OnJava8-Examples/housekeeping/TerminationCondition.java

// housekeeping/TerminationCondition.java
// (c)2020 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Using finalize() to detect an object that
// hasn't been properly cleaned up
import onjava.*;

class Book {
  boolean checkedOut = false;
  Book(boolean checkOut) {
    checkedOut = checkOut;
  }
  void checkIn() {
    checkedOut = false;
  }
  @SuppressWarnings("deprecation")
  @Override
  public void finalize() {
    if(checkedOut)
      System.out.println("Error: checked out");
    // Normally, you'll also do this:
    // super.finalize(); // Call the base-class version
  }
}

public class TerminationCondition {
  public static void main(String[] args) {
    Book novel = new Book(true);
    // Proper cleanup:
    novel.checkIn();
    // Drop the reference, forget to clean up:
    new Book(true);
    // Force garbage collection & finalization:
    System.gc();
    new Nap(1); // One second delay
  }
}
/* Output:
Error: checked out
*/