Java How to Program Fourth Edition - DCC

462 Object-Oriented Programming Chapter 9
Application class Test (Fig. 9.9) uses this Point/Circle inheritance hierarchy.
The application begins in method main by instantiating Circle object circle1 (line
11). This invokes the Circle constructor at line 15 of Fig. 9.8, which immediately
invokes the Point constructor at line 15 of Fig. 9.7. The Point constructor outputs the
values received from the Circle constructor by implicitly calling method toString
and returns program control to the Circle constructor. Then the Circle constructor
outputs the complete Circle by calling method toString. Notice that the first two
lines of the output from this program both show values for x, y and radius. Polymorphism
is once again causing the Circle’s toString method to execute because it is a
Circle object that is being created. When toString is invoked from the Point constructor,
0.0 is displayed for the radius because the radius has not yet been initialized
in the Circle constructor.
Circle object circle2 is instantiated next. Again, the Point and Circle constructors
both execute. Notice, in the command-line output window, that the body of the
Point constructor is performed before the body of the Circle constructor, showing that
objects are constructed “inside out.”
1 // Fig. 9.9: Test.java
2 // Demonstrate when superclass and subclass
3 // constructors and finalizers are called.
4 public class Test {
5
6 // test when constructors and finalizers are called
7 public static void main( String args[] )
8 {
9 Circle circle1, circle2;
10
11 circle1 = new Circle( 4.5, 72, 29 );
12 circle2 = new Circle( 10, 5, 5 );
13
14 circle1 = null; // mark for garbage collection
15 circle2 = null; // mark for garbage collection
16
17 System.gc(); // call the garbage collector
18 }
19
20 } // end class Test
Point constructor: Center = [72, 29]; Radius = 0.0
Circle constructor: Center = [72, 29]; Radius = 4.5
Point constructor: Center = [5, 5]; Radius = 0.0
Circle constructor: Center = [5, 5]; Radius = 10.0
Circle finalizer: Center = [72, 29]; Radius = 4.5
Point finalizer: Center = [72, 29]; Radius = 4.5
Circle finalizer: Center = [5, 5]; Radius = 10.0
Point finalizer: Center = [5, 5]; Radius = 10.0
Fig. Fig. 9.9 9.9 Order in which constructors and finalizers are called.
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