// A Stock object represents purchases of shares of a stock.

public class Stock {
    private String symbol;        // stock symbol, e.g. "YHOO"
    private int totalShares;      // total shares purchased
    private double totalCost;     // total cost for all shares

    // Initializes a new Stock with no shares purchased.
    // Precondition: symbol != null
    public Stock(String theSymbol) {
        if (theSymbol == null) {
            throw new NullPointerException();
        }

        symbol = theSymbol;
        totalShares = 0;
        totalCost = 0.0;
    }

    // Returns the total profit or loss earned on this stock,
    // based on the given price per share.
    // pre: currentPrice >= 0.0
    public double getProfit(double currentPrice) {
        if (currentPrice < 0.0) {
            throw new IllegalArgumentException();
        }

        double marketValue = totalShares * currentPrice;
        return marketValue - totalCost;
    }

    // Records purchase of the given shares at the given price.
    // pre: shares >= 0 && pricePerShare >= 0.0
    public void purchase(int shares, double pricePerShare) {
        if (shares < 0 || pricePerShare < 0.0) {
            throw new IllegalArgumentException();
        }

        totalShares += shares;
        totalCost += shares * pricePerShare;
    }
}