Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 609

Really, the Employee class introduced earlier in this chapter should also have been
an abstract class. We did not especially want client code to construct Employee
objects. No one is just an employee; the Employee class merely represented a general
category that we wanted the other classes to extend.
Abstract classes are allowed to implement interfaces. Rather than requiring all
subclasses of ShareAsset to implement the Asset interface, we can specify that
ShareAsset implements Asset :
public abstract class ShareAsset implements Asset {
...
}
This indication will save ShareAsset subclasses from having to write implements
Asset in their class headers.
ShareAsset does not implement the getMarketValue method required by
Asset ; that functionality is left for its subclasses. We can instead declare
getMarketValue as an abstract method in the ShareAsset class. Abstract methods
declared in abstract classes need to have the keyword abstract in their headers in
order to compile properly. Otherwise, the syntax is the same as when we declare an
abstract method in an interface, with a semicolon replacing the method's body:
// returns the current market value of this asset
public abstract double getMarketValue();
The general syntax for an abstract method declaration in an abstract class is the
following:
public abstract <type> <name> (<type> <name>, ..., <type> <name>);
Another benefit of this design is that code in the abstract class can actually call
any of its abstract methods, even if they don't have implementations in that file. This
is allowed because the abstract class can count on its subclasses to implement the
abstract methods. Now that ShareAsset implements Asset , we can move the
common redundant getProfit code up to ShareAsset and out of Stock and
MutualFund :
// returns the profit earned on shares of this asset
public double getProfit() {
// calls an abstract getMarketValue method
// (the subclass will provide its implementation)
return getMarketValue() - totalCost;
}
ShareAsset objects can call getMarketValue from their getProfit methods
even though that method isn't present in ShareAsset . The code compiles because