Inheritance - Java - page 434

PITFALL: The Terms Subclass and Superclass

Many programmers and authors use the term subclass for a derived class and superclass
for its base class (or any of its ancestor classes). This is logical. For example, the collection of
all hourly employees in the world is a subclass of all employees. Similarly, the collection of all
objects of type HourlyEmployee is a subcollection of the collection of all objects of the class
Employee . As you add more instance variables and methods, you restrict the number of
objects that can satisfy the class definition. Despite this logic, people often reverse the terms
subclass and superclass. Remember that these terms refer to the collections of objects of the
derived class and the base class and not to the number of instance variables or methods. A
derived class is a subclass ( not a superclass ) of its base class. Another way to remember
which is a superclass is as follows: Recall that the super constructor invocation is an invoca-
tion of the base class and so the base class is the superclass .

subclass and
superclass

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Self-Test Exercises
5. Is the following program legal? The relevant classes are defined in Displays 7.2,
7.3, and 7.5.

public class EmployeeDemo
{
public static void main( String[] args)
{
HourlyEmployee joe =
new HourlyEmployee("Joe Young",
new Date("February", 1, 2004), 10.50, 40);
SalariedEmployee boss =
new SalariedEmployee("Mr. Big Shot",
new Date("January", 1, 1999), 100000);
printName(joe);
printName(boss);
}

public void printName(Employee object)
{
System.out.println(object.getName());
}
}

6. Give a definition for a class TitledEmployee that is a derived class of the base class
SalariedEmployee given in Display 7.5. The class TitledEmployee has one addi-
tional instance variable of type String called title . It also has two additional meth-
ods: getTitle , which takes no arguments and returns a String , and setTitle ,
which is a void method that takes one argument of type String . It also overrides
(redefines) the method definition for getName , so that the string returned includes
the title as well as the name of the employee.