求助 代码能运行 但是运行时有问题 输入数据不能输出结果 要怎么修改呢

import java.util.Scanner;

public class Test {
    /*实现一个函数，可统计任一整数中某个数字出现的次数。例如21252中，2出现了3次，则该函数应该返回3。int countDigit (int n, int d); */
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入一个整数:");
        String str = sc.nextLine();
        System.out.print("请输入所求数字:");
        String str2 = sc.nextLine();
        
        String n = str2.trim().charAt(0)+"";
        
        int count = 0;
        for(;;) {
            if(str.contains(n)) {
                str = str.replaceFirst(n,",,,");
                count++;
            }else {
                break;
            }
        }
        System.out.println("数字:"+str2+"出现了:"+count+"次");
        sc.close();
    }
}

import java.util.Scanner;
/*给定两个均不超过9的正整数a和n，要求编写函数求a+aa+aaa++⋯+aa⋯a（n个a）之和。函数接口定义：int fn( int a, int n ); int SumA( int a, int n );

其中函数fn须返回的是n个a组成的数字，比如，fn(2,3)返回的是222；
SumA返回要求的和，比如sumA（2,3）返回2 + 22 + 222。*/

public class Test {
    @SuppressWarnings("resource")
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("请输入a:");
        int a = sc.nextInt();
        System.out.print("请输入n:");
        int n = sc.nextInt();

        String str1 = fn(a, n);
        String str2 = sumA(a, n);

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

    public static String sumA(int a, int n) {
        String str = "";
        for (int i = n; i > 0; i--) {
            for (int j = 0; j < n; j++) {
                str = ("" + a) + str;
            }
            n--;
            str = "+" + str;
        }
        return str;
    }

    public static String fn(int a, int n) {
        String str = "";
        for (int i = 0; i < n; i++) {
            str += ("" + a);
        }
        return str;
    }
}