public class Test {

    public static void main(String[] args) throws Exception {

        Book b=new Book(true);

        b.checkIn();

        new Book(true);

        System.gc();

    }

}

class Book{

    boolean checkdOut = false;

    Book (boolean checkOut) {

        checkdOut = checkOut;

    }

    protected void checkIn() {

        checkdOut = false;

    }

    protected void finalize() {

        if (checkdOut) {

            System.out.println("Error : check Out");

        }

    }

}