Learning Java, 4th Edition

Classes

Classes are the building blocks of a Java application. A class can contain methods (functions), variables, initialization code, and, as we’ll discuss later, other classes. It serves as a blueprint for making class instances, which are runtime objects (individual copies) that implement the class structure. You declare a class with the class keyword. Methods and variables of the class appear inside the braces of the class declaration:

    class Pendulum {
        float mass;
        float length = 1.0f;
        int cycles;

        float getPosition ( float time ) {
            ...
        }
        ...
    }

The Pendulum class contains three variables: mass, length, and cycles. It also defines a method called getPosition(), which takes a float value as an argument and returns a float value as a result. Variables and method declarations can appear in any order, but variable initializers can’t make “forward references” to other variables that appear later. Once we’ve defined the Pendulum class, we can create a Pendulum object (an instance of that class) as follows:

    Pendulum p;
    p = new Pendulum();

Recall that our declaration of the variable p doesn’t create a Pendulum object; it simply creates a variable that refers to an object of type Pendulum. We still had to create the object, using the new keyword, as shown in the second line of the preceding code snippet. Now that we’ve created a Pendulum object, we can access its variables and methods, as we’ve already seen many times:

    p.mass = 5.0;
    float pos = p.getPosition( 1.0 );

Two kinds of variables ...