Classes, Data Classes and Objects

Primary Constructors

class Book(val title: String, var price: Int) {
    init { require(price >= 0) { "price must be positive" } }
}

Data Classes

data class User(val name: String, val email: String)
val u1 = User("Ravi", "ravi@test.com")
val u2 = u1.copy(email = "r@test.com")  // changed copy
println(u1)  // auto toString, equals, hashCode

object: Singletons and Companions

object Config { const val BASE_URL = "https://groovygrails.in" }
class Order {
    companion object {
        fun guestCheckout() = Order()
    }
}

Use object for singletons and companion object for factory methods and constants - the clean static replacement.

Properties, Getters, Lateinit

• Custom accessors: val slug get() = title.slug().
• lateinit var for injected fields set after construction.
• by lazy for expensive values computed once on first use.

Key Points

• Constructors are concise; init blocks validate.
• data class ends POJO boilerplate forever.
• object plus companion replace statics cleanly.