Defining Classes - Beginning Java - page 219

Sphere ball; // Declare a variable
no constructor is called because no object is created. All you have created at this point is the variable ball ,
which can store a reference to an object of type Sphere , if and when you create one. Figure 5-5 shows this.
FIGURE 5-5
Recall from the discussion of String objects and arrays that the variable and the object it references are
distinct entities. To create an object of a class you must use the keyword new followed by a call to a con-
structor. To initialize ball with a reference to an object, you could write:
ball = new Sphere(10.0, 1.0, 1.0, 1.0); // Create a sphere
Now you have a Sphere object with a radius of 10.0 located at the coordinates (1.0, 1.0, 1.0). The object
is created in memory and occupies a sufficient number of bytes to accommodate all the data necessary to
define the object. The variable ball records where in memory the object is — it acts as a reference to the
object. This is illustrated in Figure 5-5 .
Of course, you can do the whole thing in one step with the following statement:
Sphere ball = new Sphere(10.0, 1.0, 1.0, 1.0); // Create a sphere
This declares the variable ball and defines the Sphere object to which it refers.
You can create another variable that refers to the same object as ball :
Sphere myBall = ball;
Now the variable myBall refers to the same object as ball . You still have only one object, but you have
two different variables that reference it. You could have as many variables as you like referring to the same
object.
As I mentioned earlier, the separation of the variable and the object has an important effect on how ob-
jects are passed to a method, so let's look at that in more detail.
Passing Objects to a Method