Delving Further into Object-Oriented Concepts - Beginning Java Programming: The Object-Oriented Approach - page 240

8.

Finally, you used the newly created getter and setter methods to read and modify the variables in a
safer way.

class inheritance

Java classes are organized in a hierarchical inheritance structure. Subclasses are derived from super-
classes. The class Object is the highest-level superclass; it has no parent class above it. All other
classes are either subclasses of Object or subclasses of subclasses of Object . This is illustrated in
Figure 7-2.

Object

ClassA ClassB1ii ClassC

ClassA1

ClassB1ii

ClassA2

ClassB1ii

ClassB1ii

figure 7-2

Inheritance represents “is a” relationship. For example, a Student is a Person , so the class Student
could be a subclass of the Person class. An Employee is a Person , too, so Employee could be
another subclass of Person . An undergraduate is a student and a graduate is a student, so you can
create two additional subclasses of the class Student . Staff and Employee might be subclasses of
the class Employee . A possible class hierarchy for this inheritance example is shown in Figure 7-3.

To indicate that a class is a subclass of another class, use the extends keyword in the class declara-
tion, for example, public class Employee extends Person{ } . Variables and methods that are
shared by all types of Persons can be placed in the Person class, and then each subclass can have
specialized variables and methods that are only used by that type or its subclasses. For example,