Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 612

25 public void purchase( int shares, double pricePerShare) {
26 totalShares += shares;
27 addCost(shares * pricePerShare);
28 }
29 }

1 // A MutualFund object represents a mutual fund asset.
2 public class MutualFund extends ShareAsset {
3 private double totalShares;

4
5 // constructs a new MutualFund investment with the given
6 // symbol and price per share
7 public MutualFund(String symbol, double currentPrice) {
8 super (symbol, currentPrice);
9 totalShares = 0.0;
10 }
11
12 // returns the market value of this mutual fund, which
13 // is the number of shares times the price per share
14 public double getMarketValue() {
15 return totalShares * getCurrentPrice();
16 }
17
18 // returns the number of shares of this mutual fund
19 public double getTotalShares() {
20 return totalShares;
21 }
22
23 // records purchase of the given shares at the given price
24 public void purchase( double shares, double pricePerShare) {
25 totalShares += shares;
26 addCost(shares * pricePerShare);
27 }
28 }

Abstract classes can do everything interfaces can do and more, but this does not
mean that it is always better to use them than interfaces. One important difference
between interfaces and abstract classes is that a class may choose to implement arbi-
trarily many interfaces, but it can extend just one abstract class. That is why an inter-
face often forms the top of an inheritance hierarchy, as our Asset interface did in this
design. Such placement allows classes to become part of the hierarchy without having
it consume their only inheritance relationships.