MCQs on Null Safety | Kotlin

Mastering Null Safety in Kotlin: 30 Essential MCQs
Dive into the critical concept of null safety in Kotlin, including nullable types, non-null types, Elvis operator, safe calls, and null assertions. Sharpen your skills with these practice questions!

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MCQs on Null Safety in Kotlin

1. Nullable Types (?) and Non-Null Types

1. Which of the following declarations represents a nullable type in Kotlin?
   a) val name: String
   b) val name: String?
   c) var name = null
   d) val name: ?String
2. What will be the output of the following code?
   val name: String? = null println(name)
   a) null
   b) Error
   c) ""
   d) undefined
3. How can you assign a nullable value to a non-nullable variable in Kotlin?
   a) Using !!
   b) Directly assigning
   c) Using as keyword
   d) It’s not allowed
4. What is the default type for a variable in Kotlin if no type is explicitly declared?
   a) Nullable
   b) Non-null
   c) Inferred nullable
   d) Inferred non-null
5. When declaring a variable, adding ? after the type implies:
   a) It is a nullable type
   b) It is a generic type
   c) It can hold any type
   d) It supports typecasting

2. Elvis Operator (?:)

6. What is the primary purpose of the Elvis operator ?: in Kotlin?
   a) To provide a default value for nullable expressions
   b) To throw exceptions on null
   c) To combine two nullable variables
   d) None of the above
7. What is the output of the following code?
   val name: String? = null val result = name ?: "Unknown" println(result)
   a) null
   b) Unknown
   c) Error
   d) ""
8. Which of the following expressions demonstrates the correct use of the Elvis operator?
   a) val x = a ?: b
   b) val x = a ? b
   c) val x = a ?? b
   d) val x = a :? b
9. How does the Elvis operator behave when the left-hand operand is not null?
   a) Returns the left-hand operand
   b) Returns the right-hand operand
   c) Throws an exception
   d) Assigns null
10. If x is null and y is "default", what will val z = x ?: y return?
    a) null
    b) "default"
    c) Error
    d) undefined

3. Safe Calls and Null Assertions

11. What does the safe call operator ?. do in Kotlin?
    a) Executes a block of code on non-null values
    b) Prevents null pointer exceptions
    c) Forces null values to be non-null
    d) Allows nullable types to hold non-null values
12. What is the result of the following code?
    val length = str?.length
    a) Length of str or null
    b) Always throws an exception
    c) Null pointer exception if str is null
    d) Undefined behavior
13. What will happen if !! operator is used on a null variable?
    a) Throws a NullPointerException
    b) Assigns a default value
    c) Silently ignores the null
    d) None of the above
14. Which operator should be avoided to prevent runtime exceptions in Kotlin?
    a) !!
    b) ?.
    c) :
    d) =
15. How does safe call chaining work in Kotlin?
    a) Stops at the first null value
    b) Executes all calls regardless of nulls
    c) Always returns null
    d) Combines all non-null values
16. Can the null assertion operator !! be used with safe calls?
    a) No, they are mutually exclusive
    b) Yes, but with caution
    c) Yes, it forces nulls to non-null
    d) None of the above
17. What is the correct way to safely access a nullable property?
    a) Using ?.
    b) Using !!
    c) Using as
    d) Using val
18. What will user?.address?.city return if user or address is null?
    a) Throws an exception
    b) Returns null
    c) Returns "city"
    d) Undefined
19. Which of these statements can lead to NullPointerException in Kotlin?
    a) Using !! on a null variable
    b) Using safe calls
    c) Using Elvis operator with non-null values
    d) Assigning null to a nullable type
20. How do you avoid null pointer exceptions effectively in Kotlin?
    a) By using safe calls and Elvis operator
    b) By avoiding !! operator
    c) By using nullable types
    d) All of the above

Comprehensive

21. Can a non-null type be explicitly assigned null?
    a) No
    b) Yes
    c) Only using !!
    d) None of the above
22. What does the following code snippet do?
    val result = myString!!.toUpperCase() a) Converts myString to uppercase if not null
    b) Throws an exception if myString is null
    c) Converts to lowercase if null
    d) None of the above
23. What is the purpose of combining ?. with ?:?
    a) To safely access and provide defaults
    b) To force null to non-null
    c) To assign types dynamically
    d) None of the above
24. What will the code listOf(null)?.size ?: 0 return?
    a) 1
    b) 0
    c) null
    d) Throws an error
25. When is a nullable type preferable over a non-null type?
    a) When dealing with optional values
    b) Always
    c) Never
    d) In primitive types only
26. Can !! be applied to a non-nullable type?
    a) No, it’s redundant
    b) Yes, it forces a null check
    c) No, it throws an exception
    d) None of the above
27. What is the output of this code? val x: String? = "Hello" println(x ?: "World") a) Hello
    b) World
    c) null
    d) Error
28. Which operator is best suited for null-safe operations?
    a) ?.
    b) !!
    c) :
    d) =
29. What is a potential drawback of using !!?
    a) Can cause NullPointerException
    b) Slows down code execution
    c) Ignores null values
    d) Makes types dynamic
30. Which statement about null safety in Kotlin is correct?
    a) Null safety is enforced at compile-time
    b) Null safety is optional
    c) Null safety applies only to primitives
    d) None of the above

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Answers Table

Qno	Answer (Option with Text)
1	b) val name: String?
2	a) null
3	a) Using !!
4	b) Non-null
5	a) It is a nullable type
6	a) To provide a default value
7	b) Unknown
8	a) val x = a ?: b
9	a) Returns the left-hand operand
10	b) "default"
11	b) Prevents null pointer exceptions
12	a) Length of str or null
13	a) Throws a NullPointerException
14	a) !!
15	a) Stops at the first null value
16	b) Yes, but with caution
17	a) Using ?.
18	b) Returns null
19	a) Using !! on a null variable
20	d) All of the above
21	a) No
22	b) Throws an exception if null
23	a) To safely access and provide defaults
24	a) 1
25	a) When dealing with optional values
26	a) No, it’s redundant
27	a) Hello
28	a) ?.
29	a) Can cause NullPointerException
30	a) Null safety is enforced at compile-time