Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 331

stance variable of a class BankAccount ) by a method computeInterest . Set methods are
also commonly called mutator methods , because they typically change an object's state—
i.e., modify the values of instance variables. Get methods are also commonly called accessor
methods or query methods .
Set and Get Methods vs. public Data
It would seem that providing set and get capabilities is essentially the same as making a
class's instance variables public . This is one of the subtleties that makes Java so desirable
for software engineering. A public instance variable can be read or written by any method
that has a reference to an object containing that variable. If an instance variable is declared
private , a public get method certainly allows other methods to access it, but the get meth-
od can control how the client can access it. For example, a get method might control the
format of the data it returns, shielding the client code from the actual data representation.
A public set method can—and should—carefully scrutinize attempts to modify the vari-
able's value and throw an exception if necessary. For example, attempts to set the day of
the month to 37 or a person's weight to a negative value should be rejected. Thus, al-
though set and get methods provide access to private data, the access is restricted by the
implementation of the methods. This helps promote good software engineering.
Software Engineering Observation 8.6
Classes should never have public nonconstant data, but declaring data public static
final enables you to make constants available to clients of your class. For example, class
Math offers public static final constants Math.E and Math.PI .
Error-Prevention Tip 8.3
Do not provide public static final constants if the constants' values are likely to change
in future versions of your software.
Validity Checking in Set Methods
The benefits of data integrity do not follow automatically simply because instance vari-
ables are declared private —you must provide validity checking. A class's set methods
could determine that attempts were made to assign invalid data to objects of the class. Typ-
ically set methods have void return type and use exception handling to indicate attempts
to assign invalid data. We discuss exception handling in detail in Chapter 11.
Software Engineering Observation 8.7
When appropriate, provide public methods to change and retrieve the values of private
instance variables. This architecture helps hide the implementation of a class from its cli-
ents, which improves program modifiability.
Error-Prevention Tip 8.4
Using set and get methods helps you create classes that are easier to debug and maintain.
If only one method performs a particular task, such as setting an instance variable in an
object, it's easier to debug and maintain the class. If the instance variable is not being set
properly, the code that actually modifies instance variable is localized to one set method.
Your debugging efforts can be focused on that one method.