Accessors

In most, if not all, examples in this book, the attributes are defined as private so that a second object cannot access another object's attributes. It would be ridiculous to create an object in isolation ”we want to share the appropriate information with other objects. Isn't it necessary to inspect and sometimes change another class's attribute? The answer is yes, of course. There are times when an object needs to access another object's attributes; however, it does not need to do it directly.

A class should be very protective about its attributes. For example, you do not want object A to have the capability to inspect or change the attributes of object B without object B having control. There are several reasons for this; the most important reasons really boil down to data integrity and efficient debugging.

Assume that there is a bug in the Cab class. You have tracked the problem to the Name attribute. Somehow it is getting overwritten, and garbage is turning up in some name queries. If Name were public and any class could change it, you would have to go searching through all the possible code, trying to find places that reference and change Name . However, if you let only a Cabbie object change Name , you'd only have to look in the Cabbie class. This access is provided by a type of method called an accessor . Sometimes accessors are referred to as getters and setters, and sometimes they're simply called get() and set() . By convention, in this book we'll name the methods with the set and get prefixes, as in the following:

 // Set the Name of the Cabbie public void setName(String iName) {     name = iName; } // Get the Name of the Cabbie public String getName() {     return name; } 

In this snippet, a Supervisor object must ask the Cabbie object to return its name (see Figure 4.4). The important point here is that the Supervisor object can't simply retrieve the information on its own, it must ask the Cabbie object for the information. This concept is important at many levels. For example, you might have a setAge() method that checks to see whether the age entered was 0 or below. If the age is less than 0, the setAge() method can refuse to set this incorrect value. In general, the setters are used to ensure a level of data integrity.

Figure 4.4. Asking for information.

Notice that the getCompanyName method is declared as static , as a class method; class methods are described in more detail in Chapter 3. Remember that the attribute companyName is also declared as static . A method, like an attribute, can be declared static to indicate that there is only one copy of the method for the entire class.

Objects

Actually, there isn't a physical copy of each nonstatic method for each object. Each object would point to the same physical code. However, from a conceptual level, you can think of objects as being wholly independent and having their own attributes and methods.

The following code fragment illustrates how to define a static method, and Figure 4.5 shows how more than one object points to the same code.

Figure 4.5. Method memory allocation.

Static Attributes

If an attribute is static, and the class provides a setter for that attribute, any object that invokes the setter will change the single copy. Thus, the value for the attribute will change for all objects.

 // Get the Name of the Cabbie public static String getCompanyName() {     return companyName; }