Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 601

Stock
symbol
total share: int
total cost
current price
getMarketValue()
getProfit()

DividendStock
symbol
total share: int
total cost
current price
dividends
getMarketValue()
getProfit()

MutualFund
symbol
total share: double
total cost
current price
getMarketValue()
getProfit()

Cash
amount
getMarketValue()

Figure 9.7

Financial classes with state and behavior

has a market value, so it would be nice to be able to compute the total market
value of all assets in an investor's portfolio, without worrying about the different
types of assets.
Because different assets compute their market values in different ways, we should
consider using an interface to represent the notion of an asset and have every class
implement the asset interface. Our interface will demand that all assets have methods
to calculate the market value and profit. The interface is a way of saying, “Classes
that want to consider themselves assets must have getMarketValue and getProfit
methods.” Our interface for financial assets would be saved in a file called
Asset.java and would look like this:

1 // Represents financial assets that investors hold.
2 public interface Asset {
3 // how much the asset is worth
4 public double getMarketValue();
5
6 // how much money has been made on this asset
7 public double getProfit();
8 }

We'll have our various classes certify that they are assets by making them imple-
ment the Asset interface. For example, let's look at the Cash class. We didn't write a
getProfit method in our previous diagram of Cash , because the value of cash doesn't
change and therefore it doesn't have a profit. To indicate this quality, we can write a
getProfit method for Cash that returns 0.0 . The Cash class should look like this:

1 // A Cash object represents an amount of money held by an investor.
2 public class Cash implements Asset {
3 private double amount; // amount of money held

4
5 // constructs a cash investment of the given amount
6 public Cash( double amount) {

7 this .amount = amount;