Objects and classes

In ActionScript 3.0, every object is defined by a class. A class can be thought of as a template or a blueprint for a type of object. Class definitions can include variables and constants, which hold data values, and methods, which are functions that encapsulate behavior bound to the class. The values stored in properties can be primitive values or other objects. Primitive values are numbers, strings, or Boolean values.

ActionScript contains a number of built-in classes that are part of the core language. Some of these built-in classes, such as Number, Boolean and String, represent the primitive values available in ActionScript. Others, such as the Array, Math, and XML classes, define more complex objects.

All classes, whether built-in or user-defined, derive from the Object class. For programmers with previous ActionScript experience, it is important to note that the Object data type is no longer the default data type, even though all other classes still derive from it. In ActionScript 2.0, the following two lines of code were equivalent because the lack of a type annotation meant that a variable would be of type Object:

var someObj:Object; 
var someObj;

ActionScript 3.0, however, introduces the concept of untyped variables, which can be designated in the following two ways:

var someObj:*; 
var someObj;

An untyped variable is not the same as a variable of type Object. The key difference is that untyped variables can hold the special value undefined, while a variable of type Object cannot hold that value.

You can define your own classes using the class keyword. You can declare class properties in three ways: constants can be defined with the const keyword, variables are defined with the var keyword, and getter and setter properties are defined by using the get and set attributes in a method declaration. You can declare methods with the function keyword.

You create an instance of a class by using the new operator. The following example creates an instance of the Date class called myBirthday.

var myBirthday:Date = new Date();