Scala Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is Scala and why is it used?
Scala is a programming language that combines object-oriented and functional programming. It is used for building scalable and concise applications on the Java Virtual Machine (JVM).
Ques 2. Explain the difference between val and var in Scala.
val is a keyword used for defining immutable variables, while var is used for defining mutable variables.
Ques 3. What is the significance of 'Option' in Scala?
'Option' is used to represent optional values. It can either be 'Some(value)' if a value is present or 'None' if the value is absent.
Ques 4. What is a higher-order function?
A higher-order function is a function that takes other functions as parameters or returns functions as results.
Ques 5. How does Scala handle null values?
Scala encourages the use of 'Option' types to represent optional values instead of using null. 'Some' is used for a present value, and 'None' is used for an absent value.
Ques 6. Explain the 'yield' keyword in Scala.
In a 'for' comprehension, 'yield' is used to produce a value that is included in the resulting collection. It is often used to transform and filter data.
Ques 7. How does Scala handle exceptions?
Scala uses the 'try', 'catch', and 'finally' blocks to handle exceptions. However, it does not distinguish between checked and unchecked exceptions.
Ques 8. What is the purpose of the 'case' keyword in Scala?
The 'case' keyword is used to define classes for pattern matching. It automatically generates equals, hashCode, and toString methods, making it suitable for use in pattern matching and collections.
Ques 9. What are the advantages of using the 'Option' type over null in Scala?
Using 'Option' type forces the developer to handle the absence of a value explicitly, reducing the likelihood of null pointer exceptions. It leads to safer and more robust code.
Ques 10. Explain the 'yield' keyword in the context of Scala collections.
In the context of Scala collections, 'yield' is used within a 'for' comprehension to produce elements for the resulting collection. It is often used for transforming and filtering data.
Ques 11. What is the 'type' keyword used for in Scala?
The 'type' keyword is used to define type aliases in Scala. It allows developers to create alternative names for existing types, improving code readability and maintainability.
Ques 12. What is the purpose of the 'Seq' trait in Scala?
'Seq' is a trait in Scala that represents sequences—ordered collections of elements. It is a subtrait of 'Iterable' and provides methods for indexing, length, and more.
Ques 13. Explain the 'copy' method in case classes.
The 'copy' method is automatically generated for case classes in Scala. It allows you to create a new instance of the case class with some fields modified, providing a convenient way to copy and update objects.
Ques 14. Explain the 'partition' method in Scala collections.
The 'partition' method in Scala collections divides a collection into two parts based on a predicate. Elements that satisfy the predicate are placed in one part, while the rest go into the other part.
Ques 15. Explain the 'zip' method in Scala collections.
The 'zip' method in Scala collections combines two collections element-wise, creating a new collection of pairs. If the original collections have different lengths, the resulting collection will have the length of the shorter one.
Intermediate / 1 to 5 years experienced level questions & answers
Ques 16. Explain pattern matching in Scala.
Pattern matching is a powerful feature in Scala that allows you to match complex data structures, making code more concise and readable.
Ques 17. What is the purpose of the 'apply' method in Scala?
The 'apply' method is a special method in Scala that is invoked when an object is called like a function. It is often used to create objects without using the 'new' keyword.
Ques 18. What is the difference between 'map' and 'flatMap' in Scala?
'map' applies a function to each element of a collection and returns a new collection of the results, while 'flatMap' applies a function that returns a collection to each element and flattens the results into a single collection.
Ques 19. Explain the concept of immutability in Scala.
Immutability means that once an object is created, its state cannot be changed. In Scala, 'val' and immutable collections are used to create immutable data structures.
Ques 20. What is a companion object in Scala?
A companion object is an object with the same name as a class and is defined in the same file. It is used to contain static methods and properties related to the class.
Ques 21. What is a trait in Scala?
A trait is a collection of abstract and concrete methods that can be mixed into classes to provide additional functionality. Unlike classes, a class can extend multiple traits.
Ques 22. Explain lazy evaluation in Scala.
Lazy evaluation is a strategy where the evaluation of an expression is delayed until its value is actually needed. It can improve performance by avoiding unnecessary computations.
Ques 23. What are case classes in Scala?
Case classes are regular classes with some additional features. They are often used for immutable data modeling and come with built-in support for pattern matching.
Ques 24. What is a partial function in Scala?
A partial function is a function that is not defined for every input value. It is defined only for a subset of possible input values using the 'isDefinedAt' method.
Ques 25. What is the 'implicit' keyword used for in Scala?
The 'implicit' keyword is used to declare that a value can be automatically passed to a method or function parameter. It is often used in combination with implicits.
Ques 26. Explain the concept of type bounds in Scala.
Type bounds restrict the possible types that can be used as type parameters. They include 'Upper bounds' denoted by <: and 'Lower bounds' denoted by >:.
Ques 27. Explain the concept of implicit conversions in Scala.
Implicit conversions allow the compiler to automatically convert one type to another if needed. They are defined using the 'implicit' keyword and are commonly used for enhancing existing types with new functionality.
Ques 28. What is the 'sealed' keyword used for in Scala?
The 'sealed' keyword is used to restrict the inheritance of a class to the same file. It helps the compiler exhaustively check pattern matches, ensuring that all possible subclasses are covered.
Ques 29. What is the 'View' in Scala collections?
The 'View' in Scala collections provides a lazy and non-strict version of a collection. It allows you to create a lightweight view on a collection without creating a new data structure.
Ques 30. How does Scala support multiple inheritance?
Scala supports multiple inheritance through the use of traits. A class can extend multiple traits, allowing it to inherit behavior from each of them.
Ques 31. What is the purpose of the 'withFilter' method in Scala?
The 'withFilter' method is used in conjunction with 'for' comprehensions to enable lazy filtering of elements in a collection. It is similar to 'filter' but is evaluated lazily.
Ques 32. What is the purpose of 'scalatest' in Scala?
'scalatest' is a popular testing framework for Scala. It provides a rich set of features for writing and executing tests, including support for behavior-driven development (BDD) and various testing styles.
Ques 33. What is the purpose of the 'unapply' method in Scala?
The 'unapply' method is used in the context of pattern matching. It allows you to extract values from objects, making it a key component in creating custom extractors for pattern matching.
Experienced / Expert level questions & answers
Ques 34. What is the 'for' comprehension in Scala?
The 'for' comprehension is a syntactic sugar for combining sequences of 'map', 'flatMap', and 'filter' operations. It makes code more readable and expressive.
Ques 35. How does Scala support concurrency?
Scala provides actor-based concurrency using the Akka library. Actors are lightweight, concurrent entities that communicate through message passing.
Ques 36. Explain currying in Scala.
Currying is the technique of transforming a function that takes multiple arguments into a series of functions that take one argument each. It is supported natively in Scala.
Ques 37. What is type inference in Scala?
Type inference is the ability of the Scala compiler to automatically deduce the data types of expressions, making it unnecessary to explicitly specify types in most cases.
Ques 38. How are implicits used in Scala?
Implicits are used for automatic type conversions, adding new methods to existing classes, and resolving ambiguities. They are a powerful and controversial feature in Scala.
Ques 39. What is the 'Future' class in Scala?
The 'Future' class represents a value or error computation that may be running asynchronously. It is commonly used for handling concurrent and parallel programming.
Ques 40. Explain the concept of variance in Scala.
Variance is a way to express how subtyping between more complex types relates to subtyping between their components. It is denoted by the use of 'covariant', 'contravariant', or 'invariant' keywords.
Ques 41. What is tail recursion optimization in Scala?
Tail recursion optimization is a compiler optimization that eliminates the overhead of function calls when the last action of a function is a call to itself (tail call). Scala supports this optimization for tail-recursive functions.
Ques 42. Explain the concept of implicits in Scala.
Implicits are a mechanism in Scala that allows the compiler to automatically insert extra arguments, convert types, or provide default values. They are used for concise and flexible programming.
Ques 43. What is the 'implicitly' method in Scala?
The 'implicitly' method is used to summon an implicit value of a given type. It is often used in implicit conversions and parameter injection.
Ques 44. Explain the 'self' type in Scala.
The 'self' type is a way to declare the type of 'this' within a class or trait. It is often used in situations where the type of 'this' needs to be specified explicitly.
Ques 45. Explain the concept of covariance and contravariance in Scala.
Covariance allows a type parameter to vary in the same direction as the container type, while contravariance allows it to vary in the opposite direction. They are denoted by + and - symbols, respectively.
Ques 46. Explain the 'yield' keyword in the context of Scala futures.
In the context of Scala futures, 'yield' is used within a 'for' comprehension to produce a value that will be included in the future's result. It is often used for combining and transforming asynchronous computations.
Ques 47. What is the purpose of the 'implicit class' feature in Scala?
The 'implicit class' feature in Scala allows developers to add new methods to existing classes without modifying their source code. It is often used to extend functionality in a clean and concise manner.
Ques 48. What is the purpose of the 'implicit parameter' in Scala?
An implicit parameter is a parameter marked with the 'implicit' keyword. It allows the compiler to automatically provide a value for the parameter if there is an implicit value of the corresponding type in scope.
Most helpful rated by users:
- Explain the difference between val and var in Scala.
- What is a higher-order function?
- Explain the 'yield' keyword in Scala.
- What is the purpose of the 'case' keyword in Scala?
- What are the advantages of using the 'Option' type over null in Scala?
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