Problem Statement
What does the !! (not-null assertion) operator do and when should you use it?
Explanation
The not-null assertion operator double exclamation converts a nullable type to non-nullable, explicitly telling the compiler you guarantee the value is not null, and it will throw a KotlinNullPointerException if the value is actually null. Use it sparingly only when you're absolutely certain the value cannot be null but the compiler cannot infer that, such as after null checks in complex scenarios or when working with Java code that doesn't have null annotations. Overusing double exclamation defeats Kotlin's null safety and is considered a code smell, so prefer safe call or Elvis operators when possible, and use let, also, or require for better alternatives.