Collections interop behavior in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Kotlin list from a Java list.

Kotlin

val javaList = java.util.ArrayList<String>()
javaList.add("apple")
val kotlinList: List<String> = [1](javaList)

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

AArrayList

BlistOf

CmutableListOf

DCollections

Attempts:

3 left

💡 Hint

Common Mistakes

Using listOf() which creates an immutable list and does not accept a Java list directly.

Using mutableListOf() which creates an empty list.

Use ArrayList(javaList) to create a Kotlin list from a Java list.

2fill in blank

medium

Complete the code to convert a Kotlin list to a Java list.

Kotlin

val kotlinList = listOf("one", "two", "three")
val javaList: java.util.List<String> = kotlinList.[1]()

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

AtoCollection

BtoMutableList

CtoList

DtoJavaList

Attempts:

3 left

💡 Hint

Common Mistakes

Using toList() which returns an immutable Kotlin list, not a Java list.

Using toJavaList() which does not exist.

Use toMutableList() to get a Java mutable list from a Kotlin list.

3fill in blank

hard

Fix the error in the code to add an element to a Java list obtained from a Kotlin list.

Kotlin

val kotlinList = listOf("a", "b")
val javaList: java.util.List<String> = kotlinList.[1]()
javaList.add("c")

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

AtoList

BtoSet

CasList

DtoMutableList

Attempts:

3 left

💡 Hint

Common Mistakes

Using toList() which returns an immutable list causing add() to fail.

Using asList() which returns a fixed-size list.

Use toMutableList() to get a mutable Java list that supports adding elements.

4fill in blank

hard

Fill both blanks to create a Kotlin map from a Java map and access a value.

Kotlin

val javaMap = java.util.HashMap<String, Int>()
javaMap["x"] = 10
val kotlinMap: Map<String, Int> = [1](javaMap)
val value = kotlinMap[[2]]

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

AHashMap

B"x"

CmapOf

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using mapOf() which creates an empty map here.

Using 10 as a key instead of "x".

Use HashMap(javaMap) to create a Kotlin map from a Java map, and access the value with the key "x".

5fill in blank

hard

Fill all three blanks to filter a Kotlin list converted from a Java list.

Kotlin

val javaList = java.util.ArrayList<Int>()
javaList.add(1)
javaList.add(2)
javaList.add(3)
val kotlinList = [1](javaList)
val filtered = kotlinList.filter { it [2] [3] }

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

AArrayList

B>

C1

DlistOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using listOf which creates an immutable list.

Using '<' instead of '>' in the filter.

Convert the Java list to a Kotlin ArrayList, then filter elements greater than 1.

Practice

1. In Kotlin, when you use a Java List inside Kotlin code, what happens to the collection?

easy

A. Kotlin wraps the Java list directly without copying it.

B. Kotlin creates a new copy of the Java list.

C. Kotlin converts the Java list to a Kotlin Set.

D. Kotlin throws an error because Java collections are incompatible.

Solution

1. Step 1: Understand Kotlin-Java collection interop

   Kotlin collections can wrap Java collections directly, meaning no new copy is made.

2. Step 2: Identify behavior for Java List in Kotlin

   Using a Java List in Kotlin results in a wrapper around the original Java list, preserving changes.

3. Final Answer:

   Kotlin wraps the Java list directly without copying it. -> Option A

4. Quick Check:

   Kotlin wraps Java collections directly = D [OK]

Hint: Kotlin wraps Java collections directly, no copying needed [OK]

Common Mistakes:

• Thinking Kotlin copies Java collections
• Assuming Java collections convert to Kotlin sets
• Believing Java collections cause errors in Kotlin

2. Which of the following Kotlin code snippets correctly creates a mutable Kotlin list from a Java ArrayList without copying?

easy

A. val javaList = ArrayList<String>(); val kotlinList = javaList as MutableList<String>

B. val javaList = ArrayList<String>(); val kotlinList = javaList.toMutableList()

C. val javaList = ArrayList<String>(); val kotlinList = javaList.toList()

D. val javaList = ArrayList<String>(); val kotlinList = javaList.copy()

Solution

1. Step 1: Review Kotlin-Java mutable list interop

   Kotlin mutable lists can be created by casting Java mutable lists directly without copying.

2. Step 2: Analyze each option

   val javaList = ArrayList<String>(); val kotlinList = javaList as MutableList<String> uses as MutableList<String> to wrap the Java ArrayList directly. toList() and toMutableList() create copies, and there is no copy() method.

3. Final Answer:

   val javaList = ArrayList<String>(); val kotlinList = javaList as MutableList<String> -> Option A

4. Quick Check:

   Cast Java list to MutableList for no copy = C [OK]

Hint: Cast Java list to MutableList to avoid copying [OK]

Common Mistakes:

• Using toList() or toMutableList() which copies
• Trying to call non-existent copy() method
• Not casting and expecting automatic conversion

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

val javaList = java.util.ArrayList<String>()
javaList.add("Kotlin")
val kotlinList: List<String> = javaList
println(kotlinList[0])
javaList.add("Java")
println(kotlinList.size)

medium

A. Kotlin
0

B. Kotlin
1

C. Kotlin
2

D. Compilation error

Solution

1. Step 1: Understand Kotlin wrapping Java list

   The Kotlin List wraps the Java ArrayList directly, so changes to javaList reflect in kotlinList.

2. Step 2: Trace the code output

   First print prints "Kotlin" (element at index 0). Then adding "Java" to javaList increases size to 2, so second print outputs 2.

3. Final Answer:

   Kotlin 2 -> Option C

4. Quick Check:

   Wrapped list reflects changes = A [OK]

Hint: Kotlin List wraps Java list, reflects changes instantly [OK]

Common Mistakes:

• Assuming kotlinList is a copy and size stays 1
• Expecting compilation error due to Java interop
• Confusing index access with size

4. Consider this Kotlin code snippet:

val javaList = java.util.ArrayList<String>()
val kotlinList: MutableList<String> = javaList
kotlinList.add("Hello")
println(javaList.size)

What is the problem with this code?

medium

A. It compiles but javaList.size remains 0.

B. It compiles and javaList.size becomes 1.

C. It causes a runtime ClassCastException.

D. It causes a compilation error due to type mismatch.

Solution

1. Step 1: Analyze Kotlin-Java mutable list assignment

   Assigning a Java ArrayList to a Kotlin MutableList variable works because Kotlin treats Java mutable collections as mutable Kotlin collections.

2. Step 2: Trace mutation effect

   Adding "Hello" to kotlinList modifies the underlying javaList, so javaList.size becomes 1.

3. Final Answer:

   It compiles and javaList.size becomes 1. -> Option B

4. Quick Check:

   Mutable Kotlin list wraps Java list, changes reflect = A [OK]

Hint: Mutable Kotlin list wraps Java list; changes affect both [OK]

Common Mistakes:

• Expecting runtime ClassCastException
• Thinking javaList.size stays 0 after add
• Assuming compilation error on assignment

5. You have a Java HashMap<String, Integer> and want to use it in Kotlin as a mutable map without copying. Which Kotlin code correctly achieves this and allows modifying the map?

hard

A. val javaMap = java.util.HashMap()
val kotlinMap: Map = javaMap.toMap()

B. val javaMap = java.util.HashMap()
val kotlinMap: Map = javaMap as Map

C. val javaMap = java.util.HashMap()
val kotlinMap: MutableMap = javaMap.toMutableMap()

D. val javaMap = java.util.HashMap()
val kotlinMap: MutableMap = javaMap

Solution

1. Step 1: Understand Kotlin-Java map interop

   Kotlin mutable maps can directly wrap Java mutable maps like HashMap without copying.

2. Step 2: Evaluate options for mutability and copying

   The direct assignment val kotlinMap: MutableMap<String, Int> = javaMap wraps the Java HashMap without copying and preserves mutability. In contrast, toMap() and casting to Map provide immutable access, while toMutableMap() creates a new mutable copy.

3. Final Answer:

   val javaMap = java.util.HashMap<String, Int>() val kotlinMap: MutableMap<String, Int> = javaMap -> Option D

4. Quick Check:

   Assign Java HashMap to MutableMap for no copy and mutability = B [OK]

Hint: Assign Java HashMap directly to MutableMap for no copy [OK]

Common Mistakes:

• Using toMap() or toMutableMap() which copies
• Casting to Map loses mutability
• Assuming Kotlin creates a new map automatically