下面这种方法是歪果仁写的，来自于stackoverflow

public static String formatFileSize(long size) {// 转换文件大小
		if (size < = 0)
			return "0";
		final String[] units = new String[] { "B", "kB", "MB", "GB", "TB" };
		int digitGroups = (int) (Math.log10(size) / Math.log10(1024));
		return new DecimalFormat("#.00").format(size / Math.pow(1024, digitGroups)) + " " + units[digitGroups];
}

要转化的size为25 * 1024 * 1024 * 1024，运行100000次耗时300+

下面这种方法是国内搜索常见到的：

public static String formatFileSize(long size) {// 转换文件大小
		 DecimalFormat df = new DecimalFormat("#.00");
		 String fileSizeString = "";
		 if (size < 1024) {
		 fileSizeString = df.format((double) size) + "B";
		 } else if (size < 1048576) {
		 fileSizeString = df.format((double) size / 1024) + "K";
		 } else if (size < 1073741824) {
		 fileSizeString = df.format((double) size / 1048576) + "M";
		 } else {
		 fileSizeString = df.format((double) size / 1073741824) + "G";
		 }
		 return fileSizeString;
	}

虽然代码行数比较多，如果要转化的size同样为25 * 1024 * 1024 * 1024，运行100000次耗时280+

各位自己挑着用