charAt，与字符’0’,做差值。

import java.util.Scanner;public class Main3 {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);String num = scanner.nextLine();char[] arr = num.toCharArray();int sum = 0;for(char c : arr){int tmp = c - '0';sum+=tmp;}System.out.println(sum);}
}

第二种解法

import java.util.Scanner;public class Main10 {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);int i = scanner.nextInt();
//        System.out.println(i);int sum = 0;//各位数字while(i > 0){int tmp = i%10;i = i /10;sum+=tmp;}System.out.println(sum);}
}