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);
        }
    }

    /**

    @Override
    public void readExternal(final ObjectInput in) throws IOException, ClassNotFoundException {
        final int num = in.readInt();
        mapTable = new Entry[num];
        listTable = new Entry[num];
        threshold = (int) (num * LOAD_FACTOR);
        final int size = in.readInt();
        for (int i = 0; i < size; i++) {
            final K key = (K) in.readObject();
            final V value = (V) in.readObject();

        }
    }

    @SuppressWarnings("unchecked")
    @Override
    public void readExternal(final ObjectInput s) throws IOException, ClassNotFoundException {
        final int size = s.readInt();
        header = new Entry(null, null, null);
        header.next = header;
        header.previous = header;
        for (int i = 0; i < size; i++) {
            addLast((E) s.readObject());
        }
    }