Collections (List, Map, Set) in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a List in Kotlin collections?

A List is an ordered collection of elements that can contain duplicates. You can access elements by their index.

beginner

How does a Set differ from a List in Kotlin?

A Set is a collection that holds unique elements only. It does not maintain insertion order and does not allow duplicates.

beginner

What is a Map in Kotlin collections?

A Map stores key-value pairs. Each key is unique and maps to exactly one value.

beginner

How do you create an immutable List in Kotlin?

Use listOf() function. For example: val myList = listOf("apple", "banana") creates a read-only list.

intermediate

What function creates a mutable Map in Kotlin?

Use mutableMapOf(). For example: val myMap = mutableMapOf("key1" to 1, "key2" to 2) creates a map you can change.

Which Kotlin collection type allows duplicate elements and maintains order?

A List

B Set

C Map

D None of the above

A List allows duplicates and keeps the order of elements.

What happens if you add a duplicate element to a Kotlin Set?

A It adds the duplicate

B It ignores the duplicate

C It throws an error

D It replaces the existing element

Sets ignore duplicates and keep only unique elements.

How do you access a value in a Kotlin Map?

A By index

B By value

C By key

D By position

You access values in a Map using their unique keys.

Which function creates a mutable List in Kotlin?

A listOf()

B mapOf()

C setOf()

D mutableListOf()

mutableListOf() creates a list you can change.

What is the key characteristic of a Kotlin Map?

A It stores key-value pairs with unique keys

B It stores only unique values

C It stores ordered elements

D It stores only numbers

A Map stores pairs where each key is unique and maps to a value.

Explain the differences between List, Set, and Map in Kotlin collections.

Think about how each collection stores and accesses data.

Describe how to create and use a mutable List and a mutable Map in Kotlin.

Focus on the functions and what mutability means.

Practice

1. Which Kotlin collection type should you use to store a list of names where order matters and duplicates are allowed?

easy

A. List

B. Map

C. Set

D. Array

Solution

1. Step 1: Understand collection types

   List keeps items in order and allows duplicates, Map stores key-value pairs, Set stores unique items without duplicates.

2. Step 2: Match requirement to collection

   Since order matters and duplicates are allowed, List fits best.

3. Final Answer:

   List -> Option A

4. Quick Check:

   Order + duplicates = List [OK]

Hint: Remember: List = ordered + duplicates allowed [OK]

Common Mistakes:

• Choosing Set which disallows duplicates
• Choosing Map which stores key-value pairs
• Confusing Array with List in Kotlin

2. Which of the following is the correct syntax to create a mutable map in Kotlin?

easy

A. val map = mutableListOf()

B. val map = mapOf()

C. val map = mutableMapOf()

D. val map = setOf()

Solution

1. Step 1: Identify mutable map syntax

   mutableMapOf creates a mutable map; mapOf creates an immutable map.

2. Step 2: Check other options

   mutableListOf creates a list, setOf creates a set; both are incorrect for map.

3. Final Answer:

   val map = mutableMapOf() -> Option C

4. Quick Check:

   Mutable map uses mutableMapOf [OK]

Hint: Mutable map uses mutableMapOf() function [OK]

Common Mistakes:

• Using mapOf which is immutable
• Using mutableListOf instead of mutableMapOf
• Using setOf which creates a set, not a map

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

val set = setOf(1, 2, 2, 3)
println(set.size)

medium

A. 3

B. 2

C. 4

D. Error

Solution

1. Step 1: Understand set behavior

   Set stores unique elements only, so duplicates are removed.

2. Step 2: Count unique elements

   Elements are 1, 2, 3 so size is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Set removes duplicates, size = 3 [OK]

Hint: Set size counts unique elements only [OK]

Common Mistakes:

• Counting duplicates in set size
• Assuming set keeps duplicates
• Expecting runtime error for duplicates

4. Identify the error in this Kotlin code snippet:

val map = mapOf("a" to 1, "b" to 2)
map["c"] = 3

medium

A. map["c"] = 3 is correct syntax

B. Syntax error in map declaration

C. Key "c" already exists

D. mapOf creates an immutable map, cannot add elements

Solution

1. Step 1: Check mapOf mutability

   mapOf creates an immutable map, so you cannot add or change elements.

2. Step 2: Analyze assignment

   map["c"] = 3 tries to add a new key-value pair, which is not allowed on immutable map.

3. Final Answer:

   mapOf creates an immutable map, cannot add elements -> Option D

4. Quick Check:

   mapOf = immutable map, no additions allowed [OK]

Hint: mapOf is immutable; use mutableMapOf to add items [OK]

Common Mistakes:

• Assuming mapOf is mutable
• Thinking syntax is wrong
• Ignoring immutability of mapOf

5. Given a list of words, how can you create a map that counts how many times each word appears? Choose the correct Kotlin code snippet.

hard

A. val counts = words.filter { it.length > 1 }

B. val counts = words.groupingBy { it }.eachCount()

C. val counts = words.distinct()

D. val counts = words.toSet().map { it.length }

Solution

1. Step 1: Understand the goal

   We want a map with words as keys and their counts as values.

2. Step 2: Analyze options

   val counts = words.groupingBy { it }.eachCount() uses groupingBy and eachCount to count occurrences correctly. Others do not count frequencies.

3. Final Answer:

   val counts = words.groupingBy { it }.eachCount() -> Option B

4. Quick Check:

   groupingBy + eachCount counts items [OK]

Hint: Use groupingBy and eachCount to count list items [OK]

Common Mistakes:

• Using toSet which removes duplicates but doesn't count
• Using map to get lengths instead of counts
• Filtering instead of counting