code: static String humanReadableUnits(long bytes, DecimalFormat df) {
      if (bytes / ONE_GB > 0) {
        return df.format((float) bytes / ONE_GB) + " GB";
      } else if (bytes / ONE_MB > 0) {
        return df.format((float) bytes / ONE_MB) + " MB";
      } else if (bytes / ONE_KB > 0) {
        return df.format((float) bytes / ONE_KB) + " KB";
      } else {
        return bytes + " bytes";
      }
    }
docs: Returns <code>size</code> in human-readable units (GB, MB, KB or bytes).