User Guide

154 Chapter 5: ActionScript Core Language Elements
The following example shows a second AS file, called Car.as, in the same directory. This class
extends the Vehicle class, modifying it in three ways. First, the Car class adds a variable
fullSizeSpare to track whether the car object has a full-size spare tire. Second, it adds a new
method specific to cars,
activateCarAlarm(), that activates the car’s anti-theft alarm. Third, it
overrides the
stop() function to add the fact that the Car class uses an anti-lock braking system
to stop.
class Car extends Vehicle {
var fullSizeSpare:Boolean;
function Car(param_numDoors:Number, param_color:String,
param_fullSizeSpare:Boolean) {
this.numDoors = param_numDoors;
this.color = param_color;
this.fullSizeSpare = param_fullSizeSpare;
}
function activateCarAlarm():Void {
trace("[Car] activateCarAlarm");
}
function stop():Void {
trace("[Car] stop with anti-lock brakes");
}
}
The following example instantiates a Car object, calls a method defined in the Vehicle class
(
start()), then calls the method overridden by the Car class (stop()), and finally calls a method
from the Car class (
activateCarAlarm()):
var myNewCar:Car = new Car(2, "Red", true);
myNewCar.start(); // output: [Vehicle] start
myNewCar.stop(); // output: [Car] stop with anti-lock brakes
myNewCar.activateCarAlarm(); // output: [Car] activateCarAlarm
A subclass of the Vehicle class can also be written using the keyword super, which the subclass
can use to access properties and methods of the superclass. The following example shows a third
AS file, called Truck.as, again in the same directory. The Truck class uses the
super keyword in
the constructor and again in the overridden
reverse() function.
class Truck extends Vehicle {
var numWheels:Number;
function Truck(param_numDoors:Number, param_color:String,
param_numWheels:Number) {
super(param_numDoors, param_color);
this.numWheels = param_numWheels;
}
function reverse():Void {
beep();
super.reverse();
}
function beep():Void {
trace("[Truck] make beeping sound");
}
}