Classes

So far, all of our classes have been a collection of methods - self-contained programs that do some type of processing, which Java can run because there is a public static void main(String[] args) method.

class as a Program

This class prompts a user to enter a radius, which it will use to calculate the volume of a sphere, and prints the result.

public class SphereVolume {
  public static void main(String[] args) {
    double radius = getRadius();

    double volume = calculateVolume(radius);

    printVolumeForRadius(radius, volume);
  }

  public static double getRadius() {
    java.util.Scanner scanner = new java.util.Scanner(System.in);
    double radius = 0.0;
    System.out.print("Enter the radius of the sphere: ");
    radius = scanner.nextDouble();
    scanner.close();
    return radius;
  }

  public static double calculateVolume(double radius) {
    double vol = 4.0 / 3.0 * 3.14159 * radius * radius * radius;
    return vol;
  }

  public static void printVolumeForRadius(double radius, double volume) {
    System.out.print("A sphere of radius " + radius);
    System.out.println(" has a volume of " + volume);
  }
}

We have only had to think about what our code does.

Going forward, we will think about what things in our code represent.

class as a Blueprint

This class models a sphere, which has a radius and a method we can call to ask for its volume.

public class Sphere {
  public double radius;

  public double getVolume() {
    double vol = 4.0 / 3.0 * 3.14159 * radius * radius * radius;
    return vol;
  }
}

model

Create a representation of something in a program. In Java, this is a class.

The Sphere class is a blueprint we can use to create Sphere objects in a program.

public class SphereObjectProgram {

  public static void main(String[] args) {
    double rad = getRadius();

    // create a Sphere object
    Sphere s = new Sphere();
    s.radius = rad;

    // get the object's volume
    double volume = s.getVolume();

    // print the radius using this object's data
    printVolumeForRadius(s.radius, volume);
  }

  public static double getRadius() {
    java.util.Scanner scanner = new java.util.Scanner(System.in);
    double radius = 0.0;
    System.out.print("Enter the radius of the sphere: ");
    radius = scanner.nextDouble();
    scanner.close();
    return radius;
  }

  public static void printVolumeForRadius(double radius, double volume) {
    System.out.print("A sphere of radius " + radius);
    System.out.println(" has a volume of " + volume);
  }
}

Using the Sphere class as a blueprint, we can create several Sphere objects.

Drill

IntroToObjects/src/drills/SphereCreation.java * Create three Sphere "objects." 1. A baseball, radius 3.7 2. A basketball, radius 12.0 3. Mercury, radius 244 million centimeters. (Try writing this value as 244_000_000.0 or 244e5.) * Print each object's volume to the screen using System.out.println.

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