All classes in Kotlin have a common superclass,
Any, which is the default superclass for a class with no supertypes declared:
Any has three methods:
toString(). Thus, these methods are defined for all Kotlin classes.
By default, Kotlin classes are final – they can’t be inherited. To make a class inheritable, mark it with the
To declare an explicit supertype, place the type after a colon in the class header:
If the derived class has a primary constructor, the base class can (and must) be initialized in that primary constructor according to its parameters.
If the derived class has no primary constructor, then each secondary constructor has to initialize the base type using the
super keyword or it has to delegate to another constructor which does. Note that in this case different secondary constructors can call different constructors of the base type:
Kotlin requires explicit modifiers for overridable members and overrides:
override modifier is required for
Circle.draw(). If it were missing, the compiler would complain. If there is no
open modifier on a function, like
Shape.fill(), declaring a method with the same signature in a subclass is not allowed, either with
override or without it. The
open modifier has no effect when added to members of a final class – a class without an
A member marked
override is itself open, so it may be overridden in subclasses. If you want to prohibit re-overriding, use
The overriding mechanism works on properties in the same way that it does on methods. Properties declared on a superclass that are then redeclared on a derived class must be prefaced with
override, and they must have a compatible type. Each declared property can be overridden by a property with an initializer or by a property with a
You can also override a
val property with a
var property, but not vice versa. This is allowed because a
val property essentially declares a
get method, and overriding it as a
var additionally declares a
set method in the derived class.
Note that you can use the
override keyword as part of the property declaration in a primary constructor:
Derived class initialization order
During the construction of a new instance of a derived class, the base class initialization is done as the first step (preceded only by evaluation of the arguments for the base class constructor), which means that it happens before the initialization logic of the derived class is run.
This means that when the base class constructor is executed, the properties declared or overridden in the derived class have not yet been initialized. Using any of those properties in the base class initialization logic (either directly or indirectly through another overridden
open member implementation) may lead to incorrect behavior or a runtime failure. When designing a base class, you should therefore avoid using
open members in the constructors, property initializers, or
Calling the superclass implementation
Code in a derived class can call its superclass functions and property accessor implementations using the
Inside an inner class, accessing the superclass of the outer class is done using the
super keyword qualified with the outer class name:
In Kotlin, implementation inheritance is regulated by the following rule: if a class inherits multiple implementations of the same member from its immediate superclasses, it must override this member and provide its own implementation (perhaps, using one of the inherited ones).
To denote the supertype from which the inherited implementation is taken, use
super qualified by the supertype name in angle brackets, such as
It's fine to inherit from both
Polygon, but both of them have their implementations of
draw(), so you need to override
Square and provide a separate implementation for it to eliminate the ambiguity.