Non-nullable types by default in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing a nullable variable

What is the output of this Kotlin code snippet?

Kotlin

fun main() {
    val name: String? = null
    println(name?.length ?: "No length")
}

A null

B No length

C 0

D Throws NullPointerException

💡 Hint

Look at the safe call operator and the Elvis operator usage.

The variable name is nullable and set to null. Using name?.length safely returns null instead of throwing an error. The Elvis operator ?: then provides the fallback string "No length".

❓ Predict Output

intermediate

Behavior of non-nullable variable assignment

What happens when you try to assign null to a non-nullable variable in Kotlin?

Kotlin

fun main() {
    val number: Int = null
    println(number)
}

A Compilation error

B Prints null

C Prints 0

D Throws NullPointerException at runtime

💡 Hint

Remember Kotlin's default type nullability.

Kotlin types are non-nullable by default. Assigning null to a non-nullable type causes a compilation error.

🔧 Debug

advanced

Identify the cause of NullPointerException

Why does this Kotlin code throw a NullPointerException at runtime?

Kotlin

fun main() {
    val text: String? = null
    val length: Int = text!!.length
    println(length)
}

A Because println cannot print null values

B Because length is declared as Int instead of Int?

C Because text is nullable and !! forces non-null, causing exception

D Because text is not initialized

💡 Hint

Check the use of the !! operator on a null value.

The !! operator asserts that the value is not null. Since text is actually null, this causes a NullPointerException at runtime.

🧠 Conceptual

advanced

Understanding Kotlin's type system default

Which statement best describes Kotlin's type system regarding nullability?

A All types are nullable by default, requiring explicit non-null declaration

B Kotlin does not support nullable types

C Nullability is not checked by the compiler in Kotlin

D All types are non-nullable by default, requiring explicit nullable declaration

💡 Hint

Think about how Kotlin prevents null pointer errors.

Kotlin's type system makes all types non-nullable by default. To allow null values, you must explicitly declare a type as nullable using ?.

❓ Predict Output

expert

Output of complex nullable chaining

What is the output of this Kotlin code?

Kotlin

fun main() {
    val map: Map<String, String?> = mapOf("key" to null)
    val length = map["key"]?.length ?: -1
    println(length)
}

A -1

B Throws NullPointerException

C null

D 0

💡 Hint

Check the safe call and Elvis operator on a nullable map value.

The map contains a key with a null value. The safe call ?.length returns null, so the Elvis operator returns -1.

Practice

1. In Kotlin, what happens if you declare a variable without a ? after its type?

easy

A. The variable cannot hold null values.

B. The variable can hold null values.

C. The variable is automatically initialized to null.

D. The variable must be declared as lateinit.

Solution

1. Step 1: Understand Kotlin's default type behavior

   Kotlin types are non-nullable by default, meaning variables cannot hold null unless specified.

2. Step 2: Check the effect of missing ?

   Without ?, the variable type is non-nullable, so it cannot hold null.

3. Final Answer:

   The variable cannot hold null values. -> Option A

4. Quick Check:

   Non-nullable type = no null allowed [OK]

Hint: No ? means no null allowed [OK]

Common Mistakes:

• Thinking variables are nullable by default
• Assuming variables initialize to null automatically
• Confusing lateinit with nullability

2. Which of the following Kotlin variable declarations is correct to allow null values?

easy

A. val name: String? = null

B. val name: String?

C. val name: String = null

D. val name: String = "null"

Solution

1. Step 1: Identify nullable type syntax

   Adding ? after the type allows the variable to hold null.

2. Step 2: Check initialization

   Nullable variables must be initialized or declared with null explicitly, so val name: String? = null is correct.

3. Final Answer:

   val name: String? = null -> Option A

4. Quick Check:

   Nullable type with ? and initialized to null [OK]

Hint: Use ? and initialize nullable variables [OK]

Common Mistakes:

• Declaring nullable without ?
• Not initializing nullable variables
• Assigning null to non-nullable types

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

fun main() {
    val text: String? = null
    println(text?.length ?: "No text")
}

medium

A. 0

B. Compilation error

C. null

D. No text

Solution

1. Step 1: Understand safe call operator usage

   The text?.length safely accesses length only if text is not null.

2. Step 2: Evaluate the Elvis operator

   Since text is null, text?.length is null, so the Elvis operator ?: returns "No text".

3. Final Answer:

   No text -> Option D

4. Quick Check:

   Safe call + Elvis operator returns fallback [OK]

Hint: Safe call with Elvis returns fallback if null [OK]

Common Mistakes:

• Expecting 0 instead of fallback string
• Confusing safe call with normal call
• Thinking code causes error due to null

4. Identify the error in this Kotlin code snippet:

fun main() {
    val number: Int = null
    println(number)
}

medium

A. Variable 'number' is not initialized.

B. Cannot assign null to a non-nullable type Int.

C. Missing semicolon after println statement.

D. Variable 'number' should be declared as var.

Solution

1. Step 1: Check variable type and assignment

   The variable 'number' is declared as non-nullable Int but assigned null, which is not allowed.

2. Step 2: Understand Kotlin null safety rules

   Non-nullable types cannot hold null, so this causes a compilation error.

3. Final Answer:

   Cannot assign null to a non-nullable type Int. -> Option B

4. Quick Check:

   Non-nullable types reject null assignments [OK]

Hint: Non-nullable types cannot be assigned null [OK]

Common Mistakes:

• Thinking semicolon is required
• Assuming var fixes null assignment
• Ignoring Kotlin's null safety

5. You have a nullable variable val input: String?. You want to print its length only if it is not null, otherwise print "Empty input". Which Kotlin code correctly does this?

hard

A. println(input.length ?: "Empty input")

B. println(input!!.length)

C. println(if (input != null) input.length else "Empty input")

D. println(input?.length)

Solution

1. Step 1: Understand nullable variable usage

   Since input can be null, we must check before accessing length.

2. Step 2: Evaluate each option

   println(input.length ?: "Empty input") is invalid because length is Int, not nullable, so Elvis operator on it is wrong.
   println(if (input != null) input.length else "Empty input") safely checks null and prints length or fallback.
   println(input!!.length) uses !! which throws exception if null.
   println(input?.length) prints null if input is null, not the string "Empty input".

3. Final Answer:

   println(if (input != null) input.length else "Empty input") -> Option C

4. Quick Check:

   Explicit null check with if-else for safe access [OK]

Hint: Use if-else to check null before accessing properties [OK]

Common Mistakes:

• Using !! and risking exceptions
• Misusing Elvis operator on non-nullable Int
• Printing null instead of fallback string