code: public void close() throws IOException {
          if (open) {
              if (residual.length() > 0) {
                  throw new IOException("Base64 error - data is not properly"
                          + "padded to 4-character groups.");
              }
              stream.close();
              open = false;
          }
      }
docs: Close the writer. If there are any residual characters at this point, the
  data stream was not a valid Base64 encoding.
  
  @throws IOException
  from the inner OutputStream.