400 Scala Interview Questions with Answers 2026

The Type System: Covariance, Contravariance, Type Classes, and Higher-Kinded Types.

Concurrency: Akka/Pekko Actor Model, Futures, and Supervision strategies.

Functional Effects: Monads, Functors, and Fiber-based concurrency (ZIO/Cats).

Architecture & Performance: JVM Tuning, SBT security, and Tapir/Http4s APIs.

B) It is a shorthand for a using clause that requires an instance of Ordering[A] to be available in the implicit scope.

C) It restricts the type A to be a direct subclass of Ordering.

D) It enables the use of Macros to generate the ordering logic at runtime.

E) It ensures that the method find can only be called on Numeric types.

F) It triggers a compiler error if the list is empty.

Correct Answer: B

Overall Explanation: Context bounds are a concise way to express that a generic type requires an available type class instance (using/implicit) to function.

Detail Explanation: * A: Incorrect; while it uses the implicit scope, it is modern syntax, not "legacy" only.

C: Incorrect; that would be a subtype constraint (A <: Ordering), not a context bound.

D: Incorrect; Context bounds are a type system feature, not specifically a Macro trigger.

E: Incorrect; it requires an Ordering, which could be for Strings, custom objects, or Numbers.

F: Incorrect; type bounds affect compilation, not runtime collection size.

Question 2: In the context of Scala's Variance, if class Box[+A] is defined, which statement is true?

B) Box[Any] is a subtype of Box[String].

C) The type A can only be used in "contravariant" positions (method arguments).

D) This makes the Box class invariant by default.

E) You cannot use A as a return type for any method in the class.

F) It requires the use of the opaque keyword in Scala 3.

Correct Answer: A

Overall Explanation: The + symbol denotes Covariance, meaning the subtyping relationship of the generic follows the subtyping of the type arguments.

Detail Explanation: * A: Correct; Since String is a subtype of Any, Box[String] is a subtype of Box[Any].

C: Incorrect; Covariant types (+A) are generally restricted to "covariant" positions (return types).

D: Incorrect; Invariance is the default (no prefix).

E: Incorrect; Covariant types are specifically intended to be return types.

F: Incorrect; Opaque types are for type masking, unrelated to variance prefixes.

Question 3: When using Akka or Pekko Actors, what happens to an Actor’s state if it encounters an unhandled exception and the default supervisor strategy is applied?

B) The Actor is resumed, and the state remains exactly as it was during the crash.

C) The Actor is restarted; the current instance is discarded and a new instance is created, resetting the state.

D) The Exception is bubbled up to the JVM Main thread, crashing the entire application.

E) The message that caused the error is sent back to the original sender automatically.

F) The Actor enters a "Stash" mode where it waits for manual intervention.

Correct Answer: C

Overall Explanation: The default Supervision Strategy in Akka/Pekko for a standard Exception is Restart.

Detail Explanation: * A: Incorrect; this is the Stop strategy.

C: Correct; Restarting creates a fresh instance, meaning internal mutable state is lost.

D: Incorrect; Actors are designed for isolation to prevent the whole system from crashing.

E: Incorrect; the sender is not notified unless explicitly programmed via "Ask" or status messages.

F: Incorrect; "Stashing" is a manual pattern, not a supervisor's automatic response to a crash.

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