String.prototype.padLeft =
Number.prototype.padLeft = function(total, pad) {
  return (Array(total).join(pad || 0) + this).slice(-total);
}

//補(bǔ)位
"X".padLeft(5, "Y"); //返回：YYYYX
(50).padLeft(5);   //返回：00050

var x = 1;
x.padLeft(2);     //返回：01

//截位
(123).padLeft(2);   //返回：23

/**
 * 取子字符串
 * @param oriStr  原字符串
 * @param beginIndex 取子串的起始位置
 * @param len  取子串的長(zhǎng)度
 * @return 子字符串
 */
 public String subString(String oriStr,int beginIndex,int len){
 int strlen = oriStr.length();
 beginIndex = beginIndex -1;
 if(strlen <= beginIndex){
  System.out.println("out of " + oriStr + "'s length, please recheck!");
 }else if(strlen <= beginIndex+len){
  str = oriStr.substring(beginIndex);
 }else{
  str = oriStr.substring(beginIndex, beginIndex+len);
 }
 return str;
 }
 
 /**
 * 右補(bǔ)位，左對(duì)齊
 * @param oriStr 原字符串
 * @param len 目標(biāo)字符串長(zhǎng)度
 * @param alexin 補(bǔ)位字符
 * @return 目標(biāo)字符串
 */
 public String padRight(String oriStr,int len,char alexin){
 int strlen = oriStr.length();
 if(strlen < len){
  for(int i=0;i<len-strlen;i++){
  str = str+alexin;
  }
 }
 str = str + oriStr;
 return str;
 }
 
 /**
 * 左補(bǔ)位，右對(duì)齊
 * @param oriStr 原字符串
 * @param len 目標(biāo)字符串長(zhǎng)度
 * @param alexin 補(bǔ)位字符
 * @return 目標(biāo)字符串
 */
 public String padLeft(String oriStr,int len,char alexin){
 int strlen = oriStr.length();
 if(strlen < len){
  for(int i=0;i<len-strlen;i++){
  str = str+alexin;
  }
 }
 str = oriStr + str;
 return str;
 }