how to override compareTo(). If it's possible
<pre><code>public int compareTo(Object obj){
   Books bk = (Books) bk;
   return this.id - obj.id;
}
</code></pre>

<p>my code works fine when <code>id</code> is an <code>Integer</code> and when is a positive value. Is it possible for <code>id</code> to be a negative value? How to change the code for that?</p>
