Null safety and validation

The idea

A nullable type such as String? can hold either a string or null. Kotlin requires code to account for that absence before using members that need a real string. A safe call, written ?., runs the member access only when the receiver is present and otherwise returns null. The Elvis operator ?: supplies an alternative when its left side is null.

Do not treat an empty string as equivalent to null unless the requirement says so. The non-null assertion !! removes a compiler check by requesting a runtime failure if the value is absent. Prefer validation that narrows the type, safe calls, or a meaningful early return when absence is an expected part of the input.

fun displayLength(name: String?): Int = name?.trim()?.length ?: 0
fun main() {
  println(displayLength("  Kai  "))
  println(displayLength(null))
}

3
0

The present string is trimmed before its length is read. Null passes through the safe-call chain and selects the fallback zero.