INHERITANCE AND POLYMORPHISM - Introduction to Java Programming: Comprehensive Version - page 428

casting must be used to confirm your intention to the compiler with the (SubclassName)
cast notation. For the casting to be successful, you must make sure that the object to be cast is
an instance of the subclass. If the superclass object is not an instance of the subclass, a runtime
ClassCastException occurs. For example, if an object is not an instance of Student , it
cannot be cast into a variable of Student . It is a good practice, therefore, to ensure that the
object is an instance of another object before attempting a casting. This can be accomplished
by using the instanceof operator. Consider the following code:

ClassCastException

instanceof

Object myObject = new Circle();
... // Some lines of code
/** Perform casting if myObject is an instance of Circle */
if (myObject instanceof Circle) {
System.out.println( "The circle diameter is " +
((Circle)myObject).getDiameter());
...
}

You may be wondering why casting is necessary. The variable myObject is declared
Object . The declared type decides which method to match at compile time. Using
myObject.getDiameter() would cause a compile error, because the Object
class does not have the getDiameter method. The compiler cannot find a match for
myObject.getDiameter() . Therefore, it is necessary to cast myObject into the Circle
type to tell the compiler that myObject is also an instance of Circle .
Why not define myObject as a Circle type in the first place? To enable generic program-
ming, it is a good practice to define a variable with a supertype, which can accept an object
of any subtype.

Note
instanceof is a Java keyword. Every letter in a Java keyword is in lowercase.

lowercase keywords

Tip
To help understand casting, you may also consider the analogy of fruit, apple, and
orange, with the Fruit class as the superclass for Apple and Orange . An apple is
a fruit, so you can always safely assign an instance of Apple to a variable for Fruit .
However, a fruit is not necessarily an apple, so you have to use explicit casting to assign
an instance of Fruit to a variable of Apple .

casting analogy

Listing 11.7 demonstrates polymorphism and casting. The program creates two objects
(lines 5-6), a circle and a rectangle, and invokes the displayObject method to display them
(lines 9-10). The displayObject method displays the area and diameter if the object is a
circle (line 15), and the area if the object is a rectangle (lines 21-22).

L ISTING 11.7

CastingDemo.java
1 public class CastingDemo {
2 /** Main method */
3 public static void main(String[] args) {
4 // Create and initialize two objects
5 Object object1 = new CircleFromSimpleGeometricObject( 1 );
6 Object object2 = new RectangleFromSimpleGeometricObject( 1 , 1 );
7
8 // Display circle and rectangle

9 displayObject(object1);

10 displayObject(object2);

11 }
12