Free Kotlin Interview Questions

Kotlin language essentials and the tricky corners interviewers actually probe.

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Does declaring a variable with val make the object it references immutable?
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Is a List returned by listOf actually immutable, and can it ever change under your feet?
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What is the difference between == and === in Kotlin?
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What is a platform type, and why can it cause a surprise NullPointerException even in null-safe Kotlin?
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When you call copy() on a data class holding a mutable list, is the copy independent of the original?
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What is the practical difference between lateinit var and by lazy for deferred initialization?
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Do the scope functions let, run, also, apply, and with differ only in style, or is there a real semantic distinction?
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Why can an extension function not override a member function, and what determines which one is called?
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What does the inline keyword actually do to a higher-order function, and what problem does it solve?
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What do noinline and crossinline do, and when are they each required?
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What does a reified type parameter enable that a normal generic type parameter cannot, and what is the requirement?
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Why should you not add or remove elements from a MutableList while iterating it, and how does Kotlin behave?
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What is the difference between const val and a regular val, and where can const be used?
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When are you forced to override both equals and hashCode together, and what breaks if you do not?
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Why might comparing two Double values with == give a surprising result, especially with NaN?
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What is the difference between a sealed class and an enum, and when do you choose each?
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What is the difference between a companion object and a plain object declaration?
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Why is a Kotlin companion object member not automatically a Java static, and how do you fix interop?
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What is the difference between a Sequence and a List when chaining map and filter, and why does it matter for performance?
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Why can the compiler refuse to smart-cast a nullable var even right after a null check?
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What does declaration-site variance with out and in mean, and how does it differ from Java wildcards?
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What is a star projection, and how is List of star different from List of Any?
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What is type erasure in Kotlin generics, and what specifically can you not do because of it?
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What is a value class (formerly inline class), and when does it actually avoid allocation?
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Can an extension function be called on a null receiver, and how does that work?
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What is the difference between an inner class and a nested class in Kotlin?
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How does a suspend function work under the hood, and why can you only call it from a coroutine?
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What makes a when expression exhaustive, and when are you not required to write an else branch?
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What is the Nothing type in Kotlin and where does it show up usefully?
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What does a Kotlin contract do, and why is require or isNullOrEmpty able to help smart-casting?
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How does destructuring work in Kotlin, and why is it positional rather than by name?
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What is a tailrec function, and what silently prevents the optimization?
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How do property delegates like by lazy and Delegates.observable actually work?
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What is a typealias, and what does it not give you compared to a value class?
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How does operator overloading resolve in Kotlin, and what are the constraints on which functions you can overload?
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What is the difference between JvmField, JvmStatic, and JvmOverloads, and when do you need each?
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What does a labeled return like return at a label do, and why is it needed inside a lambda?
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Why does adding a non-constructor property to a data class exclude it from equals, hashCode, and copy?
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How do default arguments interact with copy on a data class, and what is the pitfall?
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Why can you not smart-cast a property declared with a custom getter, and what is the safe alternative?
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What is the difference between associate, associateBy, and associateWith, and where do you lose data?
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What is the difference between flatMap and map plus flatten, and when does a wrong transform surprise you?
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How does Kotlin's elvis operator combined with return or throw enable early exit, and why does it type-check?
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What is the difference between a reified is T check and a plain Class comparison for generics, and what does filterIsInstance rely on?
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Why does modifying a captured var inside a Kotlin lambda work, unlike Java's effectively-final rule?
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What is the difference between an anonymous object expression and a lambda when implementing an interface, and when must you use each?
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How does the by keyword implement interface delegation, and what surprising override behavior can it produce?
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What does the in and out variance actually forbid you from doing, illustrated with a covariant class trying to consume its type?
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Why can two functions with the same name but different generic parameters clash on the JVM, and how do JvmName and mangling help?
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How do coroutine structured concurrency, the Nothing type, and suspend combine to make cancellation cooperative, and what breaks it?
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