Scala Interview Questions

val declares an immutable reference that cannot be reassigned; var declares a mutable one. Idiomatic Scala prefers val and immutable collections.

A class for immutable data. The compiler generates a constructor without new, field accessors, equals/hashCode based on values, toString, copy and pattern-matching support.

1case class User(name: String, age: Int)
2val a = User("Asha", 25)
3val b = a.copy(age = 26)   // User(Asha,26)
4a == User("Asha", 25)      // true

A type that is either Some(value) or None, used instead of null to represent a value that may be missing. Use map, getOrElse, fold or pattern matching to handle both cases.

A powerful match expression that checks a value against patterns — constants, types, case class shapes, tuples, lists — and can extract parts of the value. With sealed types the compiler warns about missing cases.

A reusable unit of fields and methods that classes can mix in; a class can extend several traits. Traits replace Java interfaces and also hold implemented methods.

Syntax sugar over map, flatMap and withFilter that makes chaining Options, Lists, Futures or Eithers readable, stopping early on None or Left.

Functions that take other functions as parameters or return them, such as map, filter and foldLeft. They enable concise, reusable data transformations.

Contextual abstractions: a given defines a canonical instance of a type (for example an Ordering), and a using clause asks the compiler to supply it automatically. They replace Scala 2 implicits for type classes and context passing.

A Future represents a value computed asynchronously on an ExecutionContext. Compose them with map, flatMap and for comprehensions; handle failures with recover. Avoid Await in production code.

Spark itself is written in Scala, its DataFrame/Dataset APIs feel natural with Scala’s functional collections and case classes, and the typed Dataset API uses case classes for compile-time schema checks.