Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 605

We stole some code from the Stock class and then made a few changes so that the
code would fit this interface. Our Stock code accepted the current share price as a
parameter to its getProfit method. Since the getProfit method cannot accept any
parameters if we wish to implement the interface, we'll instead store the current
share price as a field in the ShareAsset class and supply a setCurrentPrice muta-
tor method that can be called to set its proper value. We also include a constructor
that can initialize a Stock object with any number of shares and a total cost.
One last modification we made in creating ShareAsset was to include an
addCost method, which we'll use to add a given amount to the asset's total cost. We
will need this because purchases of Stock sand MutualFund s need to update the
totalCost field, but they cannot do so directly because it is private.
The Stock class can now extend ShareAsset to implement its remaining func-
tionality. Notice that we both extend ShareAsset and implement the Asset interface
in the class's header:

1 // A Stock object represents purchases of shares of a stock.
2 // Initial version.
3 public class Stock extends ShareAsset implements Asset {
4

private int totalShares;

5
6 // constructs a new Stock with the given symbol and
7 // current price per share
8 public Stock(String symbol, double currentPrice) {
9 super (symbol, currentPrice);
10 totalShares = 0;
11 }
12
13 // returns the market value of this stock, which is
14 // the number of total shares times the share price
15 public double getMarketValue() {
16 return totalShares * getCurrentPrice();
17 }
18
19 // returns the total number of shares purchased
20 public int getTotalShares() {
21 return totalShares;
22 }
23
24 // returns the profit made on this stock
25 public double getProfit() {
26 return getMarketValue() - getTotalCost();
27 }
28
29 // records a purchase of the given number of shares of