Scala interview questions

1) What is Scala?

Scala is object-oriented, functional programming and scripting language that supports general software applications which is designed to articulate solutions concisely.

2) What are the types of variables in Scala?

There are primarily two types of variables in Scala. They are:

• Mutable Variables: The mutable variables are declared with the var keyword. Values in mutable variables can be modified.
• Immutable Variables: The immutable variables are declared with the val keyword. Values in Immutable Variables cannot be modified.

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3) Explain about Scala set and Scala map.

Scala set: Scala set is a pair-wise collection of similar elements. It has no duplicate elements. It has two kinds of sets. They are mutable and immutable.

Scala map: Scala map is any set of key or value pairs. Depending on the key, all values can be recovered. Although the values are not unique, the keys are unique on the map.

4) Explain about Scala Variables.

The values and variables are two things that are available in Scala. A value variable is the constant and may not be modified when assigned. It is immutable, whereas the regular variable is mutable, and you may modify the value. The two variable types are:

Var myVar: Int=1;

Var myVal: Int=2;

5) How is an object different from a class?

A class refers to a definition of a description. It sets out a type in terms of the methods and composition of the other types. A class is an overview of the object. At the same time, an object is a singleton, an instance of the class that is unique. An unknown class is created for each object, which inherits the classes that you have declared to be an object to implement.

6) Explain recursion tail and Scala trait.

Recursion tail: Recursion is a function which calls itself. For example, a function 'A' will call function 'B' that will call function 'C'. The technique is often employed in functional programming. For a tail to be recursive, the function call has to be the last function for the execution.

Scala trait: "Traits" are useful in defining the types of objects prescribed by the signature of the supported methods. Scala enables partial implementation, but traits can have no constructor parameters. It includes the method as well as field definition by blending them into the classes it may be reused.

7) When do we use traits?

There is no particular rule for using a trait. However, there is a guideline that can be considered.

• If the behaviour is not be used again, then it must be set as a concrete class.
• To inherit them in Java code, we can use an abstract class.
• If efficiency is a top priority, choose a class.
• Make it a feature if it can be reused into multiple and independent classes. Within various parts of class hierarchy, only the traits may be combined into different parts.
• We may use the abstract class if you would like to distribute it as compiled and expect external groups to write classes inherited from it.

8) What is the use of Tuple in Scala?

A Scala tuple can combine a number of finite items in order that a programmer may send a tuple around it as a whole. In contrast to a list or an array, a tuple is immutable and may contain objects of various data types.

9) What are implicit parameters?

The implicit parameter is how the parameters of a method can be found. It looks similar to the default settings; however, it provides a different method to find the "default" value. If a parameter value is not specified, the compiler will look for the "implicit" value set in a scope.

10) What is a monad?

A monad is the object which surrounds another object. Monad functions can be passes to perform the manipulation of data of the underlying object rather than directly manipulating the objects. Monad selects the manner in which the program is applied to the underlying object.

Scala Intermediate Level Interview Questions:

11) What is function currying?

Currying is a technique to turn a function which takes more than one argument into a function which takes only one argument. Many similar techniques, such as LISP and Haskell, are supported by Scala. Function currying is the less used and the most misunderstood one.

12) Explain Infix, Prefix and Postfix operator notations.

13) How is the function println() different from print()?

println(): The function println() prints the argument in a new line that is received at input each time it is called. 

print(): The function print() will not add a new line after printing the value passed as input.

14) What is a partially applied function?

A partially applied function is the expression in which we fail to provide all the arguments that the function needs. But we provide some or no necessary arguments.

15) What is args?

Args is a Scala array which provides command-line arguments for a Scala script.

16) What is the difference between

Nil: This is a useful way to initialize an empty list because Nil is an object that extends List.

Null: This is an AnyRef type subtype in the hierarchy of Scala types. Since Scala works on JVM, it will use NULL to provide support for the Java null keyword, or in terms of Scala, for providing the type for the null keyword, the Null type exists. It shows the lack of type information for complex types which are inherited from the AnyRef.

Nothing: This is a subtype of all types which exists in the hierarchy of Scala types. It assists in providing the return type of operations that may impact the flow of a normal program. It only can be used as a type because nothing can be instantiated. It integrates all kinds of AnyVal and AnyRef. Nothing is generally used as the return type for methods with an anomalous end and give rise to an exception.

None: There are a lot of circumstances in programming in which we have received unexpectedly null for the methods that we call. In Java, they are manipulated by a try or catch or left unattended, leading to errors in the program. Scala offers a most gracious way to deal with these situations.

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17) What is the scala trait?

A trait is a specific type of class that allows multiple inheritances to be used. While a trait can be extended to a single class, a class may have multiple traits. However, in contrast to classes, features are not instantiated.

Traits are most primarily used when we need dependency injection. In contrast to Java, across the Spring framework, dependency injection is carried out by means of annotations. Scala does not import annotations or special packages. Simply initialize the class with the trait, and it is done, the dependency is injected.

18) For a Scala Programming language, what is the benefit of having immutability in design?

Scala uses default immutability in most cases because it helps to solve problems when managing simultaneous programs and all other equality problems.

19) List some packages that are used in Scala.

20) What is Pattern Matching?

The Pattern Matching in Scala includes a series of alternative sequences that begin with the keyword case. Every available alternative uses the pattern and expressions. Scala assesses these patterns when they are matching, and the "=>" arrow symbol is used for separating them from expressions.

Scala Interview Questions for Experienced:

21) What is Auxiliary Constructor?

In Scala, Auxiliary Constructor is used for Constructor overloading. Auxiliary Constructor should call the already defined primary constructors or auxiliary constructors in its first line of the body.

22) What is the difference between Array and List?

• The List is a recursive data structure that is immutable, whereas the array is a data structure that is sequentially mutable. 
• Lists are covariant, while the arrays are invariants.
• The size of a list increases or decreases automatically depending on the operations that are carried out.
• In Scala, a list is a data structure with a variable size, whereas an array is a data structure with a fixed size.

23) What are apply and unapply methods in Scala?

In Scala, the apply and unapply methods are used to map and unmap data across the form and model data.

Apply method: This method allows us to assemble an object out of its components.

For example, if you want to create an Employee object, use both first and last name components and enter the Employee object with the apply method.

Unapply Method: It is used for the decomposition of an object from its components. This method follows the reverse process of the apply method.

For example, if we have an employee object, then it can be broken down into two components as first name and last name.

24) Why do we use parentheses when accessing elements of an array in Scala?

Since Scala is a functional language, it transforms the access calls to the elements of an arr array. For example arr(arr_el) to arr.apply(arr_el). Therefore, in Scala, access to array elements calls a function, so parentheses are used.

25) Is it possible for a companion object in Scala to access private members of its companion class?

As per the private access specifier, the private members can only be accessed within that class, but the class and companion object of Scala offers special access to the private members. A companion object may have access to all the private members of that companion class. Likewise, a companion class is able to access all the private members of the companion objects.

26) What is the purpose of Scala's App?

The App is a trait that is defined within the scala package as "scala.App" that defines the main method. If a class or an object expands that trait, they automatically become Scala executable programs because they inherit the application's main method. Developers do not need to write the main method while using the App but the one and only disadvantage of using App is that the developers need to use the same name args for referring the command line arguments since Scala. App's main() method will use this name.

27) Why it is said that

Scala implements entirely object-oriented programming. That's because in Scala, each value is an object, and each operator is a function call. In Scala, moreover, the functions, strings, Integer all get the same weighting. And, rather than modifying the existing data, operations in Scala map input values into output values. That is why Scala is also an entirely functional programming language.

28) What makes Scala a scalable programming language?

29) What is type inference?

If the programmer fails to specify the data type of the variable in Scala, the Scala interpreter deduces the data type by itself.

For example: 

val words = "Hey you!"

The val data type will automatically recognize it as a string.

30) Why doesn't the List of classes in Scala provide the append function and provide prepend function?

It is due to the time required for both operations. When adding an element in a list in Scala, the time taken increases linearly along with the size of the List while preparing an element with the "::" operator requires constant time. Nevertheless, if the append function is used, they may use ListBuffer.

31) Name the different Literals used in Scala.

The following are the various Scala Literals.

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

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32) What are the various advantages of using the Scala language?

There are many benefits of using Scala language.

• It is a highly testable and scalable language.
• It uses lightweight syntax.
• Scala is highly productive and easy to maintain.
• It offers Concurrent programming features.
• Scala is functional and object-oriented
• It has Native Tuples
• It holds a less error-prone available style

33) Name the different Operators that Scala uses.

Below are the various Scala Operators.

• Arithmetic Operators
• Bitwise Operators
• Assignment Operators
• Relational Operators
• Logical Operators

34) Explain why Scala is better to use than other programming languages?

Scala language offers many advantages that make it better than other programming languages. The following are its various features:-

• Scala language allows easy writing, debugging, and running of the program, compared to other coding languages.
• It is a more crisp, error-free, and highly readable language.
• Moreover, it uses third-party libraries for some particular tasks that add as language constructs.
• Scala has no boilerplate code.

35) What is meant by Closure in Scala?

 In Scala, closure is a function that uses many free variables whose return value depends on these variables. The free variables are outside the closure function and not in the function. Also, these are not bound to be a function with valid values. 

36) Define the Anonymous function in Scala.

 In Scala, an Anonymous function is a function without any name, and these functions are also called “function literals .”But at run time, these function literals are incorporated into objects known as “function values .”These are useful for building inline functions.

37) List out the most common frameworks of Scala.

 The following are a few frameworks of Scala language:-

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

38) Explain the different Scala identifiers.

The following are the various types of Scala identifiers:-

• Alphanumeric identifiers - The Alphanumeric identifiers in Scala begin with a letter or underscore.
• Operator identifiers - The operator identifiers include one or more operator characters.
• Mixed identifiers - These identifiers include features of both Alphanumeric and operator identifiers. 
• Literal identifiers - These Scala identifiers include an arbitrary string with backticks.

39) Define higher-order functions.

The Higher-order functions in Scala are the functions that include other functions as parameters. Also, they return a function as its result. 

40) Explain about Extractor in Scala.

 In Scala, an extractor is an object that includes a method “unapply” as one of its members, which aims to match the values and take them apart.

41) Define how Scala is different from Java?

 The following are the various factors that differentiate Scala from Java.

• Scala supports DSL (Domain Specific Language), Closures, and Concurrency.
• It also supports Nested Functions.
• It consists of Type-inference.
• In Scala, all values are managed as Objects.

42) List out a few String methods in Scala.

The following are a few of the various String methods in Scala.:-

• String to uppercase
• String trim()
• Char[] to CharArray()
• String[] split(String regex)
• Int length()

43) How to format a String in Scala?

 In Scala, we use the .format() method to format a string. We can use the following syntax for this-

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

44) What do you understand about Stream in Scala?

In Scala, a Stream is a lazy list where the elements are only computed at the time of need. It is a feature of Scala that supports lazy computation.

45) What is the scope provided for Scala variables?

 There are three types of scopes provided for variables in Scala.

• Fields
• Method Parameters
• Local variables

Fields - These are the variables declared within an object and can be accessed anywhere within the program. They depend on the access modifiers. Further, these variables are expressed through Var as well as Val.

Method Parameters - These are immutable and help pass values to the methods. Moreover, the method parameters are accessible within a method, but you can also access them outside the method offered by a Reference.

Local variables - These variables are declared within a method and are accessible only within the process. The variables are accessible if you return them from the method. 

46) What is the use of “Option” in Scala?

In Scala, the option is used to cover the missing value. 

47) Define Scala Case Classes.

 In Scala, case classes are the regular classes that are immutable by default. It means they hold the feature of modeling unchangeable data. They also allow decomposition through pattern matching.

48) Define Exception handling in Scala.

An exception is an unknown event that occurs during the execution of any program. In Scala, exception handling is differently applicable. But it acts similar to Java and works flawlessly with the existing libraries of Java. All exceptions in Scala are unchecked. However, in Scala, all exceptions work similarly to C++ or Java. 

49) Define the various Access Modifiers in Scala.

The following are the different access modifiers in Scala.

• Private - A private member is only accessed within the Class or an object in which the same is stated.
• Public - This is by default where the public members are accessible from anywhere.
• Protected - These members can be accessible from the Subclass of the Class in which they are stated. 

50) What are the different types of inheritance supported by Scala?

 There are different types of inheritance that Scala supports, such as -

• Single
• Multilevel
• Multiple
• Hybrid

Here, you can use single and multilevel inheritance in your Class, but multiple inheritances can be achieved using traits only. 

Conclusion:

All of the above are commonly asked interview questions from Scala. I hope these questions and answers will help you to clear your interview. If you could not find the answer to any question related to Scala, feel free to comment in the comment section.