Mis juhtub, kui pöördute objekti välja, mis võib olla null?

NullPointerException (NPE). Occurs when you try to call a method or access a field of an object whose reference is null.

Example:

data class User(val name: String)

fun printUserName(user: User?) {
    // If user == null, a NullPointerException will occur
    println(user.name)
}

To avoid NPE, you can use:

• null checks:

  fun printUserName(user: User?) {
      if (user != null) {
          println(user.name)
      } else {
          println("User is null")
      }
  }
  

• Safe call operator (?.):

  fun printUserName(user: User?) {
      println(user?.name) // Will print null if user == null, otherwise the name
  }
  

• Elvis operator (?:):

  fun printUserName(user: User?) {
      val userName = user?.name ?: "Default Name" // Assigns "Default Name" if user?.name is null
      println(userName)
  }
  

• Non-null types (in Kotlin):

  // This method accepts only non-null User objects
  fun printUserName(user: User) {
      println(user.name) // Guaranteed not null
  }