What is Kotlin?

Kotlin is a general-purpose language from JetBrains, open source since 2011 and stable for production on JVM, JS, and Native backends. Google endorsed it for Android in 2017; Spring and Ktor use it on servers.

Core characteristics

• Static typing with inference — val count = 10 infers Int; types checked at compile time
• Null safety — nullable types use ?; the compiler rejects many NPE-prone calls
• Concise syntax — data classes, properties, and expression-oriented control flow
• Coroutines — structured concurrency without callback pyramids (full library on Gradle projects)

Typical JVM build-run flow (local)

1. Write main.kt with fun main()
2. kotlinc main.kt -include-runtime -d app.jar && java -jar app.jar or use Gradle
3. Deploy JVM bytecode alongside existing Java libraries

Important interview questions and answers

1. Q: Is Kotlin garbage collected on JVM?
   A: Yes—same JVM GC as Java; Kotlin adds value types on Native but JVM Kotlin uses heap objects like Java.
2. Q: Who uses Kotlin in production?
   A: Android apps, Spring Boot services, Gradle plugins, multiplatform shared modules (KMP).

Self-check

1. What file extension do Kotlin sources use?
2. Name one major platform where Kotlin is the default language.

Tip: Kotlin source uses .kt files compiled to JVM bytecode—think Android, Spring, and Gradle plugins.