Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 576

refers to. More specifically, it is legal for a superclass variable to refer to an object of
its subclass. The following is a legal assignment statement:
Employee ed = new Lawyer();
When we were studying the primitive types, we saw cases in which a variable of
one type could store a value of another type (for example, an int value can be stored
in a double variable). In the case of primitive values, Java converts variables from
one type to another: int values are automatically converted to double s when they
are assigned.
When a subclass object is stored in a superclass variable, no such conversion
occurs. The object referred to by ed really is a Lawyer object, not an Employee
object. If we call methods on it, it will behave like a Lawyer object. For example, the
call of ed.getVacationForm() returns " pink " , which is the Lawyer 's behavior, not
the Employee 's.
This ability for variables to refer to subclass objects allows us to write flexible
code that can interact with many types of objects in the same way. For example, we
can write a method that accepts an Employee as a parameter, returns an Employee ,
or creates an array of Employee objects. In any of these cases, we can substitute a
Secretary , Lawyer , or other subclass object of Employee , and the code will still
work. Even more importantly, code will actually behave differently depending on
which type of object is used, because each subclass overrides and changes some of
the behavior from the superclass. This is polymorphism at work.
Here is an example test file that uses Employee objects polymorphically as param-
eters to a static method:
1 // Demonstrates polymorphism by passing many types of employees
2 // as parameters to the same method.
3 public class EmployeeMain3 {
4 public static void main(String[] args) {
5 Employee edna = new Employee();
6 Lawyer lucy = new Lawyer();
7 Secretary stan = new Secretary();
8 LegalSecretary leo = new LegalSecretary();
9
10 printInfo(edna);
11 printInfo(lucy);
12 printInfo(stan);
13 printInfo(leo);
14 }
15
16 // Prints information about any kind of employee.
17 public static void printInfo(Employee e) {
18 System.out.print(e.getHours() + ", ");