Classes and properties

A Kotlin class groups state and behaviour. class Person(val name: String) declares a class whose primary constructor property name is a read-only val on every instance.

Properties declared with val expose a getter only. Properties declared with var expose a getter and a setter. You can also declare properties in the class body: var age: Int = 0.

Methods are ordinary fun members inside the class. Call them on an instance: p.greet(). Inside a method, this refers to the current instance; you may omit this. when the name is unambiguous.

Create an instance with the constructor call syntax — no new keyword: val p = Person("Ada"). Each call allocates a distinct object unless you use an object singleton (covered later).

Misses: writing new Person(...) (Java), putting types before names (String name), or treating a class name as a value without constructing an instance.

class Person(val name: String) {
    var age: Int = 0
    fun greet(): String = "hello, $name"
}

fun main() {
    val p = Person("Ada")
    p.age = 36
    println(p.greet())
    println(p.age)
}