Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 610

the compiler knows that whatever class extends ShareAsset will have to implement
getMarketValue .
The following is the final version of the ShareAsset abstract class:
1 // A ShareAsset represents a general asset that has a symbol and
2 // holds shares.
3 public abstract class ShareAsset implements Asset {
4 private String symbol;
5 private double totalCost;
6 private double currentPrice;
7
8 // constructs a new share asset with the given symbol
9 // and current price
10 public ShareAsset(String symbol, double currentPrice) {
11 this. symbol = symbol;
12 this. currentPrice = currentPrice;
13 totalCost = 0.0;
14 }
15
16 // adds a cost of the given amount to this asset
17 public void addCost( double cost) {
18 totalCost += cost;
19 }
20
21 // returns the price per share of this asset
22 public double getCurrentPrice() {
23 return currentPrice;
24 }
25
26 // returns the current market value of this asset
27 public abstract double getMarketValue();
28
29 // returns the profit earned on shares of this asset
30 public double getProfit() {
31 // calls an abstract getMarketValue method
32 // (the subclass will provide its implementation)
33 return getMarketValue() - totalCost;
34 }
35
36 // returns this asset's total cost for all shares
37 public double getTotalCost() {
38 return totalCost;
39 }