Classes - Building Java Programs: A Back to Basics Approach - page 541

per share must not be negative numbers. To perform this test, we can insert the fol-
lowing lines at the start of our purchase method:

if (shares < 0 || pricePerShare < 0) {
throw new IllegalArgumentException();

}

Next, we'll write the body of the getProfit method. As we noted previously, the
profit of a Stock is equal to its current market value minus the amount that was paid
for it:

profit ((total shares) * (current share price)) (total cost)

We can implement this formula in a straightforward manner using the
totalShares and totalCost fields and the currentPrice parameter:

// Returns the total profit or loss earned on this stock,
// based on the given price per share.
public double getProfit(double currentPrice) {
return totalShares * currentPrice - totalCost;

}

Note that parentheses are not needed in the code because multiplication has a
higher precedence than subtraction.
As we did for the other methods, we should check for illegal parameter values. In
this case, we shouldn't allow a negative current price per share. To ensure that this
doesn't happen, we can place the following code at the start of the method:

if (currentPrice < 0.0) {
throw new IllegalArgumentException();

}

After we've written all the fields, the constructor, and the methods of our Stock ,
the class will look like this:

1 // A Stock object represents purchases of shares of a stock.
2
3 public class Stock {
4 private String symbol; // stock symbol, e.g. "YHOO"
5 private int totalShares; // total shares purchased
6 private double totalCost; // total cost for all shares
7
8 // initializes a new Stock with no shares purchased
9 // pre: symbol != null