Calendar calendar =  Calendar.getInstance();
calendar.set(year,month,date);
int dayOfYear = calendar.get(Calendar.DAY_OF_YEAR);

------解决方案--------------------
Java code

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    //获取键盘输入的年月日
    int year = input.nextInt();
    int month = input.nextInt();
    int date = input.nextInt();
    
    Calendar calendar = Calendar.getInstance();
    //设置calendar表示的日期，这里注意在Calendar中月是从0开始，也就是1月的时候month应设为0;
    calendar.set(year, month-1, date);
    int dayOfYear = calendar.get(Calendar.DAY_OF_YEAR);
    System.out.println(dayOfYear);
}