Why null safety is Kotlin's defining feature - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a nullable variable in Kotlin.

Kotlin

var name: String[1] = null

Drag options to blanks, or click blank then click option'

A*

B!

C?

D#

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!' instead of '?' to mark nullable types.

In Kotlin, adding ? after a type makes it nullable, allowing it to hold null.

2fill in blank

medium

Complete the code to safely access the length of a nullable string.

Kotlin

val length = name[1]length

Drag options to blanks, or click blank then click option'

A..

B!!.

C?.?

D?.

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!.' which throws an exception if null.

The safe call operator ?. allows accessing a property only if the object is not null.

3fill in blank

hard

Fix the error in the code to throw an exception if the nullable variable is null.

Kotlin

val nonNullName = name[1]

Drag options to blanks, or click blank then click option'

A?

B!!

C?.

D?:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '?' which allows nulls without exception.

The !! operator asserts that the value is not null and throws an exception if it is.

4fill in blank

hard

Fill both blanks to provide a default value if the nullable variable is null.

Kotlin

val displayName = name [1] "Unknown"

Drag options to blanks, or click blank then click option'

A?:

B==

C?

D!!

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!!' which throws instead of providing a default.

The Elvis operator ?: returns the left value if not null, otherwise the right value.

5fill in blank

hard

Fill all three blanks to filter a list of nullable strings and print their lengths safely.

Kotlin

val names: List<String?> = listOf("Anna", null, "Bob")
val lengths = names.filterNotNull().map { it[1]length }
lengths.forEach { println(it[2]toString()[3]) }

Drag options to blanks, or click blank then click option'

A.

B+

C()

D?.

Attempts:

3 left

💡 Hint

Common Mistakes

Using safe call operator after filtering nulls, which is unnecessary.

After filtering nulls, it.length accesses length safely. Then println(it + "") prints the length as string.

Practice

1. What is the main purpose of Kotlin's null safety feature?

easy

A. To allow all variables to be null by default

B. To prevent errors caused by accessing null values

C. To make Kotlin code run faster

D. To remove the need for variable declarations

Solution

1. Step 1: Understand null safety concept

   Kotlin's null safety is designed to avoid null pointer exceptions by making null handling explicit.

2. Step 2: Identify the main purpose

   The main goal is to prevent errors caused by accessing null values, which are common bugs in many languages.

3. Final Answer:

   To prevent errors caused by accessing null values -> Option B

4. Quick Check:

   Null safety = prevent null errors [OK]

Hint: Null safety means avoiding null errors in code [OK]

Common Mistakes:

• Thinking null safety allows all variables to be null
• Believing it improves performance directly
• Confusing null safety with removing declarations

2. Which of the following is the correct way to declare a nullable variable in Kotlin?

easy

A. val name: String? = null

B. val name: String = null

C. val name: String! = null

D. val name: String = ?null

Solution

1. Step 1: Recall Kotlin nullable syntax

   In Kotlin, adding a question mark after the type makes it nullable, allowing it to hold null.

2. Step 2: Check each option

   val name: String? = null uses String? which is correct. Others are invalid syntax or do not allow null.

3. Final Answer:

   val name: String? = null -> Option A

4. Quick Check:

   Nullable type uses ? after type [OK]

Hint: Use ? after type to allow nulls in Kotlin [OK]

Common Mistakes:

• Omitting the ? for nullable types
• Using invalid symbols like ! or ?null
• Assigning null to non-nullable types

3. What will be the output of this Kotlin code?

val name: String? = null
println(name?.length ?: "No name")

medium

A. Throws NullPointerException

B. 0

C. null

D. No name

Solution

1. Step 1: Understand safe call and Elvis operator

   The safe call ?. returns null if name is null. The Elvis operator ?: provides a default value if the left side is null.

2. Step 2: Evaluate the expression

   Since name is null, name?.length is null, so the expression returns "No name".

3. Final Answer:

   No name -> Option D

4. Quick Check:

   Safe call + Elvis = default value [OK]

Hint: Safe call ?. returns null, Elvis ?: gives default [OK]

Common Mistakes:

• Expecting NullPointerException
• Thinking length of null is 0
• Confusing safe call with normal call

4. Identify the error in this Kotlin code snippet:

val length = name.length
val name: String? = null

medium

A. Variable 'name' is used before declaration

B. No error, code runs fine

C. Missing semicolon after declaration

D. Cannot access length on nullable variable without safe call

Solution

1. Step 1: Analyze variable usage order

   The variable name is used before it is declared.

2. Step 2: Identify the compile error

   Kotlin requires variables to be declared before use, causing an unresolved reference error.

3. Final Answer:

   Variable 'name' is used before declaration -> Option A

4. Quick Check:

   Declare before use [OK]

Hint: Declare variables before using them in Kotlin [OK]

Common Mistakes:

• Thinking it's only a null safety issue
• Believing the code runs fine
• Assuming Kotlin requires semicolons

5. How can you safely call a function printLength that prints the length of a nullable String text without causing a NullPointerException?

hard

A. fun printLength(text: String) { println(text.length) }
printLength(null)

B. fun printLength(text: String?) { println(text.length) }
printLength(null)

C. fun printLength(text: String?) { println(text?.length ?: 0) }
printLength(null)

D. fun printLength(text: String) { println(text?.length) }
printLength(null)

Solution

1. Step 1: Understand function parameter nullability

   To accept null, parameter must be String?. Options A and D use non-nullable String, so passing null causes error.

2. Step 2: Use safe call and default value

   fun printLength(text: String?) { println(text?.length ?: 0) } printLength(null) uses safe call ?. and Elvis operator ?: to print 0 if text is null, avoiding exceptions.

3. Final Answer:

   fun printLength(text: String?) { println(text?.length ?: 0) } printLength(null) -> Option C

4. Quick Check:

   Safe call + Elvis handles null safely [OK]

Hint: Use ?. and ?: to safely handle nullable parameters [OK]

Common Mistakes:

• Passing null to non-nullable parameter
• Accessing length without safe call
• Ignoring default value for null case