BigInteger工具类

import java.math.BigInteger;
import java.util.*;
public class Main{
    public static void main(String[]args){
        Scanner sc=new Scanner(System.in);
        String s1=sc.next();
        String s2=sc.next();
        BigInteger a=new BigInteger(s1);
        BigInteger b=new BigInteger(s2);
        System.out.println(a.add(b).toString());
    }
}

模拟高精度

import java.math.BigInteger;
import java.util.*;
public class Main{
    public static void main(String[]args){
        Scanner sc=new Scanner(System.in);
        String s1=sc.next();
        String s2=sc.next();
        int i=s1.length()-1,j=s2.length()-1;
        StringBuilder sb=new StringBuilder();
        int carry=0;
        while(i>=0||j>=0||carry!=0){
            if(i>=0) carry+=s1.charAt(i--)-'0';
            if(j>=0) carry+=s2.charAt(j--)-'0';
            sb.append(carry%10);
            carry/=10;
        }
        System.out.println(sb.reverse().toString());
    }
}