finalize q of thinking-in-java(2e) ch7

[Claire Yang]

Here is the "Inheritance and finalize( )" example of ch7 in the book "thinking in java--2e":

//: c07:Frog.java
// Testing finalize with inheritance.
class DoBaseFinalization {
  public static boolean flag = false;
}
class Characteristic {
  String s;
  Characteristic(String c) {
    s = c;
    System.out.println(
      "Creating Characteristic " + s);
  }
  protected void finalize() {
    System.out.println(
      "finalizing Characteristic " + s);
  }
}
class LivingCreature {
  Characteristic p = 
    new Characteristic("is alive");
  LivingCreature() {
    System.out.println("LivingCreature()");
  }
  protected void finalize() throws Throwable {
    System.out.println(
      "LivingCreature finalize");
    // Call base-class version LAST!
    if(DoBaseFinalization.flag)
      super.finalize();
  }
}
class Animal extends LivingCreature {
  Characteristic p = 
    new Characteristic("has heart");
  Animal() {
    System.out.println("Animal()");
  }
  protected void finalize() throws Throwable {
    System.out.println("Animal finalize");
    if(DoBaseFinalization.flag)
      super.finalize();
  }
}
class Amphibian extends Animal {
  Characteristic p = 
    new Characteristic("can live in water");
  Amphibian() {
    System.out.println("Amphibian()");
  }
  protected void finalize() throws Throwable {
    System.out.println("Amphibian finalize");
    if(DoBaseFinalization.flag)
      super.finalize();
  }
}
public class Frog extends Amphibian {
  Frog() {
    System.out.println("Frog()");
  }
  protected void finalize() throws Throwable {
    System.out.println("Frog finalize");
    if(DoBaseFinalization.flag)
      super.finalize();
  }
  public static void main(String[] args) {
    if(args.length != 0 && 
       args[0].equals("finalize"))
       DoBaseFinalization.flag = true;
    else
      System.out.println("Not finalizing bases");
    new Frog(); // Instantly becomes garbage
    System.out.println("Bye!");
    // Force finalizers to be called:
    System.gc();
  }
} ///:~

Output by typing "java Frog":
Not finalizing bases
Creating Characteristic is alive
LivingCreature()
Creating Characteristic has heart
Animal()
Creating Characteristic can live in water
Amphibian()
Frog()
Bye!
Frog finalize
finalizing Characteristic is alive
finalizing Characteristic has heart
finalizing Characteristic can live in water
Output by typing "java Frog finalize":
Creating Characteristic is alive
LivingCreature()
Creating Characteristic has heart
Animal()
Creating Characteristic can live in water
Amphibian()
Frog()
bye!
Frog finalize
Amphibian finalize
Animal finalize
LivingCreature finalize
finalizing Characteristic is alive
finalizing Characteristic has heart
finalizing Characteristic can live in water
The book explains that--
"When you use composition to create a new class, you never worry about finalizing the member objects of that class. Each member is an independent object, and thus is garbage collected and finalized regardless of whether it happens to be a member of your class. With inheritance, however, you must override finalize( ) in the derived class if you have any special cleanup that must happen as part of garbage collection. When you override finalize( ) in an inherited class, it�s important to remember to call the base-class version of finalize( ), since otherwise the base-class finalization will not happen."
Why "the base-class finalization will not happen" since Java API says "finalize()is called by the garbage collector on an object when garbage collection determines that there are no more references to the object"?
(Sorry for the long post!)

[Ray Stojonic]

The finalize() method of a given class is simply a method.
All classes are derived from the Object class, which has a finalize method.
This makes it possible for the JVM to 'always' call a finalize method on any object it is destroying.
If you override finalize() in a class, then when an object of that class is destroyed, it's finalize method will be called, not it's superclass'.
The action is much like any other method you've overridden, for instance, you wouldn't expect the equals() method of a class to include a call to it's parent class' equals() method, unless you explicitly designed your overridding method that way.

[Claire Yang]

Thanks for your reply, Ray. I made a mistake to think that when the Frog object was created, it's parents were also created, actually were the contents of the constructors executed and there should be only 4 objects on the heap--1 Frog & 3 Characteristic.