A Kotlin class groups an object’s data and behavior. For simple data holders, a primary constructor can declare properties directly, eliminating the field assignment, constructor body, and getter boilerplate familiar from Java. section 2.2 section 2.6
A compact class
class Person(val name: String) Person has a primary constructor parameter, name, and val makes that parameter a read-only property. Public visibility is the default, so no public modifier is required. section 2.2
The primary constructor is the parenthesized part after the class name. It accepts constructor parameters and can create properties from them. When initialization needs more than a direct assignment, place that code in one or more init blocks; they run when an instance is created. section 4.2.1
Properties, defaults, and construction
class User( val nickname: String, val isSubscribed: Boolean = true ) val alice = User("Alice") val bob = User("Bob", false) val carol = User("Carol", isSubscribed = false) Default values make arguments optional, and named arguments make a call explicit without requiring positional order. section 4.2.1
A class name and a type are related but not identical. A nongeneric class name can name a type, while nullable forms create another type, such as String?. Generic classes require type arguments: List is a class, whereas List<Int> and List<String?> are types. Subtype rules determine which values can be assigned or passed as arguments. section 11.3.2 section 11.3.2
class User constructor(_nickname: String) { #1
val nickname: String
init { #2
nickname = _nickname
}
}
Compare the explicit constructor, property, and init block form with the compact primary-constructor form to see exactly what the shorthand replaces. section 4.2.1
