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.