Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 564

13
14 // returns the total profit or loss earned on this stock
15 public double getProfit( double currentPrice) {

16 double marketValue = totalShares * currentPrice;

17 return marketValue - totalCost;

18 }
19
20 // records purchase of the given shares at the given price
21 public void purchase( int shares, double pricePerShare) {
22 totalShares += shares;
23 totalCost += shares * pricePerShare;
24 }
25 }

Now let's imagine that you want to create a type of object for stocks which pay
dividends. Dividends are profit-sharing payments that a corporation pays its share-
holders. The amount that each shareholder receives is proportional to the number of
shares that person owns. Not every stock pays dividends, so you wouldn't want to
add this functionality directly to the Stock class. Instead, you should create a new
class called DividendStock that extends Stock and adds this new behavior.
Each DividendStock object will inherit the symbol, total shares, and total cost
from the Stock superclass. You'll simply need to add a field to record the amount of
the dividends paid:

public class DividendStock extends Stock {
private double dividends; // amount of dividends paid
...

}

Using the dividends field, you can write a method in the DividendStock class
that lets the shareholder receive a per-share dividend. Your first thought might be to
write code like the following, but this won't compile:

// this code does not compile
public void payDividend(double amountPerShare) {
dividends += amountPerShare * totalShares;

}

A DividendStock cannot access the totalShares field it has inherited, because
totalShares is declared private in Stock . A subclass may not refer directly to any
private fields that were declared in its superclass, so you'll get a compiler error like
the following:

DividendStock.java:17: totalShares has private access in Stock