Scala Interview Questions

1. Introduction

Scala is a modern general-purpose, multi-paradigm programming language. It integrates the features of both object-oriented and functional languages. It plays an important role in the big data world. It’s designed to provide common programming patterns in a concise, clean, and type-safe way. It was designed by Martin Odersky along with others in 2003. The language was initially designed to overcome most of the design criticisms about Java at that time.

In this tutorial, we’ll cover some of the interview questions about Scala.

2. Questions

Q1. What Are Its Main Features?

As the name signifies, Scala is a highly scalable language. Below are some of the unique features that make it stand out from other languages:

• It has a concise, readable syntax. There’s less boilerplate code.
• It supports lazy evaluations and type inference which give it a dynamic feel though it’s a statically-typed language.
• It supports both functional and object-oriented programming approaches, a fusion of both.
• It runs on the JVM as well as in the browser.
• It interacts seamlessly with Java code. In other words, we can use Java classes and libraries in Scala code and vice-versa.
• It facilitates a higher level of concurrency control and parallel processing, as, by default, the objects and variables are immutable.

Moreover, the language has a vast ecosystem of libraries and frameworks used to build anything and everything: Apache Spark, web development, big data, AI, Machine Learning, and Data Analysis.

It integrates seamlessly with various IDEs and frameworks we generally use for Java development.

Q2. What Are the Main Benefits?

Because of its high scalability, interoperability, and feature-rich nature, Scala provides a large number of benefits over other languages:

• The language is easier to learn, especially for people with a background in Java or similar languages, because of its concise, readable, expressive, and less error-prone nature.
• The complex and performance-enhancing features such as macros, tuples, singleton objects, closures, etc., ease the code-writing experience.
• We can build highly fault-tolerant, concurrent, and robust systems.
• It can be perfect for web development, big data, AI, Machine Learning, Data Analytics, etc.
• It supports a fusion of both functional programming and object-oriented programming.
• We can easily use thousands of existing Java libraries in Scala code, and hence we can create applications with integrated code from both languages.
• Additionally, we can save a lot of time by integrating dozens of pre-built, functional methods and libraries in the existing Scala or Java projects.
• We can use it on both server-side and browser-side.

Q3. List Some of the Scala Frameworks.

The language has a huge ecosystem of libraries and frameworks. Let’s have a look at a few of those:

• The Play Framework is a web application framework. It’s based on a lightweight, stateless, web-friendly architecture. Additionally, it integrates the components and APIs we need for modern web application development.
• Scalatra is a simple, high-performance, async web micro-framework that helps us build high-performance websites and APIs. It integrates the power of the JVM with its out-of-the-box features.
• The Akka Framework allows us to focus on meeting business requirements rather than writing low-level code and provides robust behavior, fault tolerance, and high performance.
• Apache Spark is a multi-language data processing engine for quickly executing data science, machine learning, and AI tasks on single-node machines or clusters.
• Scala.js is a strongly-typed replacement for JavaScript that optimizes the code into highly efficient JavaScript and provides a safer way to build robust front-end web applications.
• Lagom is an open-source framework that helps us build, test, and deploy systems of Reactive microservices. It’s built on Akka and Play.

The language has a presence in various other fields like Data Science and Analytics, Big Data, AI, ML, Serialization, HTTP and JSON libraries, Graph Traversals, and REST web services, to name a few.

Q4. Why It Is Called Both a Functional and Object-Oriented Programming Language?

The language is very famously known to be a fusion of both object-oriented and functional programming. Every value is an object in Scala. Moreover, functions can also be values, and therefore, every function is also an object.

Similar to Java, it supports objects and classes for modularity, while it provides functions for building logic. However, we can provide the behavior to the methods at runtime, pass functions as arguments, and also return functions from other functions.

The language allows us to program in an imperative style, but also encourages us to adopt a more functional style. And hence, Scala is a hybrid, imperative-functional language.

Moreover, it provides all the necessary tools and structures required for object-oriented design like traits, classes, objects, access modifiers, sub-typing, etc. Subsequently, it also provides a way to define “functional objects” – objects that don’t have a mutable state.

Q5. State Some of the Differences Between Scala and Java.

Scala was essentially developed to overcome the issues raised by criticizing Java. Both of these are general-purpose, object-oriented languages that run on JVM.

Java now supports functional programming. Moreover, we can pass functions as parameters to methods in both languages. However, both have a considerable list of differences:

• Scala is a pure object-oriented programming language, which means everything is an object. It has no concept of primitive data. On the other hand, Java has primitive data types.
• It is type-safe, the type inference happens automatically in it. While in Java, we don’t have an automatic type inference while assigning the variables.
• The language supports lazy evaluations, While Java also supports lazy evaluations, but only in the case of Streams from Java 8.
• It supports closures, while Java doesn’t have anything similar to closure.
• The language also supports nested and first-class functions. Java has no such concepts.
• Generally, the programs have relatively less boilerplate code than Java.
• The variables are by default immutable, while java variables are mutable by default.
• Finally, it has no static keyword. Instead, it supports singleton objects. Java has a static keyword.

Q6. What Are Singleton Objects?

Unlike Java, Scala doesn’t have static objects. So instead of static objects, we can create singleton objects. This means that we can access the method of a singleton object just like static methods in a Java class.

Interestingly, the object keyword is used to define a class that has exactly one instance.

Below are some of the important points about singleton objects:

• We cannot create an instance of a singleton object.
• Just like static methods in Java, we can access the methods in the singleton object globally using the object name. So, there’s no need to create an instance to access these methods. However, we must remember that these methods are not static.
• The primary constructor of the singleton object is parameterless.
• In Scala, imports may appear anywhere, and they may refer to singleton objects in addition to the packages.
• We can also divide programs into singleton objects, and we can think of those as modules.
• In Scala, the main method is always present in a singleton object.
• A singleton object can extend class and traits.

Q7. What Are the Types of Variables?

Scala has mainly two types of variables – Mutable and Immutable.

We can change the value of a mutable variable. To declare mutable variables, we use the var keyword.

We can’t change the value of an immutable variable. We use the val keyword to declare immutable variables.

The imperative style of programming has both vars and vals. Whereas, if the program contains no vars but only vals, it’s probably a functional style of programming. Thus, if we want to move towards a functional way of programming, we must try to write code without any vars.

The language supports variable type inference, which means that the Scala compiler figures out the type of variable depending on the value assigned to it.

Q8. What Are Traits and Mixins?

Traits are similar to interfaces in Java 8. In short, traits encapsulate the methods and the variables or fields. They offer a more fine-grained way to reuse the code than inheritance. They are used to share interfaces and fields between classes. Traits cannot be instantiated and have no parameters. However, normal classes and objects can extend traits.

Once a trait is defined, it can be mixed into a class using the with keyword. Moreover, a class can mix in any number of traits. However, we must also note that using traits is not the same as multiple inheritance.

Mixins are nothing but traits used to compose a class. This simply means we can extend any number of traits to compose a class. Essentially, all the traits should be extended after extending any other class or abstract class, if any.

Q9. What Is the Difference Between an Object and a Class?

The concept of object and class is similar to Java, but not exactly the same.

A class is a logical entity, a blueprint, which we can use to create objects. Objects are physical, which means we can use all the functionalities of a class with objects.

A class can be abstract or concrete. By defining a class, we establish a new type. A type restricts a variable to refer to or an expression to produce only specific values, at runtime. Moreover, we can create any number of objects using the keyword new.

A companion class has the same name as that of the singleton object defined in the same source file. We call the class and the singleton object as companions of each other. They have access to private members of each other.

However, as we know, in Scala, we cannot define static members. But, we can define singleton objects. This means that we can have only one instance of that type. We can create such singleton instances using the keyword object instead of a class keyword. Moreover, as we can’t instantiate a singleton object, we can’t pass any parameter to its primary constructor.

Q10. What Are Tuples?

A tuple is a collection that can hold objects of different types. Therefore, tuples do not inherit from the Collection. A tuple comes in handy, for instance, if we need a collection of exactly one integer, one String, and say some object type.

Tuples are very much useful in a situation when we want to return multiple values from a method. However, we must determine carefully when to use a tuple and when to use a class. When the variables don’t create meaning together, then we may need a tuple. Otherwise, for a meaningful collection of variables, defining a class becomes handy.

Q11. What Is a Closure?

Generally, when we define a function, it refers to the variables known to it. But in Scala, we can let a function refer to a variable defined somewhere else, at runtime:

(value: Int) => value + something

The above function knows to add an Int value to something, but it doesn’t know what something is. Here, the value is called a bound variable while something is called a free variable. If we don’t define something and try to use this function literal directly, it will result in a compile-time error.

However, if we define something before using this function literal, then it will work:

scala> var something = 1
something: Int = 1

scala> val addSomething = (value: Int) => value + something
addSomething: (Int) => Int = <function>

scala> addSomething(5)
res9: Int = 6

Here, the function value addSomething created from our function literal, at runtime, is known as a closure. Therefore, a closure is always formed by capturing the values bound to its free variables.

Noticeably, the value of something has been defined outside the closure.

If there are no free variables in a function literal, we may not call it closure.

Finally, the return value of closure is always dependent upon the value of its free variables.

3. Conclusion

In this article, we’ve explored some of the basic Scala questions that could come up in a technical interview for Scala developers. We should take these questions into account as a starting point for further research, since this is by no means an exhaustive list.

We wish you good luck in any upcoming interviews!