Discovering Classes and Objects - Beginning Java 7 - page 139

Nowthatyouhavetemporarilyfinishedwith Point and Circle ,youwanttotest
their draw() methods in a simulated version of the graphics application. To achieve
this objective, you write Listing 2-34 ' s Graphics class.

Listing 2-34. A Graphics class for testing Point 's and Circle 's draw() methods

class Graphics
{

public static void main(String[] args)
{

Point[] points = new Point[] { new Point(10, 20),
new Circle(10, 20,

30) };

for (int i = 0; i < points.length; i++)
points[i].draw();

}

}

Listing 2-34 ' s main() method first declares an array of Point s. Upcasting is
demonstrated by first having the array's initializer instantiate the Circle class, and
then by assigning this instance's reference to the second element in the points array.
Movingon, main() usesaforlooptocalleach Point element's draw() method.
Becausethefirstiterationcalls Point 's draw() method,whereastheseconditeration
calls Circle 's draw() method, you observe the following output:
Point drawn at (10, 20)
Circle drawn at (10, 20) with radius 30
How does Java “know” that it must call Circle 's draw() method on the second
loopiteration?Shoulditnotcall Point 's draw() methodbecause Circle isbeing
treated as a Point thanks to the upcast?
At compile time, the compiler does not know which method to call. All it can do is
verifythatamethodexistsinthesuperclass,andverifythatthemethodcall'sarguments
list and return type match the superclass's method declaration.
Inlieuofknowingwhichmethodtocall,thecompilerinsertsaninstructionintothe
compiledcodethat,atruntime,fetchesanduseswhateverreferenceisin points[1]
to call the correct draw() method. This task is known as late binding .