the yield keyword is used.

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