截取字符串

   /***
    * 截取字符串
    *
    * @param start
    *            从那里开始，0算起
    * @param num
    *            截取多少个
    * @param str
    *            截取的字符串
    * @return
    */
   public static String getSubString(int start, int num, String str) {
       if (str == null) {
           return "";
       }
       int leng = str.length();
       if (start < 0) {
           start = 0;
       }
       if (start > leng) {
           start = leng;
       }
       if (num < 0) {
           num = 1;
       }
       int end = start + num;
       if (end > leng) {
           end = leng;
       }
       return str.substring(start, end);
   }