Bags - Data Structures and Abstractions with Java - page 12

program without knowing the details of the implementation. That is, we do not need to know how
the programmer implemented the bag to be able to use it. We only need to know what the ADT bag
does. This section assumes that we have a Java class, Bag , that implements the Java interface
BagInterface given in Listing 1-1. The simple examples demonstrate how we can use Bag .

Notice that once we choose the data type of the objects to be in a bag, that data type is
enclosed in brackets that follow the interface name and the class name. All entries in the bag then
must have either that data type or a subtype of that data type. The compiler will enforce this
restriction for us. For primitive types, you can place instances of an appropriate wrapper class
into a bag. For example, instead of instances of the primitive type int , you could use instances of
the wrapper class Integer .

VideoNote

Designing a test for an ADT

1.16

Example: Shopping online. When you shop online, your selections are saved in a shopping cart,
or bag, until you are ready to check out. The program that implements the shopping website can use
the class Bag to maintain the shopping cart. After all, the order in which you choose items to pur-
chase is not important. Listing 1-2 shows a simple example of such a program.

LISTING 1-2

A program that maintains a bag for online shopping

/**

A class that maintains a shopping cart for an online store.
@author Frank M. Carrano

*/
public class OnlineShopper
{

public static void main(String[] args)
{

Item[] items = { new Item("Bird feeder", 2050),
new Item("Squirrel guard", 1547),
new Item("Bird bath", 4499),
new Item("Sunflower seeds", 1295)};
BagInterface<Item> shoppingCart = new Bag<Item>();
int totalCost = 0;

// statements that add selected items to the shopping cart:
for ( int index = 0; index < items.length; index++)
{

Item nextItem = items[index]; // simulate getting item from
// shopper

shoppingCart.add(nextItem);
totalCost = totalCost + nextItem.getPrice();
} // end for

// simulate checkout
while (!shoppingCart.isEmpty())
System.out.println(shoppingCart.remove());

System.out.println("Total cost: " +
"\t$" + totalCost / 100 + "." +
totalCost % 100);