Set and Get methods can validate attempts to modify private data and control how that data is presented to the caller, respectively. These are compelling software engineering benefits. If an instance variable were public, any client of the class could see the data and modify it, including setting it to an invalid value. Also, public data allows client-code programmers to write code that depends on the class’s data format. If the class’s owner changes that format, any client code dependent on it would “break” and would need to be adjusted to the new format, making it subject to break again.

You might think that even though a client of the class cannot directly access a private instance variable, the client can nevertheless do whatever it wants with the variable through public Set and Get methods. You’d think that you could peek at the private data (and see exactly how it’s stored in the object) any time with the public Get method and that you could modify the private data at will through the public Set method.

Actually, Set methods can be programmed to validate their arguments and reject any attempts to Set the data to bad values, such as

• a negative body temperature

• a day in March outside the range 1 through 31

• a product code not in the company’s product catalog, etc.

A Get method can present the data in a different form, while the actual data representation remains hidden from the user. For example, a Grade class might store a grade instance variable as an int between 0 and 100, but a GetGrade method might return a letter grade as a string, such as "A" for grades between 90 and 100, "B" for grades between 80 and 89, …—we’ll do this in Section 5.7 with a property. Tightly controlling the access to and presentation of private data can greatly reduce errors, while increasing the robustness, security and usability of your programs.

Conceptual View of an Account Object with private Data

You can think of an Account object as shown in Fig. 4.4. The private instance variable name is hidden inside the object (represented by the inner circle containing name) and guarded by an outer layer of public methods (represented by the outer circle containing GetName and SetName). Any client code that needs to interact with the Account object can do so only by calling the public methods of the protective outer layer.

Software Engineering Observation 4.1

Generally, instance variables should be private and methods public.

 

Software Engineering Observation 4.2

Change is the rule rather than the exception. You should anticipate that your code will be modified, and possibly often. Using public Set and Get methods to control access to private data makes programs clearer and easier to maintain.