class Book {
    boolean checkedOut = false;
    Book (boolean checkOut) {
    checkedOut = checkOut;
    }
void checkIn () {
    checkedOut = false;
}
protected void finalize () {
    if (checkedOut)
        System.out.println("Error: checked out");
    //Normally,you'll also do this;
    //supper.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();
    }
}

书该具有的属性和行为！

这个讲的是java的垃圾回收