Kotlin

Kotlin has null-safety, type inference, data classes, coroutines, and a more concise syntax. Java is more verbose but has wider legacy support.

Use val for immutable and var for mutable variables.
Example:
val name = "Alice"
var age = 30

val is like a final variable in Java (read-only). var can be reassigned.

Kotlin is a statically typed language, meaning type checking is done at compile time. However, it supports type inference to reduce verbosity.

Yes, Kotlin supports default parameter values:
fun greet(name: String = "Guest") { println("Hello, $name!") }

Kotlin wraps Java collections but adds null-safety, immutability via List, Map, Set, and has extension functions for manipulation. Java lacks many of these features directly.

Use listOf() to create an immutable list:
val names = listOf("Alice", "Bob")

Yes, Kotlin is interoperable with Java, so you can use ArrayList, HashMap, etc., but be careful with nullability.

mutableListOf() creates a list that can be modified (add/remove items), while listOf() creates an immutable list.

No, standard Kotlin collections are not thread-safe. Use concurrent collections from Java or Kotlinx libraries for thread safety.

Kotlin distinguishes between nullable and non-nullable types. String vs String?. This helps catch null pointer exceptions at compile time.

You can use safe call ?., the Elvis operator ?:, or !! to assert non-null.
Example:
val length = name?.length ?: 0

The !! operator forces a nullable variable to be non-null. If it is null, it throws a NullPointerException. Use it with caution.

Kotlin uses type inference to deduce variable or return types when not explicitly specified. This keeps code concise and readable.

Coroutines provide a way to write asynchronous, non-blocking code. They are lightweight threads managed by the Kotlin runtime.

You can use GlobalScope.launch { } or use structured concurrency with lifecycleScope or viewModelScope in Android.

launch returns Job and does not return a result. async returns Deferred, which you can await() to get a result.

Yes, Kotlin coroutines work on JVM, Android, JS, and Native platforms. Just add the appropriate coroutine library.

Yes, you can call Kotlin from Java and vice versa. Kotlin uses annotations like @JvmStatic, @JvmOverloads, etc., to enhance compatibility.

Yes, with frameworks like Ktor or Spring Boot (which supports Kotlin), you can write full-stack apps in Kotlin.

Yes. Kotlin supports calling Java code using annotations and generics, though sometimes type casts or nullability handling are required.

Android Studio has a built-in tool: Paste Java code into a Kotlin file, and it will auto-convert. Or use Code > Convert Java File to Kotlin File.