Is the taxRate for Book the same as for Toy?

A good answer might be:

Yes---by using an interface, a constant can be used by several classes. This helps keep the classes consistent.

Testing Program

Here is a tiny program that tests the classes. If you want to run the program, copy and paste all the classes and the interface to a file called Store.java. 

public class Store
{

  public static void main ( String[] args )
  {
    Goods gd = new Goods( "bubble bath", 1.40 );
    Food  fd = new Food ( "ox tails", 4.45, 1500 );
    Book  bk = new Book ( "Emma", 24.95, "Austin" );
    Toy   ty = new Toy  ( "Leggos", 54.45, 8 );

    gd.display();

    fd.display();

    ty.display();
    System.out.println("Tax is: " + ty.calculateTax() + "\n" );

    bk.display();
    System.out.println("Tax is: " + bk.calculateTax() + "\n" );

  }
}

The calculateTax() method is only used with objects who's class implements the interface Here is a picture that shows the classes and their objects:

In the picture, clouds represent classes. Arrows with pointed head connect child classes to parent classes. The dotted rectangle represents the interface; a dotted arrow shows which classes implement it. Rectangles represent objects. Arrows with square head connect an object to its class.

QUESTION 13:

  • Could these 4 objects be kept in an array?
  • What would the array type be?

Click Here after you have answered the question