以字节为单位，支持汉字双字节的subString

 

public static String subStringByByte(String str, int beginIndex, int endIndex) {
	if (beginIndex < 0) throw new StringIndexOutOfBoundsException(beginIndex);
	if (beginIndex > endIndex) throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
	if (beginIndex == endIndex) return "";
	int byteLength = 0;
	String returnString = "";
	for (int i = 0; byteLength < endIndex && i < str.length(); i++) {
		char c = str.charAt(i);
		byteLength += (31 < c && c < 128) ? 1 : 2;
		if (byteLength > beginIndex)
			returnString += c;
	}

	return returnString;
}