java 大数加起来 （StringBuffer）

import java.util.Map;

//StringBuffer 有.append方法
class test{
    public static void main(String[] args){
        String str1 = "19";
        String str2 = "2";
        System.out.println(add(str1,str2));

    }

    private static String add(String str1,String str2){
        if(str1==null||"".equals(str1)){
            return str2;
        }
        if(str2==null||"".equals(str2)){
            return str1;
        }
        int maxlength = Math.max(str1.length(),str2.length());
        StringBuffer result = new StringBuffer(maxlength+1);

        //翻转两个字符串
        str1 = new StringBuffer(str1).reverse().toString();
        str2 = new StringBuffer(str2).reverse().toString();

        System.out.println(str1);
        System.out.println(str2);

        int carry=0;
        int currentNum = 0;
        int minlength = Math.min(str1.length(),str2.length());
        int i;
        for(i = 0;i<minlength;i++){ //只能小于minlength 不然会报错

            currentNum = str1.charAt(i)+str2.charAt(i) - 2*'0' +carry;
            System.out.println(currentNum);
            carry =currentNum/10;
            System.out.println(carry);
            currentNum %=10;
            System.out.println(currentNum);
            result.append(String.valueOf(currentNum));

        }
        if(str1.length()<str2.length()){
            str1 = str2;
        }
        for(;i<str1.length();i++){
            currentNum = str1.charAt(i)-'0'+carry;
            carry =currentNum/10;
            currentNum %=10;
            result.append(String.valueOf(currentNum));

        }
        if(carry>0){
            result.append(String.valueOf(carry));
        }

//        for(;i<str1.length();i++){
//
//        }
        return result.reverse().toString();

    }
}