Kotlin Interview Questions and Answers

Kotlin's type system distinguishes nullable from non-nullable types. String cannot hold null; String? can. This moves null checks to compile time and largely eliminates NullPointerException.

var a: String = "hi"
var b: String? = null
// a.length        // compile error on b.length
println(b?.length)       // safe call, prints null
println(b?.length ?: 0)  // Elvis operator, default value
b?.let { println(it) }   // runs only when non-null
val c = b!!.length       // asserts non-null, throws if null

Tools include the safe call ?., the Elvis operator ?:, let/also, and the not-null assertion !!, which should be avoided because it reintroduces crashes. lateinit var is for non-null properties initialised after construction, and by lazy for deferred initialisation. Values coming from Java have platform types with unknown nullability, so guard or annotate them at the boundary.

val declares a read-only reference that cannot be reassigned after initialisation. var declares a mutable reference that can be reassigned.

val name = "Ada"          // cannot reassign
var count = 0
count += 1                // ok

val list = mutableListOf(1)
list.add(2)               // ok: the reference is fixed, contents are mutable

The restriction applies to the reference, not the object's internal state, which is why mutating a mutableList held by a val is legal. Prefer val by default: it makes code easier to reason about, is friendlier under concurrency and helps the compiler. Use var only when reassignment is genuinely needed. A val property can still have a custom getter that computes a fresh value on each access. For truly immutable objects, also make the underlying data immutable, since val alone does not prevent mutation through other references.