Collections interop behavior in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Kotlin List to Java List Interop

What is the output of the following Kotlin code when run on the JVM?

Kotlin

val kotlinList = listOf("a", "b", "c")
val javaList: java.util.List<String> = kotlinList
println(javaList.javaClass.name)

A java.util.Arrays$ArrayList

B java.util.ArrayList

C java.util.Collections$UnmodifiableRandomAccessList

D kotlin.collections.ArraysKt___ArraysJvmKt$asList

💡 Hint

Think about how Kotlin's listOf creates a fixed-size list backed by an array.

Kotlin's listOf creates a read-only list backed by a Java array, which is represented as java.util.Arrays$ArrayList on the JVM.

❓ Predict Output

intermediate

Modifying Kotlin MutableList via Java List Reference

Given the Kotlin code below, what will be the output?

Kotlin

val mutableList = mutableListOf(1, 2, 3)
val javaList: java.util.List<Int> = mutableList
javaList.add(4)
println(mutableList)

A Runtime exception

B [1, 2, 3]

C [1, 2, 3, 4]

D Compilation error

💡 Hint

MutableList in Kotlin is backed by a Java ArrayList, so changes via Java List reflect in Kotlin list.

The mutableListOf returns a Kotlin MutableList backed by a Java ArrayList. Adding an element via the Java List reference modifies the same underlying list.

🔧 Debug

advanced

Why does this Kotlin to Java Map interop code throw an exception?

Consider this Kotlin code snippet that passes a Kotlin Map to a Java method expecting a java.util.Map. The Java method tries to modify the map but throws an exception. Why?

Kotlin

fun main() {
    val kotlinMap = mapOf("key" to "value")
    modifyMap(kotlinMap)
}

fun modifyMap(map: java.util.Map<String, String>) {
    map.put("newKey", "newValue")
}

A Kotlin maps cannot be passed to Java methods due to type erasure.

B The Java method signature is incorrect and causes a ClassCastException.

C The JVM does not allow modification of any map passed from Kotlin to Java.

D The Kotlin mapOf returns an immutable map that throws UnsupportedOperationException on put.

💡 Hint

Check the mutability of the Kotlin map created by mapOf.

Kotlin's mapOf creates an immutable map. When passed to Java, it is seen as a java.util.Map but does not support modification methods like put, causing UnsupportedOperationException.

📝 Syntax

advanced

Correct Kotlin syntax to convert Java List to Kotlin MutableList

Which of the following Kotlin code snippets correctly converts a java.util.List to a Kotlin MutableList?

A val kotlinMutableList = javaList.toMutableList()

B val kotlinMutableList = javaList as MutableList<String>

C val kotlinMutableList = javaList.toList().toMutableList()

D val kotlinMutableList = mutableListOf(javaList)

💡 Hint

Look for the idiomatic Kotlin way to copy a Java list into a mutable Kotlin list.

The toMutableList() extension function creates a new Kotlin MutableList from any Iterable, including Java lists. Casting (option A) is unsafe and may fail at runtime.

🚀 Application

expert

Predict the size of a Kotlin Set after Java interop operations

Given the Kotlin code below, what is the size of the set after all operations?

Kotlin

val kotlinSet = mutableSetOf("a", "b", "c")
val javaSet: java.util.Set<String> = kotlinSet
javaSet.add("d")
javaSet.remove("b")
println(kotlinSet.size)

A 4

B 3

C 2

D Runtime exception

💡 Hint

Consider how Kotlin's mutableSetOf is backed and how Java Set operations affect it.

Kotlin's mutableSetOf returns a LinkedHashSet, which implements the mutable methods of java.util.Set including add and remove. Thus, add("d") succeeds (size 4), then remove("b") succeeds (size 3).