/**
* Checks if current position is valid
*
* @return boolean The return value will be casted to boolean and then evaluated.
* Returns true on success or false on failure.
*/
public function valid()
{
return !is_null($this->bookShelf->getBookAt($this->index));
}
/**
* @return Book
*/
public function next()
{
$book = $this->bookShelf->getBookAt($this->index);
$this->index++;
return $book;
}
