Writing Classes - Java Software Solutions: Foundations of Program Design - page 169

SR 4.8 What happens when you pass an object to a print or println
method?
SR 4.9 What is the scope of a variable?
SR 4.10 What are UML diagrams designed to do?

4.3 Encapsulation

We mentioned in our overview of object-oriented concepts in Chapter 1 that an
object should be self-governing. That is, the instance data of an object should be
modified only by that object. For example, the methods of the Die class should
be solely responsible for changing the value of the faceValue variable. We should
make it difficult, if not impossible, for code outside of a class to “reach in” and
change the value of a variable that is declared inside that class. This characteristic
is called encapsulation.
An object should be encapsulated from the rest of the system.
It should interact with other parts of a program only through the
specific set of methods that define the services that that object pro-
vides. These methods define the interface between that object and the
program that uses it.
Encapsulation is depicted graphically in Figure 4.5. The code that uses an
object, sometimes called the client of an object, should not be allowed to access
variables directly. The client should call an object's methods, and those methods
then interact with the data encapsulated within the object. For example, the main
method in the RollingDice program calls the roll method of the Die objects.
The main method should not (and in fact cannot) access the faceValue variable
directly.

KEY CONCEPT
An object should be encapsulated,
guarding its data from inappropriate
access.

Object

Interface

Client

Methods

Data

FIGURE 4.5 A client interacting with the methods of an object