What's the difference between KotlinNullPointerException and Java's NullPointerException

Question

As Kotlin doesn't allow for implicit null values/variables, was KotlinNullPointerException introduced to explicitly denote NPE caused by !!? Is that the only purpose of this child class of NullPointerException?

Answer 1

There is no real difference between a KotlinNullPointerException and JavaNullPointerException.

Here is how:

KotlinNullPointerException is an open class that extends NullPointerException Now this NullPointerException is a typealias of java.lang.NullPointerException.

public open class KotlinNullPointerException : NullPointerException {
  constructor()

  constructor(message: String?) : super(message)
}

This is a line extracted from TypeAlias.Kt

@SinceKotlin("1.1") public actual typealias NullPointerException = java.lang.NullPointerException

Now, if we see the declaration of java.lang.NullPointerException, we are taken to a Java.lang class that extends RuntimeException.

public
class NullPointerException extends RuntimeException {
 private static final long serialVersionUID = 5162710183389028792L;

 /**
  * Constructs a {@code NullPointerException} with no detail message.
  */
 public NullPointerException() {
     super();
 }

 /**
  * Constructs a {@code NullPointerException} with the specified
  * detail message.
  *
  * @param   s   the detail message.
  */
 public NullPointerException(String s) {
     super(s);
 }
}

In Kotlin, to make some declaration of nullable type, you have to explicitly allow it by appending ? at the end of declaration type. Example:

var nullableString: String? = null

This is a simple way of saying that this variable could be null anytime, so if you try to access this variable from any part of your code, it will throw error and will force you to take measures to prevent NPE either using !!(crash if null) or ?(skip if null). It's just a way to make it look more like 'Kotlin'.

Answer 2

As of version 1.3, Kotlin throws KotlinNullPointerException only in case of a failed !! operator check. This distinguishes it from the other cases where NullPointerException can be thrown, for example, when accessing non-initialized members during class construction.

Note, however, that there are plans to remove this distinction in Kotlin 1.4: all such failed checks will throw just NullPointerException, so its inheritor KotlinNullPointerException will become unused. You can read more about that in the announce blog post of 1.3.50 version release: https://blog.jetbrains.com/kotlin/2019/08/kotlin-1-3-50-released/