1. Lists, Sets and Maps at a Glance · Kotlin Collections and More

[THEORY]

[PREDICT]

Read the code carefully before you run it.

prediction.kt

fun main() {
    val categories = listOf("Tools", "Electronics", "Tools", "Office")
    val categoriesSet = categories.toSet()
    println(categories.size)
    println(categoriesSet.size)
}

Which two numbers does this program print?

[TASK]

A warehouse management system manages products. Outside main() (top level, which works for val just as well as inside a function) create three declarations: val productNames as a List<String> with the entries "Screwdriver", "Hammer", "Screwdriver", "Pliers" (the item "Screwdriver" really is in stock twice, so it is deliberately duplicated). val categories as a Set<String> with the entries "Tools", "Tools", "Electronics" — use setOf for that. val prices as a Map<String, Double> that maps the product "Hammer" to the price 12.5 and "Pliers" to the price 8.0. Finally print productNames.size, categories.size and prices["Hammer"] in main() (three println calls).

task.kt

// TODO: create val productNames as List<String> with the four product names (via listOf) // TODO: create val categories as Set<String> with the three categories (via setOf) // TODO: create val prices as Map<String, Double> with the two prices (via mapOf) fun main() { // TODO: print productNames.size, categories.size and prices["Hammer"] (three println calls) }

Before you get a hint: what have you tried, and where exactly are you stuck?

[COMPREHENSION CHECK]

1. What does setOf("A", "A", "B") return?

A set with one element: "B", every value that appears twice is removed completely An IllegalArgumentException at runtime, the way Java's Set.of rejects duplicates A set with three elements: "A", "A" and "B", in the order they were passed in A set with two elements: "A" and "B", the duplicate "A" is dropped silently

2. Which statement about Map is true?

Values are unique, while the same key may appear again with a different value A Map is a List of key-value pairs, so every entry can be read by its index Keys are unique, while several different keys may point to the same value Keys and values are both unique, so no two entries may hold the same value

3. What is true about the relationship between List, Set and Map?

List and Set extend Collection, while Map is a separate abstraction for key-value pairs Set extends List and adds the rule that an element may not occur twice Map extends Set, since the keys of a map already form a set of unique values List, Set and Map all extend Collection, so a Collection parameter accepts all three of them