Top 35+ Scala Interview Questions & Answers [ SPARK TRICKS ] |ACTE
Scala Interview Questions and Answers

Top 35+ Scala Interview Questions & Answers [ SPARK TRICKS ]

Last updated on 04th Jul 2020, Blog, Interview Questions

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These Scala Interview Questions have been designed specially to get you acquainted with the nature of questions you may encounter during your interview for the subject of Scala As per my experience good interviewers hardly plan to ask any particular question during your interview, normally questions start with some basic concept of the subject and later they continue based on further discussion and what you answer .we are going to cover top 100 Scala Interview questions along with their detailed answers. We will be covering Scala scenario based interview questions Scala interview questions for fresher’s as well as interview Scala questions and answers for experienced

1) Explain what is Scala?


Scala is an object functional programming and scripting language for general software applications designed to express solutions in a concise manner.

2) What is a ‘Scala set’? What are methods through which operation sets are expressed?


Scala set is a collection of pairwise elements of the same type. Scala set does not contain any duplicate elements. There are two kinds of sets, mutable and immutable.

3) What is a ‘Scala map’?


Scala map is a collection of key or value pairs. Based on its key any value can be retrieved. Values are not unique but keys are unique in the Map.

4) What is the advantage of Scala?


  • Less error prone functional style
  • High maintainability and productivity
  • High scalability
  • High testability
  • Provides features of concurrent programming

5) In what ways Scala is better than other programming language?


  • The arrays uses regular generics, while in other language, generics are bolted on as an afterthought and are completely separate but have overlapping behaviors with arrays.
  • Scala has immutable “val” as a first class language feature. The “val” of scala is similar to Java final variables. Contents may mutate but top reference is immutable.
  • Scala lets ‘if blocks’, ‘for-yield loops’, and ‘code’ in braces to return a value. It is more preferable, and eliminates the need for a separate ternary operator.
  • Singleton has singleton objects rather than C++/Java/ C# classic static. It is a cleaner solution
  • Persistent immutable collections are the default and built into the standard library.
  • It has native tuples and a concise code
  • It has no boiler plate code

6) What are the Scala variables?


Values and variables are two shapes that come in Scala. A value variable is constant and cannot be changed once assigned. It is immutable, while a regular variable, on the other hand, is mutable, and you can change the value.

7) Mention the difference between an object and a class?


A class is a definition for a description. It defines a type in terms of methods and composition of other types. A class is a blueprint of the object. While,an object is a singleton, an instance of a class which is unique. An anonymous class is created for every object in the code, it inherits from whatever classes you declared object to implement.

8) What is recursion tail in scala?


Recursion’ is a function that calls itself. A function that calls itself, for example, a function ‘A’ calls function ‘B’, which calls the function ‘C’. It is a technique used frequently in functional programming. In order for a tail recursive, the call back to the function must be the last function to be performed.

9) What is ‘scala trait’ in scala?


Traits’ are used to define object types specified by the signature of the supported methods. Scala allows to be partially implemented but traits may not have constructor parameters. A trait consists of method and field definition, by mixing them into classes it can be reused.

10) When can you use traits?


There is no specific rule when you can use traits, but there is a guideline which you can consider.

  • If the behaviour will not be reused, then make it a concrete class. Anyhow it is not a reusable behaviour.
  • In order to inherit from it in Java code, an abstract class can be used.
  • If efficiency is a priority then lean towards using a class
  • Make it a trait if it might be reused in multiple and unrelated classes. In different parts of the class hierarchy only traits can be mixed into different parts.
  • You can use abstract class, if you want to distribute it in compiled form and expects outside groups to write classes inheriting from it.

11) What is Case Classes?


Case classes provides a recursive decomposition mechanism via pattern matching, it is a regular classes which export their constructor parameter. The constructor parameters of case classes can be accessed directly and are treated as public values.

12) What is the use of tuples in scala?


Scala tuples combine a fixed number of items together so that they can be passed around as whole. A tuple is immutable and can hold objects with different types, unlike an array or list.

13) What is function currying in Scala?


Currying is the technique of transforming a function that takes multiple arguments into a function that takes a single argument Many of the same techniques as language like Haskell and LISP are supported by Scala. Function currying is one of the least used and misunderstood one.

14) What are implicit parameters in Scala?


Implicit parameter is the way that allows parameters of a method to be “found”. It is similar to default parameters, but it has a different mechanism for finding the “default” value. The implicit parameter is a parameter to method or constructor that is marked as implicit. This means if a parameter value is not mentioned then the compiler will search for an “implicit” value defined within a scope.

15) What is a closure in Scala?


A closure is a function whose return value depends on the value of the variables declared outside the function.

16) What is Monad in Scala?


A monad is an object that wraps another object. You pass the Monad mini-programs, i.e functions, to perform the data manipulation of the underlying object, instead of manipulating the object directly. Monad chooses how to apply the program to the underlying object.

17) What is Scala anonymous function?


In a source code, anonymous functions are called ‘function literals’ and at run time, function literals are instantiated into objects called function values. Scala provides a relatively easy syntax for defining anonymous functions.

18) Explain ‘Scala higher order’ functions?


Scala allows the definition of higher order functions. These are functions that take other functions as parameters, or whose result is a function. In the following example, apply () function takes another function ‘f’ and a value ‘v’ and applies function to v.

19)What is the difference between var and value?


In scala, you can define a variable using either a, val or var keywords. The difference between val and var is, var is much like java declaration, but val is little different. We cannot change the reference to point to another reference, once the variable is declared using val. The variable defined using var keywords are mutable and can be changed any number of times.

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    20) What are option, some and none in scala?


      ‘Option’ is a Scala generic type that can either be ‘some’ generic value or none. ‘Queue’ often uses it to represent primitives that may be null.

    21) How do I append to the list?


    • In scala to append into a list, use “:+” single value
    • var myList = List.empty[String]
    • myList :+= “a”
    • myList :+= “b”
    • myList :+= “c”
    • use++ for appending a list
    • var myList = List.empty[String]
    • myList ++= List(“a”, “b”, “c”)

    22) How can you format a string?


    To format a string, use the .format () method, in scala you can use

    Val formatted= “%s %i”.format (mystring.myInt)

    23) Why scala prefers immutability?


    Scala prefers immutability in design and in many cases uses it as default. Immutability can help when dealing with equality issues or concurrent programs.

    24) What are the four types of scala identifiers?


    The four types of identifiers are:

    • Alpha numeric identifiers
    • Operator identifiers
    • Mixed identifiers
    • Literal identifiers

    25) What are the different types of Scala literals?


    The different types of literals in scala are:

    • Integer literals
    • Floating point literals
    • Boolean literals
    • Symbol literals
    • Character literals
    • String literals
    • Multi-Line strings

    26)How is Scala a programming language with a combination of both functional and object-oriented programming?


    Scala programming language treats every single value as an object, which also includes Functions. This way, it is a combination of both functional and object-oriented programming.

    27)What are the frameworks supported by Scala?


    There are various frameworks supported by Scala that include the following.

    • Spark Framework
    • Play  Framework
    • Akka Framework
    • Neo4j Framework
    • Bowler Framework
    • Scalding Framework
    • Lift Framework
    • Scala Frameworks

    28)What are the different kinds of variables in Scala?


    There are mainly two types of variables in Scala, which include Mutable variables and Immutable Variables.

    29) Define the features of Mutable Variables?


    The Mutable variables can be declared by using the var keyword. The values in these variables support changes.

    30) Define the features of Immutable Variables?


    The Immutable Variables can be declared by using the val keyword. The values in these variables do not support changes.

    31) Define Stream in Scala?


    A stream is defined as a Lazy list, which helps in the evaluation of the elements only when they are needed.

    32)What is the benefit of Streams in Scala?


    The benefit of Streams in Scala is that it helps in enhancing the performance of the program.

    33)What are the advantages of Scala?


    There are several advantages of Scala, which include the following.

    • Scalable
    • Maintainable
    • Productive
    • Concurrent programming
    • Consists of Native Tuples codes Consists of Testable codes
    • Concise code
    • No Boilerplate code
    • Singleton objects are clearer in the solution than static

    34)What are the different operators in Scala?


    The different operators in Scala include the following.

    • Assignment operators
    • Relational operators
    • Logical operators
    • Arithmetic operators
    • Bitwise operators

    35)What is Recursion in Scala?


    Recursion is referred to as the function in Scala that calls itself.

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    36) Give an example of Recursion in Scala?


    When  Function A calls function B, which further calls function C, then it is called recursion in Scala and is mostly used in Functional Programming.

    37)What is Tail Recursive?


    Tail recursive is a call back to the function that should be the end task function that is to be performed.

    38)What are Tuples in Scala?


    Tuples in Scala combine the finite numbers of items all together so that the programmer can Pass tuple around as a whole.

    39) Is Tuple immutable?


    Yes, Tuple is immutable mostly in the case of Array or List wherein it can hold objects with different datatypes.

    40)What is Class in Scala?


    Class in Scala combines the data and its methods in Scala.

    41)What is an Object in Scala?


    An object in Scala is one particular instance in a class.

    42) Do we need App in Scala?


    Yes, we do need App in Scala so that it could act as a helper class that holds the main method and its members together. 

    43)What is the benefit of App trait? Give an example?


    An App trait can be used for quickly turning the objects into executable programs. For example, we can have our classes extend App with the purpose of rendering the executable code.

    44)Define Higher-order functions?


    Higher-order functions are defined as a function that does one or more of the functions as arguments, returns a function as its result.

    45)What are the different scopes for variables in Scala?


    There are three different scopes for variables in Scala, which include Fields, Method Parameters, and Local Variables.

    46)What are Fields in Scala?


    Fields are variables that are declared inside an object. They can be accessed from any point inside the program, depending upon the access modifiers. It can be declared using val or var.

    47)What are the method parameters?


    Method parameters are Pass values to the methods. They are strictly immutable and can be accessed from inside a method. However, the use of Reference can be made for accessing them from outside the method provided.

    48)What are the local variables?


    Local variables can be accessed if we return them from the method. They are declared inside a method and accessible from there only.

    50) Define Traits in Scala?


    Traits in Scala is a unit that encapsulates the method and its variables or field.

    51)What is the difference between Scala and Java?


    The difference between Scala and Java include the following.

    All the values in Scala are treated as Objects. All the values in Java are not treated as Objects.

    Scala java
    It supports closures             It does not support closures  
    It has Type-inference           It does not have Type-inference
    It supports nested functions         It does not support nested functions
    It supports concurrency           It does not concurrency  
    It has different traits                      It does not support traits  
    It has Domain-Specific Language Or DSL support.  

    It does not have Domain Specific Language or DSL support.

    52)What is an extend keyword in Scala?


    An extend keyword in Scala helps in extending a base Scala class so that you can design an inherited class just like it is done in Java by use of extending keywords. 

    53)What are the restrictions in extend key keywords in Scala?


    In Scala, there are two restrictions to extend keywords. This includes the first one as method overriding, which requires override keywords and the second one as a primary constructor that can pass parameters to the base constructor.

    54)Define Implicit classes with syntax in Scala?


    Implicit classes with syntax in Scala supports the implicit conversation with the class’s primary constructor when the class is in scope. It is marked with the “implicit” keyword and introduced in Scala 2.10 version.

    55)What are the different types of Access Modifiers available in Scala?


    There are three types of Access Modifiers available in Scala, which include Private, Public, and Protected.

    56)What is a Private Access Modifier?


    A Private Access Modifier supports the restriction of the accessibility of a private member to the class or object as a set or declared in advance.

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    57)What is Public Access Modifier?


    Public Access Modifier does not require any explicit modifier to allow the public members to get access, and the members can access from anywhere.

    58)What is Protected Access Modifier?


    A Protected Access Modifier supports accessibility only from the subclass of the class where the member is defined and authorized.

    59) Define Monad in Scala?


    Monad in Scala is an object. It helps in wrapping another object as per the mini-program, which can be a function to perform data manipulation, particularly on the underlying object. It indirectly manipulates the object and chooses the method to apply for the program on the underlying object.

    61)Why is Immutability preferred in Scala?


    Immutability is preferred in Scala because it supports the design and uses it as a default. It helps in dealing with the Concurrent programs as well as Equality issues.

    62) Define different packages in Scala?


    There are three different packages in Scala. These are,

    Java.lang._: It is a package that provides classes that are fundamental for the design of the Java programming language. It is a package that imports every class in Scala for input-output resources.

    PreDef: It offers type aliases for types that are used regularly used in Scala. These include Safe, Map, and the List constructors.

    63)What is the role of Options in Scala?


    Options have a vital role in Scala, which is to Wrap the Missing value.

    64)Define types of Scala Identifiers?


    There are four types of Scala Identifiers, which include,

    • Literal identifiers
    • Alphanumeric identifiers
    • Mixed identifiers
    • Operator identifiers

    65)What are the procedures to compile Scala code?


    The procedure to compile Scala Code starts with the writing of the Code in Scala IDE or Scala REPL, which is later converted into the Byte Code and thereby transferred to Java Virtual Machine or JVM for compilation purpose.

    66)What are the features of Yield in Scala?


    Yield has several features as,

    • It is used as a Loop.
    • It produced value for each iteration.
    • It supports the use of Map, Flat Map, and Filters along with nomads.

    67)Define Null, Nill, None, and Nothing in Scala?


    Null, Nill, None, and Nothing in Scala can be defined as follows.

    • Null denotes the absence of a value.
    • Nil represents the end of a List.
    • None is the value of an option that has no value.
    • Nothing is the lowest type in the type system.

    68)What are the different Loops in Scala?


    There are three different types of Loops in Scala.

    • While Loop helps in repeating the statement or group of the statement when the condition comes out to be true, this way, it tests the conditions before the execution of the Loop body.
    • Do-While helps in testing the condition at the end of the Loop body.
    • For, helps in executing a sequence of statement number of times and abbreviates the code that manages in the Loop variable.
    • Break acts as a Loop control statement that terminates the Loop statement and transfers the execution to the statement that soon follows the Loop.

    69)What is an Infinite Loop?


    An Infinite Loop appears when a condition never becomes a false statement.

    70)What are the different String Methods?


    • There are five different String Methods which include,
    • String trim(): It returns the copy of the string with leading and trailing of the whitespace omitted.
    • String to Uppercase: It converts all of the features in the String to the Uppercase using the given Locale rules.
    • Char[] to CharArray(): It converts the string to a new character array.
    • String[] split(String regext): It splits the string around the matches of the given regular expression.
    • Int length(): It returns the length of the string.

    71) Mention The Difference Between An Object And A Class ?


    A class is a definition for a description.  It defines a type in terms of methods and composition of other types.  A class is a blueprint of the object. While, an object is a singleton, an instance of a class which is unique. An anonymous class is created for every object in the code, it inherits from whatever classes you declared object to implement.

    72) What Are The Four Types Of Scala Identifiers ?


    The four types of identifiers are

    • Alpha numeric identifiers
    • Operator identifiers
    • Mixed identifiers
    • Literal identifiers

    73)What Is Primary Constructor? What Is Secondary Or Auxiliary Constructor In Scala? What Is The Purpose Of Auxiliary Constructor In Scala? Is It Possible To Overload Constructors In Scala?


    Scala has two kinds of constructors:

    • Primary Constructor
    • Auxiliary Constructor
    • Primary Constructor: In Scala, Primary Constructor is a constructor which is defined with class definition itself. Each class must have one Primary Constructor: Either Parameter constructor or Parameterless constructor.


    • class Person
    • Above Person class has one Zero-parameter or No-Parameter or Parameterless Primary constructor to create instances of this class.
    • class Person (firstName: String, lastName: String)
    • Above Person class has a two Parameters Primary constructor to create instances of this class.
    • Auxiliary Constructor: Auxiliary Constructor is also known as Secondary Constructor. We can declare a Secondary Constructor using ‘def’ and ‘this’ keywords as shown below:
    • class Person (firstName: String, middleName:String, lastName: String){
    • def this(firstName: String, lastName: String){
    • this(firstName, “”, lastName)}

    74) What Is The Use Of Auxiliary Constructors In Scala?please Explain The Rules To Follow In Defining Auxiliary Constructors In Scala?


    In Scala, The main purpose of Auxiliary Constructors is to overload constructors. Like Java, We can provide various kinds of constructors so that use can choose the right one based on his requirement.

    Auxiliary Constructor Rules:

    • They are like methods only. Like methods, we should use ‘def’ keyword to define them.
    • We should use same name ‘this’ for all Auxiliary Constructors.
    • Each Auxiliary Constructor should start with a call to previous defined another Auxiliary Constructor or Primary Constructor. Otherwise compile-time error.
    • Each Auxiliary Constructor should differ with their parameters list: may be by number or types.
    • Auxiliary Constructors cannot call a super class constructors. They should call them through Primary Constructor only.
    • All Auxiliary Constructors call their Primary Constructor either directly or indirectly through other Auxiliary Constructors.

    NOTE:- If you want to learn about Scala’s Constructors, please refer my Scala posts at: Primary Constructor and Auxiliary Constructor.

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    75)What Are The Differences Between Array And Arraybuffer In Scala?


    Differences between Array and Array Buffer in Scala:

    • Array is fixed size array. We cannot change its size once its created.
    • Array Buffer is variable size array. It can increase or decrease it’s size dynamically.
    • Array is something similar to Java’s primitive arrays.
    • Array Buffer is something similar to Java’s Array List.

    76) What Is Case Class? What Is Case Object? What Are The Advantages Of Case Class?


    • Case class is a class which is defined with “case class” keywords. Case object is an object which is defined with “case object” keywords. Because of this “case” keyword, we will get some benefits to avoid boilerplate code.
    •         We can create case class objects without using “new” keyword. By default, Scala compiler prefixes “val” for all constructor parameters. That’s why without using val or var, Case class’s constructor parameters will become class members, it is not possible for normal classes.

    Advantages of case class:

    • By default, Scala Compiler adds toString, hashCode and equals methods. We can avoid writing this boilerplate code.
    • By default, Scala Compiler adds companion object with apply and unapply methods that’s why we don’t need new keyword to create instances of a case class.
    • By default, Scala Compiler adds copy method too.
    • We can use case classes in Pattern Matching.
    • By default, Case class and Case Objects are Serializable.

    77) When Compare To Normal Class, What Are The Major Advantages Or Benefits Of A Case-class?


    The following are the major advantages or benefits of a Case class over Normal Classes:

    • Avoids lots of boiler-plate code by adding some useful methods automatically.
    • By default, supports Immutability because it’s parameters are ‘val’
    • Easy to use in Pattern Matching.
    • No need to use ‘new’ keyword to create instance of Case Class.
    • By default, supports Serialization and Deserialization.

    78) What Is The Usage Of Isinstance of And Asinstance of Methods In Scala? Is There Anything Similar Concept Available In Java?


    • Both isInstanceOf and asInstanceOf methods are defined in Any class. So no need import to get these methods into any class or object.
    • “isInstanceOf” method is used to test whether the object is of a given type or not. If so, it returns true. Otherwise returns false.
    • scala> val str = “Hello”
    • scala>str.isInstanceOf[String]
    • res0: Boolean = false  
    • “asInstanceOf” method is used to cast the object to the given a type. If the given object and type are of same type, then it cast to given type. Otherwise, it throws java.lang.ClassCastException.
    • scala> val str = “Hello”.asInstanceOf[String]
    • str: String = Hello

    In Java, ‘instanceof’ keyword is similar to Scala’s ‘isInstanceOf’ method. In Java, the following kind of manual type casting is similar to Scala’s ‘asInstanceOf’ method.

    • AccountService service = (AccountService)
    • context.getBean(“accountService”);

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