0.2 Encapsulation - the big picture  (Page 2/12)

Supplemental material

I recommend that you also study the other lessons in my extensive collection of online programming tutorials. You will find a consolidated index at www.DickBaldwin.com .

General background information

In addition to the three explicit characteristics of encapsulation , inheritance , and polymorphism , an object-oriented program also has an implicit characteristic of abstraction .

What is abstraction?

Abstraction is the process by which we specify a new data type, often referred to an abstract data type or ADT.

How does abstraction relate to encapsulation?

Encapsulation is the process of gathering an ADT's data representation and behavior into one encapsulated entity. In other words, encapsulation converts from the abstract to the concrete.

Some analogies

You might think of this as being similar to converting an idea for an invention into a set of blueprints from which it can be built, or converting aset of written specifications for a widget into a set of drawings that can be used by the machine shop to build the widget.

Automotive engineers encapsulated the specifications for the steering mechanism of my car into a set of manufacturing drawings. Thenmanufacturing personnel used those drawings to produce an object where they exposed the interface (steering wheel) and hid the implementation (levers, bolts, etc.) .

In all likelihood, the steering mechanism object contains a number of other more-specialized embedded objects, each of which has state and behavior and also has an interface and an implementation .

The interfaces for those embedded objects aren't exposed to me, but they are exposed to the other parts of the steering mechanism that use them.

Abstraction

Abstraction is the specification of an abstract data type, which includes a specification of the type's data representation and its behavior . In particular,

• What kind of data can be stored in an entity of the new type, and
• What are all the ways that the data can be manipulated?

A new type

For our purposes, an abstract data type is a new type (not intrinsic to the ActionScript language) . It is not one of the primitive data types that are built into the programming language (such as Boolean, int, Number, String, and uint) .

Already known to the compiler

The distinction in the previous paragraph is very important. The data representation and behavior of the intrinsic or primitive types is already knownto the compiler and cannot normally be modified by the programmer.

Not known to the compiler

The representation and behavior of an abstract type is not known to the compiler until it is defined by the programmer and presented to the compiler inan appropriate manner.

Define data representation and behavior in a class

ActionScript programmers define the data representation and the behavior of a new type (present the specification to the compiler) using the keyword class . In other words, the keyword class is used to convert the specification of a new type into something that the compiler can work with; a set of plans as it were. To define a class is to go from the abstract to the concrete.