Kotlin Data Classes

Data Class Features

data class User(val name: String)
// Automatically gets comparison methods

val user2 = user1.copy(name = "New Name")

val (name, age) = person

data class Point(val x: Int, val y: Int)

🔹 Basic Data Class

Here's how to create and use a simple data class:

data class Student(
    val name: String,
    val age: Int,
    val grade: String
)

fun main() {
    val student1 = Student("Alice", 20, "A")
    val student2 = Student("Alice", 20, "A")
    val student3 = Student("Bob", 21, "B")
    
    // Automatic toString()
    println(student1)
    
    // Automatic equals()
    println("student1 == student2: ${student1 == student2}")
    println("student1 == student3: ${student1 == student3}")
    
    // Automatic hashCode()
    println("student1 hashCode: ${student1.hashCode()}")
    println("student2 hashCode: ${student2.hashCode()}")
}

🔹 Copy Function

Create modified copies of data class instances:

data class Product(
    val name: String,
    val price: Double,
    val category: String,
    val inStock: Boolean = true
)

fun main() {
    val laptop = Product("Gaming Laptop", 1299.99, "Electronics")
    
    println("Original: $laptop")
    
    // Copy with price change
    val discountedLaptop = laptop.copy(price = 999.99)
    println("Discounted: $discountedLaptop")
    
    // Copy with multiple changes
    val outOfStockLaptop = laptop.copy(
        price = 1199.99,
        inStock = false
    )
    println("Out of stock: $outOfStockLaptop")
    
    // Original remains unchanged
    println("Original unchanged: $laptop")
}

🔹 Destructuring Declarations

Extract properties from data classes easily:

data class Coordinate(val x: Int, val y: Int, val z: Int)

data class Person(val firstName: String, val lastName: String, val age: Int)

fun main() {
    val point = Coordinate(10, 20, 30)
    
    // Destructuring assignment
    val (x, y, z) = point
    println("Point coordinates: x=$x, y=$y, z=$z")
    
    // Partial destructuring
    val (xOnly, yOnly) = point
    println("Only x and y: x=$xOnly, y=$yOnly")
    
    // Using underscore to skip values
    val (_, _, zOnly) = point
    println("Only z: z=$zOnly")
    
    // Destructuring in function parameters
    fun printPersonInfo(person: Person) {
        val (first, last, age) = person
        println("Name: $first $last, Age: $age")
    }
    
    val person = Person("John", "Doe", 25)
    printPersonInfo(person)
}

🔹 Data Classes in Collections

Data classes work great with collections due to proper equals() and hashCode():

data class Book(val title: String, val author: String, val year: Int)

fun main() {
    val books = listOf(
        Book("1984", "George Orwell", 1949),
        Book("To Kill a Mockingbird", "Harper Lee", 1960),
        Book("1984", "George Orwell", 1949), // Duplicate
        Book("The Great Gatsby", "F. Scott Fitzgerald", 1925)
    )
    
    println("All books:")
    books.forEach { println("  $it") }
    
    // Remove duplicates using Set
    val uniqueBooks = books.toSet()
    println("\nUnique books:")
    uniqueBooks.forEach { println("  $it") }
    
    // Find books by author
    val orwellBooks = books.filter { it.author == "George Orwell" }
    println("\nBooks by George Orwell:")
    orwellBooks.forEach { println("  $it") }
    
    // Group by decade
    val booksByDecade = books.groupBy { (it.year / 10) * 10 }
    println("\nBooks by decade:")
    booksByDecade.forEach { (decade, bookList) ->
        println("  ${decade}s: ${bookList.size} books")
    }
}

🔹 Data Class Requirements

Understanding data class constraints and best practices:

// ✅ Valid data class
data class ValidUser(val name: String, val email: String)

// ✅ Data class with default values
data class UserProfile(
    val username: String,
    val isActive: Boolean = true,
    val joinDate: String = "2024-01-01"
)

// ✅ Data class with additional properties
data class Employee(val id: Int, val name: String) {
    var department: String = "Unassigned" // Not part of equals/hashCode
    
    fun getDisplayName() = "Employee #$id: $name"
}

fun main() {
    val user1 = ValidUser("Alice", "[email protected]")
    val user2 = ValidUser("Alice", "[email protected]")
    
    println("Users equal: ${user1 == user2}")
    
    val profile = UserProfile("john_doe")
    println("Profile: $profile")
    
    val emp1 = Employee(1, "John")
    val emp2 = Employee(1, "John")
    emp1.department = "IT"
    emp2.department = "HR"
    
    // Still equal because department is not in primary constructor
    println("Employees equal: ${emp1 == emp2}")
    println("Employee 1: ${emp1.getDisplayName()}, Dept: ${emp1.department}")
    println("Employee 2: ${emp2.getDisplayName()}, Dept: ${emp2.department}")
}