private Level findCorrespondingLevel(String levelName,
                                         SortedSet<Level> levels)
            throws CorrespondingLevelNotExistsException {
        if (null == levelName || "".equals(levelName)) {
            throw new CorrespondingLevelNotExistsException();
        }
        for (Level level : levels) {
            if (levelName.equals(level.getName())) {
                return level;
            }
        }
        throw new CorrespondingLevelNotExistsException();
    }