generate file name

private String generate() {

    int len = 6;

    int rnum = (int) Math.floor(Math.random() * Math.pow(10, len));

    StringBuffer sb = new StringBuffer(String.valueOf(rnum));

    while (sb.toString().length() < len) {

        sb.append("0");

    }

    return System.currentTimeMillis() + "_" + sb.toString();

}