Inheritance

Class inheritance enables you to create a new class that derives from an existing class. The derived class is called a descendant; the class it derives from is called the ancestor or base class. A descendant inherits all fields and methods from the ancestor and can add new fields and methods. Also, the descendant can change the behavior of the ancestor by overriding one or more methods.

To derive from a class, specify its name within parentheses after the name of the class. Example:

class Base {
  int x
  int func getXPlusOne()
    return x + 1
  endfunc
}

class Derived(Base) {
  int y
  int func getXPlusY()
    return x + y
  endfunc
}

The Derived class contains both the members from itself, and the members from Base. Let's test this:

Derived d = new Derived
d.x = 2
d.y = 3
print(d.getXPlusOne())  ; Prints 3
print(d.getXPlusY())    ; Prints 5

You can assign a derived object to a variable that has the type of the ancestor class, because a derived object is always compatible with its ancestor:

Derived d = new Derived
d.x = 4
d.y = 5
Base b = d
print(b.x)  ; Prints 4
print(b.y)  ; Compilation error!

• Use inheritance sparingly. Inheritance is only suitable if the descendant class has an "is-a" relationship with the ancestor. For example, you should never derive a Rect class (representing a rectangle) from a Point class, because a rectangle is not a point. You could however derive a Car class from a Vehicle ancestor, because a car is a type of vehicle. If there is not an "is-a" relationship, use the other class as a field. For example, a Rect class could have two fields of type Point: leftTop and bottomRight.
• In Ultra Fractal, inheritance is especially useful in combination with plug-in parameters.
• You can include a file name with the ancestor if it is declared in another file. See Importing classes.
• The chain of classes that is built with inheritance is called a class hierarchy.
• If no ancestor is specified, the class descends from the internal Object base class. This means that all classes ultimately have Object as a common ancestor. See also Casting.
• See Overriding for more information about overriding methods and changing the behavior of a descendant class.

Next: Fields

See Also
Classes
Casting