Collections (List, Map, Set) in Android Kotlin - Mini App: Build & Ship

Build: Collections Demo

This screen shows examples of Kotlin collections: List, Map, and Set. It displays each collection's content in a simple vertical list.

Target UI

-------------------------
| Collections Demo       |
|-----------------------|
| List:                 |
| - Apple               |
| - Banana              |
| - Cherry              |
|                       |
| Map:                  |
| Key: 1, Value: One    |
| Key: 2, Value: Two    |
|                       |
| Set:                  |
| - Red                 |
| - Green               |
| - Blue                |
-------------------------

Display a List of strings with three fruit names.

Display a Map with integer keys and string values.

Display a Set of strings with three color names.

Use Kotlin collection types: List, Map, Set.

Show each collection's items in a vertical Column.

Use Text composables for displaying items.

Starter Code

Android Kotlin

import androidx.compose.foundation.layout.Column
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable

@Composable
fun CollectionsDemo() {
    // TODO: Define List, Map, and Set collections
    // TODO: Display collections using Text inside Column
}

Task 1

Task 2

Task 3

Task 4

Task 5

Task 6

Solution

Android Kotlin

import androidx.compose.foundation.layout.Column
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable

@Composable
fun CollectionsDemo() {
    val fruits: List<String> = listOf("Apple", "Banana", "Cherry")
    val numbers: Map<Int, String> = mapOf(1 to "One", 2 to "Two")
    val colors: Set<String> = setOf("Red", "Green", "Blue")

    Column {
        Text(text = "List:")
        for (fruit in fruits) {
            Text(text = "- $fruit")
        }

        Text(text = "")
        Text(text = "Map:")
        for ((key, value) in numbers) {
            Text(text = "Key: $key, Value: $value")
        }

        Text(text = "")
        Text(text = "Set:")
        for (color in colors) {
            Text(text = "- $color")
        }
    }
}

This solution defines three Kotlin collections: a List of fruits, a Map of numbers, and a Set of colors.

It uses a Column composable to arrange the text vertically.

Each collection is displayed with a header and then its items are shown using Text composables in a simple loop.

This shows how to use Kotlin collections and display their contents in a Compose UI.

Final Result

Completed Screen

-------------------------
| Collections Demo       |
|-----------------------|
| List:                 |
| - Apple               |
| - Banana              |
| - Cherry              |
|                       |
| Map:                  |
| Key: 1, Value: One    |
| Key: 2, Value: Two    |
|                       |
| Set:                  |
| - Red                 |
| - Green               |
| - Blue                |
-------------------------

User sees the three collections displayed clearly.

No interactive elements; static display of collections.

Stretch Goal

Add a button that when clicked, adds a new fruit to the List and updates the UI.

💡 Hint

Use a mutableStateListOf for the List and remember { mutableStateListOf(...) } to hold state. Update the list on button click to trigger recomposition.

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