import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sca = new Scanner(System.in);
        String sh;
        int id = 1, co,n;
        while (sca.hasNext()) {
            sh = sca.nextLine();
             n = sca.nextInt();//当第二次运行时这行会出错，想问下怎么解决？？？
            System.out.printf("Case %d:", id++);
            for (int i = 0; i < n; i++) {
                String s = sca.next();
                char c = s.charAt(0);
                co = 0;
                for (int j = 0; j < sh.length(); j++) {
                    if (sh.charAt(j) == c) {
                        co++;
                    }
                }
                System.out.print(c + "--" + co + " ");
            }
            System.out.println();
        }
    }

}

定义的变量需要实例化之后才能用
int id = 1, co,n;

给代码加点注释吧

实例化是啥意思？？

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sca = new Scanner(System.in);
        String sh;
        int id = 1, co,n;
        while (sca.hasNext()) {
            sh = sca.nextLine();  // 输入一行字符串
             n = sca.nextInt();// 正整数（当第二次运行时这行会出错，想问下怎么解决？？？）
            System.out.printf("Case %d:", id++);
            for (int i = 0; i < n; i++) {  
                String s = sca.next();
                char c = s.charAt(0); //接着循环输入N个字符，表示要统计的字符
                co = 0;
                for (int j = 0; j < sh.length(); j++) {
                    if (sh.charAt(j) == c) {
                        co++;     //统计在字符串中出现的次数
                    }
                }
                System.out.print(c + "--" + co + " ");  //输出结果
            }
            System.out.println();
        }
    }
       //输入样例：yang  you klo 
        //         2 y o
        // 输出样例：Case 1:y--2 o--2
}