Inheritance - Data Structures and Problem Solving Using Java - page 122

figure 4.8
The complete Student
and Employee classes,
using both forms of
super

1 class Student extends Person
2 {
3 public Student( String n, int ag, String ad, String p,
4 double g )
5 { super( n, ag, ad, p ); gpa = g; }
6
7 public String toString( )
8 { return super.toString( ) + getGPA( ); }
9
10 public double getGPA( )
11 { return gpa; }
12
13 private double gpa;
14 }
15
16 class Employee extends Person
17 {
18 public Employee( String n, int ag, String ad,
19 String p, double s )
20 { super( n, ag, ad, p ); salary = s; }
21
22 public String toString( )
23 { return super.toString( ) + " $" + getSalary( ); }
24
25 public double getSalary( )
26 { return salary; }
27
28 public void raise( double percentRaise )
29 { salary *= ( 1 + percentRaise ); }
30
31 private double salary;
32 }

In the example, suppose that prior to completing the printing, we want to
give p[3] —which we know is an employee—a raise? Since p[3] is an
Employee , it might seem that

p[3].raise( 0.04 );

would be legal. But it is not. The problem is that the static type of p[3] is a Person ,
and raise is not defined for Person . At compile time, only (visible) members of
the static type of the reference can appear to the right of the dot operator.
We can change the static type by using a cast:

((Employee) p[3]).raise( 0.04 );

The above code makes the static type of the reference to the left of the dot
operator an Employee . If this is impossible (for instance, p[3] is in a completely