Welcome to our exploration of sorted maps using custom classes and comparators in Kotlin. In today's lesson, we'll learn how to use custom data classes as keys in sorted maps. This approach enhances data organization and access. With the addition of comparators, we can dictate the order in such maps.

A sorted map is a collection with its keys always in order. This arrangement makes operations like searching for keys within a range more efficient. In Kotlin, we often use the sortedMapOf function or TreeMap from Java's standard library for sorted maps:

Kotlin
Copy to clipboard
1import java.util.TreeMap
2
3fun main() {
4    val sMap = sortedMapOf("a" to 1, "b" to 2, "c" to 3)
5
6    println(sMap)  // Outputs {a=1, b=2, c=3}
7}

Custom classes enable us to create objects that fit our data — for instance, a Person data class for employee information or a Book data class for a library database. In Kotlin, data classes provide a concise and idiomatic approach to creating such objects.

Consider this simple data class, for example:

Kotlin
Copy to clipboard
1data class Person(val name: String, val age: Int)
2
3fun main() {
4    val person = Person(name = "John Doe", age = 30)
5    println(person.name)  // Outputs "John Doe"
6    println(person.age)   // Outputs 30
7}

Using custom classes as map keys helps organize complex multivariate keys in a sorted map. Consider the following example using the Person data class as a key in a sorted map. However, this will not work yet without a comparator.

Kotlin
Copy to clipboard
1import java.util.TreeMap
2
3data class Person(val name: String, val age: Int)
4
5fun main() {
6    val people = TreeMap<Person, String>()
7
8    val john = Person("John", 30)
9    val alice = Person("Alice", 25)
10
11    people[john] = "Programmer"
12    people[alice] = "Designer"
13}

In this code, John is assigned the value "Programmer", and Alice is assigned the value "Designer." However, this code will produce an error because TreeMap requires a way to compare the Person objects to maintain its order.

Kotlin uses a comparator to determine the order of two keys. To make this comparison, we implement the Comparable interface or use a custom Comparator. Without these methods, TreeMap can't compare its Person class keys. Here’s how to modify the Person class to use a comparator:

Kotlin
Copy to clipboard
1import java.util.TreeMap
2
3data class Person(val name: String, val age: Int) : Comparable<Person> {
4    override fun compareTo(other: Person): Int {
5        return compareValuesBy(this, other, { it.age }, { it.name })
6    }
7}
8
9fun main() {
10    val people = TreeMap<Person, String>()
11
12    val john = Person("John", 30)
13    val alice = Person("Alice", 25)
14
15    people[john] = "Programmer"
16    people[alice] = "Designer"
17
18    for ((person, profession) in people) {
19        println("${person.name} is a $profession")
20    }
21    // Output:
22    // Alice is a Designer
23    // John is a Programmer
24}

In the code above, we implement the compareTo method using compareValuesBy to ensure that Person objects are initially sorted by age and, if ages are the same, then by name. The data class automatically provides equals and hashCode implementations, ensuring consistency in collections like TreeMap.

We've explored how to use custom data classes as keys in sorted maps and how comparators work in this context using Kotlin's Comparable interface. Now, prepare for some hands-on exercises to reinforce these concepts.