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1️⃣ Kotlin Fundamentals (Beginner Level)

👉 Used for fresher / junior interviews

• What is Kotlin?
• Kotlin vs Java
• Statically typed language
• var vs val
• const val
• Data types
• Type inference
• fun keyword
• Comments
• String templates
• Control flow (if, when, for, while)
• Ranges
• Packages & imports

📌 Interview focus: syntax, readability, Java comparison

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2️⃣ Null Safety (Very High Priority 🔥)

👉 Asked in almost every Kotlin interview

• Nullable vs non-nullable types (?)
• Safe call operator (?.)
• Elvis operator (?:)
• Not-null assertion (!!)
• Safe casts (as?)
• lateinit
• lazy
• Smart casts
• NPE scenarios in Kotlin

📌 Interview focus: preventing NullPointerException

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3️⃣ Functions & Functional Programming

👉 Core Kotlin strength

• Function declaration
• Default arguments
• Named arguments
• Single-expression functions
• Higher-order functions
• Lambda expressions
• Inline functions
• noinline & crossinline
• Function types
• Anonymous functions
• Recursion & tail recursion

📌 Interview focus: lambdas, inline, higher-order functions

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4️⃣ Object-Oriented Programming in Kotlin

👉 Java developers are tested heavily here

• Classes & constructors
• Primary vs secondary constructor
• open, final, abstract
• Inheritance
• Interfaces
• override keyword
• Visibility modifiers
• Polymorphism
• Composition vs inheritance

📌 Interview focus: differences from Java OOP

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5️⃣ Special Kotlin Classes

👉 Very frequently asked

• data class
• sealed class
• enum class
• object (Singleton)
• companion object
• Value classes (@JvmInline)
• Inner vs nested classes

📌 Interview focus: data vs sealed vs enum

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6️⃣ Collections & Generics

👉 Used in backend + Android interviews

• Immutable vs mutable collections
• List, Set, Map
• Collection operations (map, filter, reduce)
• map vs flatMap
• associate
• Sequences
• Generics
• Variance (in, out)
• Star projection (*)

📌 Interview focus: immutability & performance

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7️⃣ Extension Functions & Properties

👉 Kotlin-specific feature

• Extension functions
• Extension properties
• Static resolution
• Limitations
• Use cases

📌 Interview focus: how extensions work internally

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8️⃣ Coroutines & Concurrency (Advanced 🔥🔥)

👉 Must for senior roles

• What are coroutines?
• suspend functions
• Coroutine builders (launch, async)
• Coroutine scopes
• Dispatchers
• Structured concurrency
• withContext
• Job & Deferred
• Exception handling
• Cancellation
• Flow vs Channel

📌 Interview focus: coroutines ≠ threads

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9️⃣ Error Handling

👉 Frequently combined with coroutines

• try-catch-finally
• throw
• Custom exceptions
• Checked vs unchecked (Java vs Kotlin)
• runCatching
• Exception handling in coroutines

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🔟 Kotlin + Java Interoperability

👉 Critical for real-world projects

• Calling Java from Kotlin
• Calling Kotlin from Java
• @JvmStatic
• @JvmField
• @JvmOverloads
• @JvmName
• SAM conversions
• Platform types

📌 Interview focus: mixed codebases

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1️⃣1️⃣ Kotlin Standard Library Internals

👉 Senior-level interviews

• Scope functions (let, run, apply, also, with)
• Inline functions
• Contracts
• Delegation (by)
• Property delegation
• Lazy initialization

📌 Interview focus: let vs apply vs also

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1️⃣2️⃣ Android-Specific Kotlin (If Applicable)

👉 Android interviews

• ViewBinding / DataBinding
• ViewModel + Kotlin
• LiveData
• StateFlow vs LiveData
• Coroutines in Android
• Kotlin DSL
• Jetpack Compose basics

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1️⃣3️⃣ Backend Kotlin (Spring / Ktor)

👉 Backend developer interviews

• Kotlin with Spring Boot
• Null safety in REST APIs
• Data classes in APIs
• Coroutines with Spring WebFlux
• Ktor basics

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1️⃣4️⃣ Kotlin Performance & JVM Internals

👉 Architect / Staff-level

• JVM bytecode generation
• Inline vs non-inline
• Memory optimization
• Tail recursion
• Boxing & unboxing
• Kotlin vs Java performance

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1️⃣5️⃣ Design Patterns in Kotlin

👉 System design rounds

• Singleton (object)
• Factory
• Builder
• Observer
• Strategy
• Functional patterns

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🎯 How to Use This for Interview Questions

You can create:

• Beginner: Sections 1–4
• Intermediate: Sections 5–7
• Advanced: Sections 8–11
• Senior/Architect: Sections 12–15

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Read other awesome articles in Medium.com or in akcoding’s posts.

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