컬렉션과 맵 키로 사용하는 데이터 클래스

class Key(val id: Int)  // no equals/hashCode
val map = mutableMapOf<Key, String>()
val k1 = Key(1)
val k2 = Key(1)  // same id, different object
map[k1] = "value"
println(map[k2])  // null! k2 != k1 by reference

data class Key(val id: Int)
val map = mutableMapOf<Key, String>()
val k1 = Key(1)
val k2 = Key(1)
map[k1] = "value"
println(map[k2])  // "value"! Equal by value

data class Tag(val name: String)
val tags = mutableSetOf<Tag>()
tags.add(Tag("kotlin"))
tags.add(Tag("kotlin"))  // duplicate
tags.add(Tag("java"))
println(tags.size)  // 2 (not 3)

data class Category(val name: String, val parent: String)
val items = listOf(
    "List" to Category("Collections", "stdlib"),
    "Map" to Category("Collections", "stdlib"),
    "Flow" to Category("Coroutines", "kotlinx")
)
val grouped = items.groupBy { it.second }
println(grouped.keys.size)  // 2

data class Pair2(val x: Int, val y: Int)
val points = listOf(Pair2(1,2), Pair2(1,2), Pair2(3,4))
val counts = points.groupingBy { it }.eachCount()
println(counts)  // {Pair2(x=1, y=2)=2, Pair2(x=3, y=4)=1}

data class Address(val city: String, val country: String)
data class Person(val name: String, val address: Address)
val p1 = Person("Alice", Address("Berlin", "DE"))
val p2 = Person("Alice", Address("Berlin", "DE"))
println(p1 == p2)  // true

data class Student(val name: String, val grade: Double)
val students = listOf(
    Student("Bob", 3.2),
    Student("Alice", 3.8),
    Student("Charlie", 2.9)
)
println(students.sortedBy { it.grade })

data class CacheKey(val userId: Long, val resourceType: String)
val cache = HashMap<CacheKey, ByteArray>()
val key = CacheKey(42, "avatar")
cache[key] = byteArrayOf(1, 2, 3)
println(cache[CacheKey(42, "avatar")] != null)  // true

data class Event(val type: String, val ts: Long)
val events = listOf(
    Event("click", 1000),
    Event("click", 1000),  // duplicate
    Event("scroll", 2000)
)
println(events.distinct().size)  // 2

data class ImmutableKey(val id: Int)  // val properties only
// DON'T: data class MutableKey(var id: Int)  // changing id breaks map!