Java C编译通过正常，但是Java运行不出来是为什么

cmd 命令行编译javac Demo.java
java Demo之后，又出来让我编译java Demo
运行不成功

img

import java.util.Arrays;
import java.util.Scanner;

public class Demo {
private final int num;
private final int[] q;

public Demo(int num) {
    this.num = num;
    this.q = new int[num + 1];
}

public void n_queens(int k) {
    if (k > num)
        System.out.println(Arrays.toString(q).substring(4, q.length * 3 - 1));
    else {
        for (int j = 1; j <= num; j++) {
            boolean isSafe = true;

            for (int i = 1; i < k; i++) {
                if (q[i] == j || Math.abs(i - k) == Math.abs(q[i] - j)) {
                    isSafe = false;
                    break;
                }
            }

            if (isSafe) {
                q[k] = j;
                n_queens(k + 1);
            }
        }
    }
}

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    new Demo(n).n_queens(1);
}

}

你程序代码是

你贴的代码是demo的你执行的却是另外一个类，这两个类内容一样？