INHERITANCE AND POLYMORPHISM - Java Programming: From Problem Analysis to Program Design - page 668

Consider the following application program:
// This program illustrates how polymorphic reference variables
// work.

public class Polymorphism //Line 1

{ //Line 2

public static void main(String[] args) //Line 3

{ //Line 4

RectangleFigure rectangle, shapeRef; //Line 5

BoxFigure box; //Line 6

rectangle = new RectangleFigure(8, 5); //Line 7

box = new BoxFigure(10, 7, 3); //Line 8

shapeRef = rectangle; //Line 9

System.out.println("Line 10: Rectangle:\n"
+ shapeRef); //Line 10

System.out.println(); //Line 11

shapeRef = box; //Line 12

System.out.println("Line 13: Box:\n"
+ shapeRef);

//Line 13

System.out.println(); //Line 14

} //end main //Line 15

} //Line 16

Sample Run:
Line 10: Rectangle:
Length = 8.0; Width = 5.0
Area = 40.0

Line 13: Box:
Length = 10.0; Width = 7.0; Height = 3.0
Surface Area = 242.0; Volume = 210.0
In the preceding program, shapeRef is a reference variable of the RectangleFigure
type. Because the class BoxFigure is derived from the class RectangleFigure , the
reference variable shapeRef can point to an object of the class RectangleFigure or
to an object of the class BoxFigure .
The statement in Line 7 instantiates a RectangleFigure object and stores the address of this
object in the reference variable rectangle . Similarly, the statement in Line 8 instantiates a
BoxFigure object and stores the address of this object in the reference variable box .
After the statement in Line 9 executes, shapeRef points to the object rectangle . The
statement in Line 10 executes the method toString . Because shapeRef points to
an object of the class RectangleFigure , the method toString of the class
RectangleFigure executes. When the method toString of the class
RectangleFigure executes, it also executes the method area . In this case, the method
area of the class RectangleFigure executes.