import java.util.Scanner;
public class Yearpanding {
    Scanner scan=new Scanner(System.in);
    public static void main(String[] args) {
        
        System.out.println("请输入年份");
        
        long year=scan.nextLong();
        if(year%4==0&&year%100!=0||year%400==0) {
            System.out.println(year+"年是闰年");
        }
        else {System.out.println(year+"年不是闰年");
    }
    }
}

看出来了！原来Scanner scan=new Scanner(System.in);这句应该放在主方法以内。