public class BigIntAdd {
 private int[] array;
 //声明一个数组 
 //计算大数相加的函数 
 public static String bigAdd(BigIntAdd fisrtNum, BigIntAdd secondNum) {
  String result = "";
  Boolean falg = false;
  //判断数组是否越界的标志 
  int[] arrayOne;
  int[] arrayTwo;
  int[] arrayThree = null;
  //把较长的字符串放到arrayOne数组中，因为要把计算结果放到arrayOne数组中 
  if (fisrtNum.instance().length >= secondNum.instance().length) {
   arrayOne = fisrtNum.instance();
   arrayTwo = secondNum.instance();
  } else {
   arrayOne = secondNum.instance();
   arrayTwo = fisrtNum.instance();
  }
  for (int i = 0; i < arrayTwo.length; i++) {
   if (arrayOne[i] + arrayTwo[i] < 10) {
    //不需要进位时 
    arrayOne[i] = arrayOne[i] + arrayTwo[i];
   } else if (arrayOne[i] + arrayTwo[i] >= 10) {
    //需要进位时 
    arrayOne[i] = arrayOne[i] + arrayTwo[i] - 10;
    if ((i + 1) < arrayOne.length) {
     arrayOne[i + 1] = arrayOne[i + 1] + 1;
     //下一位的值加1
    } else {
     //当arrayOne的长度不够时要把其复制到arrayThree中 
     falg = true;
     arrayThree = new int[arrayOne.length + 1];
     System.arraycopy(arrayOne, 0, arrayThree, 0,arrayOne.length);
     arrayThree[arrayOne.length] = 1;
     //把arrayThree最高位赋值1，等价于进位的1
    }
   }
  }
  //把数组arrayThree中的元素全部的拼接为字符串 
  if (falg) {
   for (int i : arrayThree) {
    result += i;
   }
  } else {
   for (int i : arrayOne) {
    result += i;
   }
  }
  //把结果反转返回 
  return new StringBuffer(result).reverse().toString();
 }
 //初始化数组方法 
 private int[] instance() {
  return array;
 }
 //初始化构造方法 
 public BigIntAdd(String num) {
  StringBuffer sb = new StringBuffer(num);
  String string = sb.reverse().toString();
  //实现字符串反转，便于计算 
  array = new int[string.length()];
  //把字符串转化为数组 
  for (int i = 0; i < string.length(); i++) {
   array[i] = Integer.valueOf(string.substring(i, i + 1));
  }
 }
 public static void main(String [] args){
  String result=BigIntAdd.bigAdd(new BigIntAdd("5211111111"), new BigIntAdd("4099999999"));
  System.out.println(result);
 }
}