代码分析-哪位能给下面这段php代码写个详细分析，最好具体到每一行。

final class Flash {

<code>const FLASHES_KEY = '_flashes';private static $flashes = null;</code>

private function __construct() {
}

public static function hasFlashes() {    self::initFlashes();    return count(self::$flashes) > 0;}public static function addFlash($message) {    if (!strlen(trim($message))) {        throw new Exception('Cannot insert empty flash message.');    }    self::initFlashes();    self::$flashes[] = $message;}public static function getFlashes() {    self::initFlashes();    $copy = self::$flashes;    self::$flashes = array();    return $copy;}private static function initFlashes() {    if (self::$flashes !== null) {        return;    }    if (!array_key_exists(self::FLASHES_KEY, $_SESSION)) {        $_SESSION[self::FLASHES_KEY] = array();    }    self::$flashes = &$_SESSION[self::FLASHES_KEY];}

}
?>