What's the difference between !! and ? in Kotlin?

Question

I am new to Kotlin. I want to know the difference between this two !! and ? in below code.

Below, there are two snippets: the first uses

Answer 1

As it said in Kotlin reference, !! is an option for NPE-lovers :)

a!!.length

will return a non-null value of a.length or throw a NullPointerException if a is null:

val a: String? = null
print(a!!.length) // >>> NPE: trying to get length of null

---

a?.length

returns a.length if a is not null, and null otherwise:

val a: String? = null
print(a?.length) // >>> null is printed in the console

---

To sum up:

+------------+--------------------+---------------------+----------------------+
| a: String? |           a.length |           a?.length |           a!!.length |
+------------+--------------------+---------------------+----------------------+
|      "cat" | Compile time error |                   3 |                    3 |
|       null | Compile time error |                null | NullPointerException |
+------------+--------------------+---------------------+----------------------+

Might be useful: What is a NullPointerException?