Platform types and null safety in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding platform types and null safety

What is the output of this Kotlin code when calling a Java method that returns a platform type?

Kotlin

fun main() {
    val javaString: String = getJavaString()
    println(javaString.length)
}

// Simulated Java method returning platform type
fun getJavaString(): String? = null

A Prints 0

B Throws a NullPointerException at runtime

C Compilation error due to null assignment

D Prints 'null'

💡 Hint

Platform types allow nulls but Kotlin treats them as non-null by default.

The Java method returns a platform type, which Kotlin treats as non-nullable. Assigning null to a non-nullable variable compiles but causes a NullPointerException at runtime when accessing its members.

❓ Predict Output

intermediate

Safe call operator with platform types

Given a platform type from Java, what will this Kotlin code print?

Kotlin

fun main() {
    val javaString = getJavaString()
    println(javaString?.length ?: "null length")
}

fun getJavaString(): String? = null

A null length

B Throws NullPointerException

C 0

D Compilation error

💡 Hint

The safe call operator ?. prevents NullPointerException.

Using ?. safely accesses length only if javaString is not null. Since javaString is null, the Elvis operator ?: returns "null length".

🔧 Debug

advanced

Identify the cause of NullPointerException with platform types

Why does this Kotlin code throw a NullPointerException at runtime?

Kotlin

fun main() {
    val javaList: List<String> = getJavaList()
    println(javaList.size)
}

fun getJavaList(): List<String>? = null

A Because platform types are treated as non-nullable but can be null at runtime

B Because Kotlin does not allow null lists at all

C Because getJavaList() returns an empty list, not null

D Because size property is not available on platform types

💡 Hint

Platform types come from Java and Kotlin assumes non-null but no guarantee.

Platform types have no nullability info, so Kotlin treats them as non-nullable. If the Java method returns null, accessing members causes NullPointerException.

📝 Syntax

advanced

Correctly declaring a platform type variable

Which Kotlin declaration correctly represents a platform type variable from Java that might be null?

A val javaStr: String! = getJavaString()

B val javaStr: String? = getJavaString()

C val javaStr = getJavaString()

D val javaStr: String = getJavaString()

💡 Hint

Platform types are not explicitly declared in Kotlin code.

Platform types are inferred by Kotlin when calling Java code. You cannot explicitly declare a variable as a platform type with '!'. Using 'val javaStr = getJavaString()' lets Kotlin infer the platform type.

🚀 Application

expert

Handling platform types safely in Kotlin

You receive a platform type String! from a Java method that might be null. Which Kotlin code snippet safely handles this value to avoid runtime exceptions?

A

val safeStr: String = javaStr
println(safeStr.length)

B

val safeStr: String = javaStr ?: ""
println(safeStr.length)

C

val safeStr: String = javaStr!!
println(safeStr.length)

D

val safeStr: String? = javaStr
println(safeStr?.length ?: 0)

💡 Hint

Convert platform type to nullable type to safely check for null.

Assigning the platform type to a nullable String allows safe calls and null checks. Options A and C risk NullPointerException. Option D is invalid because ?: cannot be used directly on platform types without explicit nullability.