重写java.lang.String IndexOf()方法，实现对字符串以ASCII规则截取

    /**
     * 根据元数据和目标ascii位数截取字符串，失败返回-1
     * @param sourceStr    元数据字符串    
     * @param endIndex    截取到第几位
     * @return 结果字符串
     */
    public static String indexOf(String sourceStr,int endIndex){
        int length = 0;
        StringBuilder result = new StringBuilder();
        List<String> resultList = new ArrayList<String>(); 
        for(int i = 0; i < sourceStr.length(); i++){
            int ascii = Character.codePointAt(sourceStr,i);
            if(ascii >= 0 && ascii <=255){
                length++;
                resultList.add(String.valueOf(ascii));
            }else{
                length+=2;
                resultList.add(String.valueOf(ascii));
            }
            if(length>0){
                if((length==sourceStr.length() && length <= endIndex) || length == endIndex || length == endIndex-1){
                    for (String string : resultList) {
                        result.append(asciiToString(string));
                    }
                    return result.toString();
                }
            }
        }
        //return String.valueOf("-1");
        /**
         * 根据业务要求，如果传进来的参数为空值的时候默认返回空字符串
         */
        return "";
    }
    
    /**
     * 将ascii码转换为utf-8
     * @param value ascii编码 多个以,号分割
     * @return 结果字符串
     */
    public static String asciiToString(String value) {
        StringBuffer sbu = new StringBuffer();
        String[] chars = value.split(",");
        for (int i = 0; i < chars.length; i++) {
            sbu.append((char) Integer.parseInt(chars[i]));
        }
        return sbu.toString();
    }