Kotlin Kotlin Type System and Null Safety

Question 1

What symbol is appended to a type to make it nullable in Kotlin?

Accepted Answer

?

Answer

!

Answer

*

Answer

~

Question 2

What does the safe call operator `?.` return when the receiver is null?

Accepted Answer

null

Answer

Throws NullPointerException

Answer

Returns the default value

Answer

An empty object

Question 3

Which operator in Kotlin throws KotlinNullPointerException if the value is null?

Accepted Answer

!!

Answer

?.

Answer

?:

Answer

as?

Question 4

What is the Elvis operator `?:` used for in Kotlin?

Accepted Answer

Providing a default value when an expression is null

Answer

Performing safe casts

Answer

Type checking at runtime

Answer

Declaring nullable types

Question 5

After a null check `if (x != null)`, Kotlin automatically treats `x` as non-nullable inside the block. This is called:

Accepted Answer

Smart casting

Answer

Auto-boxing

Answer

Type inference

Answer

Type promotion

Question 6

What is the result of `val x: String? = null; val len = x?.length ?: -1`?

Accepted Answer

-1

Answer

NullPointerException

Answer

0

Answer

null

Question 7

Which of the following will cause a compilation error in Kotlin when `x` is of type `String?`?

Accepted Answer

x.doSomething()

Answer

x?.doSomething()

Answer

x ?: defaultValue

Answer

if (x != null) x.doSomething()