| Hard Prerequisites |
| IMPORTANT: Please review these prerequisites, they include important information that will help you with this content. |
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Classes in Kotlin are declared using the keyword class:
class Invoice {
}
A class in Kotlin can have a primary constructor and one or more secondary constructors.
class Person constructor(firstName: String) {
}
The class can also declare secondary constructors, which are prefixed with constructor:
class Person {
constructor(parent: Person) {
parent.children.add(this)
}
}
To create an instance of a class, we call the constructor as if it were a regular function:
val invoice = Invoice()
val customer = Customer("Joe Smith")
Note: that Kotlin does not have a new keyword.
All classes in Kotlin have a common superclass Any, that is a default super for a class with no supertypes declared:
class Example // Implicitly inherits from Any
Any is not java.lang.Object; in particular, it does not have any members other than equals(), hashCode() and toString().
To declare an explicit supertype, we place the type after a colon in the class header:
open class Base(p: Int)
class Derived(p: Int) : Base(p)
Kotlin requires explicit annotations for overridable members (we call them open) and for overrides:
open class Base {
open fun v() {}
fun nv() {}
}
class Derived() : Base() {
override fun v() {}
}
The override annotation is required for Derived.v(). If it were missing, the compiler would complain.
A class and some of its members may be declared abstract. An abstract member does not have an implementation in its class.
We can override a non-abstract open member with an abstract one
open class Base {
open fun f() {}
}
abstract class Derived : Base() {
override abstract fun f()
}