[书摘]Cleanup Issues

class Characteristic {
  private String s;
  Characteristic(String s) {
    this.s = s;
    System.out.println("Creating Characteristic " + s);
  }
  protected void dispose() {
    System.out.println("finalizing Characteristic " + s);
  }
}

class Description {
  private String s;
  Description(String s) {
    this.s = s;
    System.out.println("Creating Description " + s);
  }
  protected void dispose() {
    System.out.println("finalizing Description " + s);
  }
}

class LivingCreature {
  private Characteristic p = new Characteristic("is alive");
  private Description t =
    new Description("Basic Living Creature");
  LivingCreature() {
    System.out.println("LivingCreature()");
  }
  protected void dispose() {
    System.out.println("LivingCreature dispose");
    t.dispose();
    p.dispose();
  }
}

class Animal extends LivingCreature {
  private Characteristic p= new Characteristic("has heart");
  private Description t =
    new Description("Animal not Vegetable");
  Animal() {
    System.out.println("Animal()");
  }
  protected void dispose() {
    System.out.println("Animal dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
}

class Amphibian extends Animal {
  private Characteristic p =
    new Characteristic("can live in water");
  private Description t =
    new Description("Both water and land");
  Amphibian() {
    System.out.println("Amphibian()");
  }
  protected void dispose() {
    System.out.println("Amphibian dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
}

public class Frog extends Amphibian {
  private Characteristic p = new Characteristic("Croaks");
  private Description t = new Description("Eats Bugs");
  public Frog() {
    System.out.println("Frog()");
  }
  protected void dispose() {
    System.out.println("Frog dispose");
    t.dispose();
    p.dispose();
    super.dispose();
  }
  public static void main(String[] args) {
    Frog frog = new Frog();
    System.out.println("Bye!");
    frog.dispose();
  }
} ///:~

 

====================Pay attention to the output order============================

OUTPUT:

E:/MyDoc/Exercises>javac Frog.java

E:/MyDoc/Exercises>java Frog
Creating Characteristic is alive
Creating Description Basic Living Creature
LivingCreature()
Creating Characteristic has heart
Creating Description Animal not Vegetable
Animal()
Creating Characteristic can live in water
Creating Description Both water and land
Amphibian()
Creating Characteristic Croaks
Creating Description Eats Bugs
Frog()
Bye!
Frog dispose
finalizing Description Eats Bugs
finalizing Characteristic Croaks
Amphibian dispose
finalizing Description Both water and land
finalizing Characteristic can live in water
Animal dispose
finalizing Description Animal not Vegetable
finalizing Characteristic has heart
LivingCreature dispose
finalizing Description Basic Living Creature
finalizing Characteristic is alive

 

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Each class in the hierarchy also contains a member objects of types Characteristic and Description, which must also be disposed. The order of disposal should be the reverse of the order of initialization, in case one subobject is dependent on another. For fields, this means the reverse of the order of declaration (since fields are initialized in declaration order). For base classes (following the form that’s used in C++ for destructors), you should perform the derived-class cleanup first, then the base-class cleanup. That’s because the derived-class cleanup could call some methods in the base class that require the base-class components to be alive, so you must not destroy them prematurely. From the output you can see that all parts of the Frog object are disposed in reverse order of creation. 

From this example, you can see that although you don’t always need to perform cleanup, when you do, the process requires care and awareness.