Intro

The main points about classes and objects so far:

• A class is a blueprint for creating an instance or object.
• Objects have state and behavior - fields and methods.
• A class can have static or non-static (instance) fields and methods.
• static fields and methods belong to the blueprint.
• Each object gets its own copy of an instance field.
• We can call instance methods on an object using the . operator.
• Create an instance with the new operator.
• Use a reference variable to point to an instance.

A class (blueprint) defining state (fields) and behavior (methods) for objects that model a sphere:

public class Sphere3 {
  public static double PI = 3.14159;  // static field

  public double radius;               // instance field

  // instance method
  public double getVolume() {
    double vol = 4.0 / 3.0 * PI  * radius * radius * radius;
    return vol;
  }

  // static method
  public static double calculateCircumference(double r) {
    return PI * 2 * r;
  }
}

Below are examples of creating and using a Sphere3 object, and using the Sphere3 static method.

Sphere3 sphere = new Sphere3();   // reference points to new instance
sphere.radius = 12.0;             // use reference to access fields and methods
double vol = sphere.getVolume();

double circum = Sphere3.calculateCircumference(15.0); // static method call
                                                      // using class name

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