assertArgumentNotNull("reader", reader);
        this.reader = reader;
    }

    @Override
    public boolean hasNext() {
        if (line == EMPTY) {
            line = ReaderUtil.readLine(reader);
        }
        return line != null;
    }

    @Override
    public String next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }

     * @return 一行の文字列。終端に達した場合は{@literal null}
     * @see BufferedReader#readLine()
     */
    public static String readLine(final BufferedReader reader) {
        assertArgumentNotNull("reader", reader);

        try {
            return reader.readLine();
        } catch (final IOException e) {
            throw new IORuntimeException(e);
        }
    }

    /**