编写一个程序，从键盘上输入年、月、日数字，请判断并输出这个日期是今年的第几天。

这个程序可以使用switch-case结构或者if-else结构来实现。下面是使用switch-case结构的代码示例：

import java.util.Scanner;

public class DayOfYear {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.println("请输入year：");
        int year = scan.nextInt();
        System.out.println("请输入month：");
        int month = scan.nextInt();
        System.out.println("请输入day：");
        int day = scan.nextInt();
        int sumdays = 0;
        switch (month) {
            case 12:
                sumdays += 30;
            case 11:
                sumdays += 31;
            case 10:
                sumdays += 30;
            case 9:
                sumdays += 31;
            case 8:
                sumdays += 31;
            case 7:
                sumdays += 30;
            case 6:
                sumdays += 31;
            case 5:
                sumdays += 30;
            case 4:
                sumdays += 31;
            case 3:
                if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
                    sumdays += 29;
                } else {
                    sumdays += 28;
                }
            case 2:
                sumdays += 31;
            case 1:
                sumdays += day;
        }
        System.out.println(year + "年" + month + "月" + day + "日是当年的第" + sumdays + "天");
    }
}

使用if-else结构的代码示例：

import java.util.Scanner;

public class DayOfYear {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        System.out.println("请输入year：");
        int year = scan.nextInt();
        System.out.println("请输入month：");
        int month = scan.nextInt();
        System.out.println("请输入day：");        int day = scan.nextInt();
        int sumdays = 0;
        for (int i = 1; i < month; i++) {
            switch (i) {
                case 12:
                case 10:
                case 8:
                case 7:
                case 5:
                case 3:
                    sumdays += 31;
                    break;
                case 11:
                case 9:
                case 6:
                case 4:
                    sumdays += 30;
                    break;
                case 2:
                    if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
                        sumdays += 29;
                    } else {
                        sumdays += 28;
                    }
                    break;
            }
        }
        sumdays += day;
        System.out.println(year + "年" + month + "月" + day + "日是当年的第" + sumdays + "天");
    }
}