JAVA实现的大整数加法

public class Test{ 
  public static String addBigNum(String str1,String str2){
     //找出两字符串的长短，方便后边引用；
     String longer = str1.length() > str2.length()? str1 : str2;
     String shorter = str1.length() < str2.length()? str1 : str2;
     int[] a = new int[longer.length()];
     int temp = shorter.length();
     int rise = 0;
     int m , n ;
     StringBuffer sb = new StringBuffer();
     for(int i = a.length - 1 ; i >= 0 ; i--){
       if((i - a.length + temp) >= 0 ){
         m = Integer.parseInt(String.valueOf(longer.charAt(i)));
         n = Integer.parseInt(String.valueOf(shorter.charAt(i - a.length + temp)));
         a[i] = (m + n  + rise) % 10;
         rise = (m + n +  rise) / 10;
       } else {
          a[i] =  (Integer.parseInt(String.valueOf(longer.charAt(i))) + rise) % 10;
          rise =  (Integer.parseInt(String.valueOf(longer.charAt(i))) + rise)  / 10;
       }
     }
     for (int i = 0; i < a.length; i++) {
   sb.append(String.valueOf(a[i]));
   }
     return new String(sb);
   }
}
//没有考虑负整数的情况，需要的话需对代码做些调整。