0.2 Encapsulation - the big picture  (Page 4/12)

The user of the class (the using programmer) should only need to care that it works as advertised. Of course this assumes that the user of the classhas access to good documentation describing the interface and the behavior of objects instantiated from the class.

Should be able to change the implementation later

For a properly designed class, the class designer should be able to come back later and change the implementation, perhaps changing the type of data structureused to store data in the object, and the using programs should not be affected by the change.

Class member access control

Object-oriented programming languages usually provide the ability to control access to the members of a class. For example, ActionScript, C++ and Java alluse the keywords public , private , and protected to control access to the individual members of a class. In addition, ActionScript andJava add a fourth level of access control, which is called internal in ActionScript and is called package-private in Java. (See Class property attributes in a companion document on ActionScript Resources.)

Public, private, and protected

To a first approximation, you can probably guess what public and private mean. Public members are accessible by all code that has access to an object of the class. Private members are accessible only by membersbelonging to the class.

The protected keyword is used to provide inherited classes with special access to the members of their base classes.

A public user interface

In general, the user interface for a class consists of the public methods. (The variables in a class can also be declared public but this is generally considered to be bad programming practice unless they are actuallyconstants. )

For a properly designed class, the class user stores, reads, and modifies values in the object's data by calling the public methods on a specific instance (object) of the class. (This is sometimes referred to as sending a message to the object asking it to change its state) .

ActionScript has a special form of method, often called an implicit setter method or an implicit getter method that is specifically used for this purpose. (You will see several implicit setter methods in the program that I will explain later in this lesson.)

Normally, if the class is properly designed and the implementation is hidden, the user cannot modify the values contained in the instance variables of theobject without going through the prescribed public methods in the interface.

Not a good design by default

An object-oriented design is not a good design by default. In an attempt to produce good designs, experienced object-oriented programmers generally agree oncertain design standards for classes. For example, the data members (instance variables) are usually private unless they are constants. The user interface usually consists only of public methods and includes few if any data members.

Of course, there are exceptions to every rule. One exception to this general rule is that data members that are intended to be used as symbolicconstants are made public and defined in such a way that their values cannot be modified.