Java常用字符串工具类 字符串智能截取（3）

 public static String subStrStart(String str, int end){   return subStr(str, 0, end); }   public static String subStrEnd(String str, int start){   return subStr(str, str.length()-start, str.length()); }   public static String subStr(String str, int length) throws RuntimeException{   if(str==null){     throw new NullPointerException("字符串为null");   }   int len = str.length();   if(len<Math.abs(length)){     throw new StringIndexOutOfBoundsException("最大长度为"+len+"，索引超出范围为:"+(len-Math.abs(length)));   }   if(length>=0){     return subStr(str, 0,length);   }else{     return subStr(str, len-Math.abs(length), len);   } }    public static String subStr(String str, int start, int end) throws RuntimeException{   if(str==null){     throw new NullPointerException("");   }   int len = str.length();   int s = 0;//记录起始索引   int e = 0;//记录结尾索引   if(len<Math.abs(start)){     throw new StringIndexOutOfBoundsException("最大长度为"+len+"，索引超出范围为:"+(len-Math.abs(start)));   }else if(start<0){     s = len - Math.abs(start);   }else if(start<0){     s=0;   }else{//>=0     s = start;   }   if(len<Math.abs(end)){     throw new StringIndexOutOfBoundsException("最大长度为"+len+"，索引超出范围为:"+(len-Math.abs(end)));   }else if (end <0){     e = len - Math.abs(end);   }else if (end==0){     e = len;   }else{//>=0     e = end;   }   if(e<s){     throw new StringIndexOutOfBoundsException("截至索引小于起始索引:"+(e-s));   }    return str.substring(s, e); }