code: @Override
      public int compareTo(Object object)
      {
      	if(!(object instanceof User))
      	{
      		return -1;
      		
      	}
      	User user = (User)object;
         
          return getLastName().compareTo(user.getLastName());
          
      }
docs: Compare user's by last name
  
  @see java.lang.Comparable#compareTo(java.lang.Object)