Scala Programming Interview Questions and Answers

Scala stands for scalable language. It is a modern multi paradigm programming language. It combines functional and object oriented programming. Object oriented make it simple to design complex systems and to adapt them to new demands and functional programming make it simple to create things rapidly from simple parts and it is compatible with Java.

It is designed by Martin Oderskey, a German computer scientist. He also teaches a course, Functional Programming Principles in Scala and Functional Program Design in Scala on Coursera.

There are the two feature of Scala i.e.

• Object Oriented
• Functional

Because of these feature Scala is scalable.

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.

Scala Map is a collection of key value pairs wherein the value in a map can be retrieved using the key. Values in a Scala Map are not unique but the keys are unique. Scala supports two kinds of maps- mutable and immutable. By default, Scala supports immutable map and to make use of the mutable map, programmers have to import the scala.collection.mutable.Map class explicitly. When programmers want to use mutable and immutable map together in the same program then the mutable map can be accessed at mutable. Map and the immutable map can just be accessed with the name of the map.

Advantages of Scala Language are:-

• Simple and Concise Code
• Very Expressive Code
• More Readable Code
• 100% Type-Safe Language
• Immutability and No Side-Effects
• More Reusable Code
• More Modularity
• Do More With Less Code
• Very Flexible Syntax
• Supports all OOP Features
• Supports all FP Features. Highly Functional.
• Less Error Prone Code
• Better Parallel and Concurrency Programming
• Highly Scalable and Maintainable code
• Highly Productivity
• Distributed Applications
• Full Java Interoperability
• Powerful Scala DSLs available

• Less Readable Code
• Bit tough to Understand the Code for beginners
• Complex Syntax to learn
• Less Backward Compatibility

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.

The two types of variables are

var myVar : Int=0;

val myVal: Int=1;

‘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.

‘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.

Traits are mostly used, when we require dependency injection. Unlike Java, through Spring framework, dependency injection is achieved through annotations. In Scala, there are no annotations or no special package to be imported. We just need to initialize the class with the trait and done, dependency is injected.

Case classes provides a recursive decomposition mechanism via pattern matching, it is a

 Regular class which export their constructor parameter. The constructor parameters of case classes can be accessed directly and are treated as public values.

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.

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.

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.

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

The simplest way to define a monad is to relate it to a wrapper. Any class object is taken wrapped with a monad in Scala. Just like you wrap any gift or present into a shiny wrapper with ribbons to make them look attractive, Monads in Scala are used to wrap objects and provide two important operations

• Identity through “unit” in Scala
• Bind through “flatMap” in Scala

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.

Expression: Expression is a value that means it will evaluate to a Value. As an Expression returns a value, we can assign it to a variable.

Example:- Scala’s If condition, Java’s Ternary operator.

Statement: Statement defines one or more actions or operations. That means Statement performs actions. As it does not return a value, we cannot assign it to a Variable.

Example:- Java’s If condition.

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.

People often confuse with the terms concurrency and parallelism. When several computations execute sequentially during overlapping time periods it is referred to as concurrency whereas when processes are executed simultaneously it is known as parallelism. Parallel collection, Futures and Async library are examples of achieving parallelism in Scala.

The first difference is that a class can only extend one other class, but an unlimited number of traits. While traits only support type parameters, abstract classes can have constructor parameters. Also, abstract classes are interoperable with Java, while traits are only interoperable with Java if they do not contain any implementation.

Nil – It’s a handy way of initializing an empty list since, Nil, is an object, which extends List [Nothing].

Null – It’s a sub-type of AnyRef type in Scala Types hierarchy. As Scala runs on JVM, it uses NULL to provide the compatibility with Java null keyword, or in Scala terms, to provide type for null keyword, Null type exists. It represents the absence of type information for complex types that are inherited from AnyRef.

None – In programming, there are many circumstances, where we unexpectedly received null for the methods we call. In java these are handled using try/catch or left unattended causing errors in the program. Scala provides a very graceful way of handling those situations. In cases, where you don’t know, if you would be able to return a value as expected, we can use Option [T]. It is an abstract class, with just two sub-classes, Some [T] and none. With this, we can tell users that, the method might return a T of type Some [T] or it might return none.

Nothing – It’s a sub-type of all the types exists in Scala Types hierarchy. It helps in providing the return type for the operations that can affect a normal program’s flow. It can only be used as a type, as instantiation of nothing cannot be done. It incorporates all types under AnyRef and AnyVal. Nothing is usually used as a return type for methods that have abnormal termination and result in an exception.

‘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.

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

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

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

The four types of identifiers are

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

The different types of literals in scala are

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

High order functions are functions that can receive or return other functions. Common examples in Scala are the filter, map, and flatMap functions, which receive other functions as arguments.

A closure is also known as an anonymous function whose return value depends upon the value of the variables declared outside the function.

The ‘Unit’ is a type similar to void in Java. You can say it is a Scala equivalent of the void in Java, while still providing the language with an abstraction over the Java platform. The empty tuple ‘()’ is a term representing a Unit value in Scala.

Akka is a concurrency framework in Scala which uses Actor based model for building highly concurrent, distributed, and resilient message-driven applications on the JVM. It uses high-level abstractions like Actor, Future, and Stream to simplify coding for concurrent applications. It also provides load balancing, routing, partitioning, and adaptive cluster management. If you are interested in learning Akka, I suggest reading Akka in Action and Akka documentation; both are great resources to learn Akka.

Play is another Scala framework to build web applications in both Java and Scala. The Play Framework is based on a lightweight, stateless, web-friendly architecture. It is built on Akka, Play provides predictable and minimal resource consumption (CPU, memory, threads) for highly-scalable applications.

Slick is a modern database query and access library for Scala. It allows you to work with stored data almost as if you were using Scala collections while at the same time giving you full control over when a database access happens and which data is transferred. You can write your database queries in Scala instead of SQL, thus profiting from the static checking, compile-time safety, and compositionality of Scala. Slick features an extensible query compiler which can generate code for different backends. 

scala> def func(arg:String*)=arg.mkString(", ")
func: (arg: String*)String
scala> func("red","green","blue")
String = red, green, blue

use “:+” single value var myList = List.empty[String] myList :+= “a”

def keyword is used to define the Function in Scala.

object add {

def addInt( a:Int, b:Int ) : Int = {

var sum:Int = 0

sum = a + b

return sum

}

}

Recursion is when a function makes a call to itself. When we place this call as the last action performed in the function, we can call the function tail-recursive. scala> def factorial(n:Int):Int={

| if(n==1) return 1

| n*factorial(n-1)

| }

factorial: (n: Int)Int

scala> factorial(5)

Int = 120

Using object keyword we can create a singleton object.

object HelloWorld {

def main(args:Array[String]) {

println("Hello world!")

}

}

In the 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.

Syntax:
(z:Int, y:Int)=> z*y
Or
(_:Int)*(_Int)

Higher Order Function (HOF) is also a function but which performs one, two or both of the following things:

 Take other functions as arguments

Return functions as their results

Slick Framework

• Play Framework • Akka Framework • Spark Framework • Scalatra Framework • Sparay framework • Neo4j framework • Bowler Framework • Lift framework