code: public static String next(boolean withHyphen) {
  		byte[] array = nextBytes();
  		final StringBuilder buf = new StringBuilder(withHyphen ? 26 : 24);
  		int t;
  		for (int i = 0; i < array.length; i++) {
  			if (withHyphen && i % 4 == 0 && i != 0) {
  				buf.append("-");
  			}
  			t = array[i] & 0xff;
  			if (t < 16) {
  				buf.append('0');
  			}
  			buf.append(Integer.toHexString(t));
  
  		}
  		return buf.toString();
  	}
docs: 获取一个objectId
  
  @param withHyphen 是否包含分隔符
  @return objectId