Inheritance - Data Structures and Problem Solving Using Java - page 123

figure 4.9
An illustration of
polymorphism with
arrays

1 class PersonDemo
2 {
3 public static void printAll( Person [ ] arr )
4 {
5 for( int i = 0; i < arr.length; i++ )
6 {
7 if( arr[ i ] != null )
8 {
9 System.out.print( "[" + i + "] " );
10 System.out.println( arr[ i ].toString( ) );
11 }
12 }
13 }
14
15 public static void main( String [ ] args )
16 {
17

Person [ ] p = new Person[ 4 ];

18
19 p[0] = new Person( "joe", 25, "New York",
20 "212-555-1212" );
21 p[1] = new Student( "jill", 27, "Chicago",
22 "312-555-1212", 4.0 );
23 p[3] = new Employee( "bob", 29, "Boston",
24 "617-555-1212", 100000.0 );
25
26 printAll( p );
27 }
28 }

different hierarchy), the compiler will complain. If it is possible for the cast to
make sense, the program will compile, and so the above code will success-
fully give a 4% raise to p[3] . This construct, in which we change the static
type of an expression from a base class to a class farther down in the inheri-
tance hierarchy is known as a downcast .
What if p[3] was not an Employee ? For instance, what if we used the following?

A downcast is a
cast down the
inheritance hierar-
chy. Casts are
always verified at
runtime by the
Virtual Machine.

((Employee) p[1]).raise( 0.04 ); // p[1] is a Student

In that case the program would compile, but the Virtual Machine would throw
a ClassCastException , which is a run-time exception that signals a program-
ming error. Casts are always double-checked at run time to ensure that the
programmer (or a malicious hacker) is not trying to subvert Java's strong
typing system. The safe way of doing these types of calls is to use instanceof
first:

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