Introducing Java - Beginning Java - page 17

You might have a parameter owner , which would record the owner's name, so the value stored as the at-
tribute owner could be changed when the hat was sold or otherwise transferred to someone else. You might
also have a parameter hatOn , for example, which would indicate whether the hat was on or off the owner's
head; the value true would indicate that the owner was indeed wearing the hat, whereas the value false
would mean that the hat had been removed and was just lying about somewhere.
Operating on Objects
In spite of what you might think from looking at Figure 1-5 , a class object is not just a collection of various
items of data. In addition to the parameters that characterize an object, a class specifies what you can do with
an object — that is, it defines the operations that are possible on objects of the class. Clearly, for objects to
be of any use in a program, you need to decide what you can do with them. The operations that you specify
for objects of a given type depend on what sort of objects you are talking about, the attributes they contain,
and how you intend to use them.
For the CowboyHat class in Figure 1-5 , you may want to have operations that you could refer to as
putHatOn and takeHatOff , which would have meanings that are fairly obvious from their names, and do
make sense for CowboyHat objects. These operations on a particular CowboyHat object would set the value
of hatOn for the object. To determine whether your CowboyHat was on or off, you would just need to look
at this value. Conceivably, you might also have an operation changeOwner by which you could set the in-
stance variable recording the current owner's name to a new value. Figure 1-6 shows two operations applied
in succession to a CowboyHat object.
FIGURE 1-6