Mutable vs immutable interfaces in Kotlin - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an immutable list in Kotlin.

Kotlin

val numbers: List<Int> = [1](1, 2, 3)

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

AlistOf

BmutableListOf

CarrayListOf

DsetOf

Attempts:

3 left

💡 Hint

Common Mistakes

Using mutableListOf instead of listOf, which creates a mutable list.

In Kotlin, listOf creates an immutable list, which means you cannot change its elements after creation.

2fill in blank

medium

Complete the code to add an element to a mutable list in Kotlin.

Kotlin

val numbers = mutableListOf(1, 2, 3)
numbers.[1](4)

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

Aset

Bremove

Cclear

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' which replaces an element instead of adding.

Using 'remove' which deletes an element.

The add function adds an element to a mutable list in Kotlin.

3fill in blank

hard

Fix the error in the code by choosing the correct interface to declare a mutable list.

Kotlin

val numbers: [1]<Int> = mutableListOf(1, 2, 3)

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

AMutableList

BSet

CList

DCollection

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'List' which is immutable and causes errors when modifying.

To declare a mutable list interface in Kotlin, use MutableList. The List interface is read-only.

4fill in blank

hard

Fill both blanks to create an immutable map from a mutable map in Kotlin.

Kotlin

val mutableMap = mutableMapOf("a" to 1, "b" to 2)
val immutableMap: [1]<String, Int> = mutableMap
val readOnlyMap: [2]<String, Int> = immutableMap

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

AMap

BMutableMap

CHashMap

DLinkedHashMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using MutableMap for immutableMap causes mutability.

Both immutableMap and readOnlyMap use the Map interface, which is read-only in Kotlin.

5fill in blank

hard

Fill all three blanks to filter a mutable list and create an immutable list in Kotlin.

Kotlin

val mutableList = mutableListOf(1, 2, 3, 4, 5)
val filteredList: [1]<Int> = mutableList.[2] { it [3] 3 }

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

AList

Bfilter

C>

DMutableList

Attempts:

3 left

💡 Hint

Common Mistakes

Using MutableList as the type causes confusion about mutability.

Using '<' instead of '>' changes the filter condition.

The filter function returns a new List (immutable) containing elements greater than 3.

Practice

1. Which statement best describes the difference between mutable and immutable interfaces in Kotlin?

easy

A. Immutable interfaces are used only for primitive types.

B. Mutable interfaces only allow reading data, immutable interfaces allow modification.

C. Both mutable and immutable interfaces allow data modification.

D. Immutable interfaces allow only reading data, while mutable interfaces allow data modification.

Solution

1. Step 1: Understand interface roles

   Immutable interfaces provide read-only access to data, preventing changes.

2. Step 2: Compare mutable interfaces

   Mutable interfaces extend immutable ones and allow data to be changed.

3. Final Answer:

   Immutable interfaces allow only reading data, while mutable interfaces allow data modification. -> Option D

4. Quick Check:

   Immutable = read-only, Mutable = read-write [OK]

Hint: Immutable = read-only, mutable = can change data [OK]

Common Mistakes:

• Confusing which interface allows modification
• Thinking both interfaces allow changes
• Assuming immutable interfaces are only for primitives

2. Which of the following is the correct way to declare a mutable list interface in Kotlin?

easy

A. val list: MutableList<Int> = listOf(1, 2, 3)

B. val list: List<Int> = mutableListOf(1, 2, 3)

C. val list: MutableList<Int> = mutableListOf(1, 2, 3)

D. val list: List<Int> = listOf(1, 2, 3)

Solution

1. Step 1: Identify mutable list declaration

   MutableList is the interface for mutable lists, and mutableListOf() creates a mutable list instance.

2. Step 2: Check each option

   val list: MutableList<Int> = mutableListOf(1, 2, 3) correctly declares a MutableList and initializes it with mutableListOf().

3. Final Answer:

   val list: MutableList<Int> = mutableListOf(1, 2, 3) -> Option C

4. Quick Check:

   MutableList + mutableListOf() = mutable list [OK]

Hint: Use MutableList with mutableListOf() for mutable lists [OK]

Common Mistakes:

• Using listOf() with MutableList type
• Declaring mutable list as List type
• Mixing mutable and immutable functions

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

val immutableList: List<Int> = listOf(1, 2, 3)
val mutableList: MutableList<Int> = mutableListOf(4, 5, 6)

println(immutableList.size)
mutableList.add(7)
println(mutableList)

medium

A. 3
[4, 5, 6, 7]

B. 3
[4, 5, 6]

C. 4
[4, 5, 6, 7]

D. Compilation error

Solution

1. Step 1: Check immutable list size

   immutableList has 3 elements, so size is 3.

2. Step 2: Add element to mutable list and print

   mutableList adds 7, so it becomes [4, 5, 6, 7].

3. Final Answer:

   3 [4, 5, 6, 7] -> Option A

4. Quick Check:

   Immutable size 3, mutable list adds 7 [OK]

Hint: Immutable size fixed; mutable can add elements [OK]

Common Mistakes:

• Trying to add to immutable list causing error
• Expecting immutable list size to change
• Confusing output format

4. Identify the error in this Kotlin code snippet:

val numbers: List<Int> = listOf(1, 2, 3)
numbers.add(4)

medium

A. Cannot add to a List because it is immutable

B. Syntax error in listOf() usage

C. No error, code runs fine

D. Variable numbers should be declared as var

Solution

1. Step 1: Understand List immutability

   List interface in Kotlin is immutable; it does not support add() method.

2. Step 2: Analyze the error

   Calling add(4) on immutable List causes a compilation error.

3. Final Answer:

   Cannot add to a List because it is immutable -> Option A

4. Quick Check:

   Immutable List blocks add() method [OK]

Hint: List is immutable; use MutableList to add elements [OK]

Common Mistakes:

• Trying to add to immutable List
• Assuming var allows modification of immutable List
• Confusing syntax error with immutability error

5. You have a function that accepts a List<String> parameter but you want to modify the list inside the function. Which is the best approach to handle this in Kotlin?

hard

A. Change the parameter type to MutableList<String> and pass a mutable list.

B. Create a new mutable copy of the list inside the function and modify that.

C. Use val keyword inside the function to modify the list.

D. Keep the parameter as List<String> and cast it to MutableList inside the function.

Solution

1. Step 1: Understand parameter immutability

   List<String> is immutable, so it cannot be modified directly.

2. Step 2: Evaluate safe modification methods

   Casting to MutableList is unsafe and may cause runtime errors; changing parameter type affects API design.

3. Step 3: Use safe copy for modification

   Creating a mutable copy inside the function allows safe modifications without affecting original list.

4. Final Answer:

   Create a new mutable copy of the list inside the function and modify that. -> Option B

5. Quick Check:

   Copy immutable list to mutable for safe changes [OK]

Hint: Copy immutable list to mutable before modifying [OK]

Common Mistakes:

• Casting immutable list to mutable unsafely
• Changing parameter type breaking API contracts
• Assuming val allows modification