INHERITANCE AND POLYMORPHISM - Introduction to Java Programming: Comprehensive Version - page 423

Note
You can also pass an object to invoke System.out.println(object) or
System.out.print(object) . This is equivalent to invoking System.out
.println(object.toString()) or System.out.print(object.
toString()) . Thus, you could replace System.out.println(loan
.toString()) with System.out.println(loan) .

print object

11.7 Polymorphism
Polymorphism means that a variable of a supertype can refer to a subtype object.

Key
Point

The three pillars of object-oriented programming are encapsulation, inheritance, and poly-
morphism. You have already learned the first two. This section introduces polymorphism.
First, let us define two useful terms: subtype and supertype. A class defines a type. A
type defined by a subclass is called a subtype , and a type defined by its superclass is called
a supertype . Therefore, you can say that Circle is a subtype of GeometricObject and
GeometricObject is a supertype for Circle .
The inheritance relationship enables a subclass to inherit features from its superclass with
additional new features. A subclass is a specialization of its superclass; every instance of a
subclass is also an instance of its superclass, but not vice versa. For example, every circle
is a geometric object, but not every geometric object is a circle. Therefore, you can always
pass an instance of a subclass to a parameter of its superclass type. Consider the code in
Listing 11.5.

subtype
supertype

L ISTING 11.5

PolymorphismDemo.java
1 public class PolymorphismDemo {
2

/** Main method */

3 public static void main(String[] args) {

4 // Display circle and rectangle properties

5 displayObject( new CircleFromSimpleGeometricObject polymorphic call

6 ( 1 , "red" , false ));

7 displayObject( new RectangleFromSimpleGeometricObject polymorphic call

8 ( 1 , 1 , "black" , true ));

9 }
10
11 /** Display geometric object properties */
12 public static void displayObject(SimpleGeometricObject object) {
13 System.out.println( "Created on " + object.getDateCreated() +
14 ". Color is " + object.getColor());

15 }
16 }

Created on Mon Mar 09 19:25:20 EDT 2011. Color is red
Created on Mon Mar 09 19:25:20 EDT 2011. Color is black

The method displayObject (line 12) takes a parameter of the GeometricObject type.
You can invoke displayObject by passing any instance of GeometricObject (e.g., new
CircleFromSimpleGeometricObject(1, "red", false) and new Rectangle-
FromSimpleGeometricObject(1, 1, "black", false) in lines 5-8). An object of
a subclass can be used wherever its superclass object is used. This is commonly known as
polymorphism (from a Greek word meaning “many forms”). In simple terms, polymorphism
means that a variable of a supertype can refer to a subtype object.

what is polymorphism?