java用eclipse编写程序：用*输出一个三角的框框

public static void main(String[] args) {
    Scanner scan=new Scanner(System.in);
    System.out.print("请输入层数:");
    int a=scan.nextInt();
    System.out.println();
    String s=" ";
    for(int i=a;i>=1;i--){
        for(int j=0;j<=i;j++){
            System.out.print(s);
        }
        System.out.print("*");
        if(i!=1) {
            for (int k = 2 * a - 1; k > 2 * i; k--) {
                System.out.print(s);
            }
            if (i != a) {
                System.out.print("*");
            }
        }else{
            for (int k = 2 * a - 1; k >= 2 * i; k--) {
                System.out.print("*");
            }
        }
        System.out.println();
    }
}