Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 611

40
41 // sets the current share price of this asset
42 public void setCurrentPrice( double currentPrice) {
43 this. currentPrice = currentPrice;
44 }
45 }

An abstract class is a useful hybrid that can contain both abstract and nonabstract
methods. All methods declared in an interface are implicitly abstract; they can be
declared with the abstract keyword if you wish. Declaring them without the abstract
keyword as we have done in this chapter is a commonly used shorthand for the
longer explicit form. Unfortunately, abstract classes disallow this shorthand to avoid
ambiguity.
Nonabstract classes like Stock and MutualFund are sometimes called concrete
classes to differentiate them from abstract classes. We can modify the Stock and
MutualFund classes to take advantage of ShareAsset and reduce redundancy. The
following are the final versions of the Stock and MutualFund classes.
( DividendStock is unmodified.) Notice that the subclasses of ShareAsset must
implement getMarketValue , or we'll receive a compiler error:

1 // A Stock object represents purchases of shares of a stock.
2 public class Stock extends ShareAsset {
3 private int totalShares;

4
5 // constructs a new Stock with the given symbol and
6 // current price per share
7 public Stock(String symbol, double currentPrice) {
8 super (symbol, currentPrice);
9 totalShares = 0;
10 }
11
12 // returns the market value of this stock, which is
13 // the number of total shares times the share price
14 public double getMarketValue() {
15 return totalShares * getCurrentPrice();
16 }
17
18 // returns the total number of shares purchased
19 public int getTotalShares() {
20 return totalShares;
21 }
22
23 // records a purchase of the given number of shares of
24 // the stock at the given price per share