TopolearnClasses model your domain; Kotlin removes the getter/setter/constructor boilerplate of Java.
A class with a primary constructor declares its properties in one line:
class Account(val owner: String, private var balance: Double = 0.0)val/var in the constructor create properties; without them, parameters are only visible in initializers. Create instances without new: val acc = Account("Ada").
init { ... }blocks run during construction, in order with property initializers; use them for validation (require(balance >= 0)).- Secondary constructors (
constructor(...) : this(...)) are rarely needed thanks to default arguments. - Properties have generated getters/setters; you access them like fields (
acc.owner). Customize with accessors:val isEmpty get() = size == 0, orvar name = "" set(value) { field = value.trim() }, wherefieldis the backing field. private setlets everyone read but only the class write.
Visibility: public (default), private, protected, and internal (visible within the same module). Classes are final by default: you must mark them open to allow inheritance.
Methods are functions inside the class and can access properties directly. toString, equals and hashCode come from Any and compare identity unless overridden; data classes (next lesson) generate them.
There is no static; use top-level functions or a companion object for class-level members.
Example
class Account(val owner: String, balance: Double = 0.0) {
var balance = balance
private set // readable outside, writable only inside
val isEmpty: Boolean
get() = balance == 0.0 // computed property
init {
require(owner.isNotBlank()) { "owner required" }
}
fun deposit(amount: Double) {
require(amount > 0) { "amount must be positive" }
balance += amount
}
}
fun main() {
val acc = Account("Ada")
println(acc.isEmpty) // true
acc.deposit(50.0)
println("${acc.owner}: ${acc.balance}") // Ada: 50.0
// acc.balance = 1.0 // error: private setter
runCatching { Account(" ") }.onFailure { println(it.message) } // owner required
}Remember
- Primary constructor val/var params become properties
- init blocks validate; require() throws IllegalArgumentException
- Custom get()/set() with field for the backing field
- Classes are final unless marked open