Queues, Deques, and Priority Queues - Data Structures and Abstractions with Java - page 253

Figure 10-7 shows a CRC card for the class StockLedger . The class enables us to record stock
purchases in chronological order. At the time of sale, the class computes the capital gain and
updates the record of stocks owned. These last two steps are related, so we combine them into one
method. Thus, the class has two methods, buy and sell , as Figure 10-8 illustrates.

FIGURE 10-7

A CRC card for the class StockLedger

StockLedger

Responsibilities

Record the shares of a stock purchased, in

chronological order

Remove the shares of a stock sold, beginning

with the ones held the longest

Compute the capital gain (loss) on shares of a

stock sold

Collaborations

Share of stock

FIGURE 10-8

A diagram of the classes StockLedger and StockPurchase

StockLedger

*

ledger —a collection of shares owned, in order of their purchase

buy(sharesBought, pricePerShare)
sell(sharesSold, pricePerShare)

StockPurchase

*

cost —cost of one share

getCostPerShare()

The following statements demonstrate how we could use StockLedger to record the transac-
tions given in the problem description:

StockLedger myStocks = new StockLedger();
myStocks.buy(20, 45); // buy 20 shares at $45
myStocks.buy(20, 75); // buy 20 shares at $75
double capGain = myStocks.sell(30, 65); // sell 30 shares at $65

10.11

Implementation. In this example, StockLedger records instances of StockPurchase —which rep-
resent the shares we own—in a queue. A queue orders the shares chronologically, so we can sell
them in the order in which we purchased them. The method buy then just enqueues each share that
is bought.
The method sell removes from the queue as many shares as are sold. As it does this, it computes
the total capital gain from the sale and returns it. The class StockLedger is given in Listing 10-3.