Classes: Inheritance
Ara supports single inheritance between classes, which means that a class can only inherit from one other class.
Extending a class
A class can inherit from another class using the extends keyword.
class Base
{
}
class Derived extends Base
{
}
Overriding methods
A method in a child class can override a method in a parent class by defining a method with the same name.
class Counter
{
protected int $value = 0;
public function increment(): void
{
$this->value++;
}
public function getValue(): int
{
return $this->value;
}
}
class OneOffCounter extends Counter
{
public function getValue(): int
{
return $this->value + 1;
}
}
Ara does not support method overloading, which means that a method overriding another method must have a compatible signature.
class Counter
{
protected int $value = 0;
public function increment(): void
{
$this->value++;
}
public function getValue(): int
{
return $this->value;
}
}
class OffCounter extends Counter
{
public function getValue(int $how_much = 1): int
{
return $this->value + $how_much;
}
}
INFO
This restriction is also applied to constructors.
Calling the parent method
A child class can call the parent method using the parent keyword.
class Counter
{
protected int $value = 0;
public function increment(): void
{
$this->value++;
}
public function getValue(): int
{
return $this->value;
}
}
class OneOffCounter extends Counter
{
public function getValue(): int
{
return parent::getValue() + 1;
}
}
This also works for static methods.
class Counter
{
protected static int $value = 0;
public static function getValue(): int
{
return self::$value;
}
}
class OneOffCounter extends Counter
{
public static function getValue(): int
{
return parent::getValue() + 1;
}
}
Abstract Classes
A class can be declared as abstract, which means that it cannot be instantiated.
abstract class Counter
{
protected int $value = 0;
}
An abstract class can contain abstract methods, which means that they must be implemented by a child class.
abstract class Counter
{
protected int $value = 0;
abstract public function increment(): void;
}
An abstract class can also contain concrete methods.
abstract class Counter
{
protected int $value = 0;
public function increment(): void
{
$this->value++;
}
}
WARNING
An abstract method cannot be declared on a non-abstract class.
class Counter
{
public abstract function getValue(): int;
}
The code above will result in the following error:
error[P0040]: cannot declare abstract method `Counter::getValue` on a non-abstract class
┌─ examples/abstract-method-non-abstract-class:2:3
│
1 │ class Counter {
│ -------
2 │ public abstract function getValue(): int;
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot declare abstract method `Counter::getValue` on a non-abstract class
│
= note: abstract methods can only be declared on abstract classes.
= help: try removing the abstract modifier.
Final Classes
A class can be declared as final, which means that it cannot be extended.
final class Counter
{
}
WARNING
A final class cannot be declared as abstract.
abstract final class Counter {}
The code above will result in the following error:
error[P0025]: final class cannot be abstract
┌─ examples/abstract-final-class.ara:1:1
│
1 │ abstract final class Counter {}
│ ^^^^^^^^ -----
│ │
│ final class cannot be abstract
│
= note: a final class cannot be abstract because it cannot be extended by other classes.
= help: try removing the `abstract` modifier.
Final Methods
A method can be declared as final, which means that it cannot be overridden.
class Counter
{
protected int $value = 0;
final public function increment(): void
{
$this->value++;
}
}
WARNING
A final method cannot be declared as abstract.
abstract class Counter
{
final abstract public function increment(): void;
}
The code above will result in the following error:
error[P0026]: final class member cannot be abstract
┌─ examples/final-abstract-method.ara:2:11
│
2 │ final abstract public function increment(): void;
│ ----- ^^^^^^^^ final class member cannot be abstract
│
= note: a final class member cannot be abstract because it cannot be overridden by other classes.
= help: try removing the `abstract` modifier.