UserId.kt

val user: User?

// Get user ID

//If expression (userId is nullable)
val userId = if(user != null) user.id else null

//Elvis operator (userId is not nullable, fail fast)
val userId = user.id ?: throw IllegalStateException("User can not be null at this time")

//Any?.let (userId is nullable)
val userId = user?.let{it.id}

//Any?.let (userId is not nullable, is a String, can be empty)
val userId = user?.let{it.id}.orEmpty()

//Safe navigation (userId is nullable)
val userId = user?.id