[Chapter 3] 3.3 Class Variables

3.3 Class Variables

In our Circle class definition, we declared three "instance" variables: x, y, and r. Each instance of the class--each circle--has its own copy of these three variables. These variables are like the fields of a struct in C--each instance of the struct has a copy of the fields. Sometimes, though, we want a variable of which there is only one copy--something like a global variable in C.

The problem is that Java doesn't allow global variables. (Actually, those in the know consider this is feature!) Every variable in Java must be declared inside a class. So Java uses the static keyword to indicate that a particular variable is a class variable rather than an instance variable. That is, that there is only one copy of the variable, associated with the class, rather than many copies of the variable associated with each instance of the class. The one copy of the variable exists regardless of the number of instances of the class that are created--it exists and can be used even if the class is never actually instantiated.

This kind of variable, declared with the static keyword, is often called a static variable. I prefer (and recommend) the name "class variable" because it is easily distinguished from its opposite, "instance variable." We'll use both terms in this book.

An Example

As an example (a somewhat contrived one), suppose that while developing the Circle class we wanted to do some testing on it and determine how much it gets used. One way to do this would be to count the number of Circle objects that are instantiated. To do this we obviously need a variable associated with the class, rather than with any particular instance. Example 3.4 shows how we can do it--we declare a static variable and increment it each time we create a Circle.

Example 3.4: Static Variable Example

public class Circle {
    static int num_circles = 0; // class variable: how many circles created
    public double x, y, r;      // instance vars: the center and the radius
    public Circle(double x, double y, double r) {
        this.x = x; this.y = y; this.r = r;
        num_circles++;
    }
    public Circle(double r) { this(0.0, 0.0, r); }
    public Circle(Circle c) { this(c.x, c.y, c.r); }
    public Circle() { this(0.0, 0.0, 1.0); }
    public double circumference() { return 2 * 3.14159 * r; }
    public double area() { return 3.14159 * r*r; }
}

Accessing Class Variables

Now that we are keeping track of the number of Circle objects created, how can we access this information? Because static variables are associated with the class rather than with an instance, we access them through the class rather than through the instance. Thus, we might write: [5]

[5] Recall that System.out.println() prints a line of text, and that the string concatenation operator, +, converts non-string types to strings as necessary.

System.out.println("Number of circles created: " + Circle.num_circles);

Notice that in our definition of the constructor method in Example 3.4, we just used num_circles instead of Circle.num_circles. We're allowed to do this within the class definition of Circle itself. Anywhere else, though, we must use the class name as well.

Global Variables?

Earlier we said that Java does not support global variables. In a sense, though, Circle.num_circles behaves just like one. What is different from a global variable in C is that there is no possibility of name conflicts. If we use some other class with a class variable named num_circles, there won't be a "collision" between these two "global" variables, because they must both be referred to by their class names. Since each class variable must be part of a class and must be referred to with its class name, each has a unique name. Furthermore, each class has a unique name because, as we saw in Chapter 2, How Java Differs from C, it is part of a package with a unique name.

Constants: Another Class Variable Example

Let's try a less forced example of why you might want to use a class variable with the Circle class. When computing the area and circumference of circles, we use the value pi. Since we use the value frequently, we don't want to keep typing out 3.14159, so we'll define it as a class variable that has a convenient name:

public class Circle {
    public static final double PI = 3.14159265358979323846;
    public double x, y, r;
    // ... etc....
}

Besides the static keyword that we've already seen, we use the final keyword, which means that this variable can never have its value changed. This prevents you from doing something stupid like:

Circle.PI = 4;

which would tend to give you some pretty square-looking circles.

The Java compiler is smart about variables declared both static and final--it knows that they have constant values. So when you write code like this:

double circumference = 2 * Circle.PI * radius;

the compiler precomputes the value 2 * Circle.PI , instead of leaving it for the interpreter.

Java does not have a preprocessor with a C-style #define directive. static final variables are Java's substitute for C's #define'd constants. Note that the C convention of capitalizing constants has been carried over into Java.