1.5. Classes and Objects

Imagine you are going to build an airplane, entirely from scratch. Think about the process you would follow. You very likely wouldn't just head to a metal shop and start welding. You'd have to draw up a blueprint for the airplane first. In fact, given that you are building the airplane from scratch, you'd have to draw up not just one, but many blueprints—one for each of the airplane's many parts (the wheels, the wings, the seats, the brakes, and so on). Each blueprint would describe a specific part conceptually and correspond to an actual part in the physical incarnation of the airplane. To build the airplane, you would manufacture each of the parts individually, and then assemble them according to a master blueprint. The interoperation of the airplane's assembled parts would produce the airplane's behavior.

If that all sounds logical to you, you've got what it takes to become an ActionScript programmer. Just as an airplane flying through the sky is a group of interoperating parts based on a set of blueprints, a running ActionScript program is a group of interoperating objects, based on a set of classes. ActionScript objects represent both the tangible things and the intangible concepts in a program. For example, an object might represent a number in a calculation, a clickable button in a user interface, a point in time on a calendar, or a blur effect on an image. Objects are incarnations, or instances, of classes. Classes are the blueprints upon which objects are based.