程序分析：以3月5日为例，应该先把前两个月的加起来，然后再加上5天即本年的第几天，特殊情况，闰年且输入月份大于3时需考虑多加一天。

import java.util.Scanner;

public class test {

	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		System.out.println("请输入年、月、日：");
		int year = scanner.nextInt();
		int month = scanner.nextInt();
		int day = scanner.nextInt();
		int flag = 0;
		if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
			flag = 1;
		else
			flag = 0;

		switch (month - 1) {
		case 11:
			day += 30;
		case 10:
			day += 31;
		case 9:
			day += 30;
		case 8:
			day += 31;
		case 7:
			day += 31;
		case 6:
			day += 30;
		case 5:
			day += 31;
		case 4:
			day += 30;
		case 3:
			day += 31;
		case 2:
			day += 28 + flag;
		case 1:
			day += 31;
		case 0:
			day += 0;
			break;
		default:
			System.out.println("请输入正确的月份。");
			break;
		}

		System.out.println("这一天是这一年的第" + day + "天");

	}

}

示例输出：

另一个方法：

import java.util.Scanner;

public class test {

	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		System.out.print("请输入当前日期（年-月-日）:");
		int year = scan.nextInt();
		int month = scan.nextInt();
		int date = scan.nextInt();
		scan.close();
		System.out.println("今天是"+year+"年的第"+analysis(year,month,date)+"天");
	}
	//判断天数
	private static int analysis(int year, int month, int date){
		int n = 0;
		int[] month_date = new int[] {0,31,28,31,30,31,30,31,31,30,31,30};
		if((year%400)==0 || ((year%4)==0)&&((year%100)!=0))
		  month_date[2] = 29;
		for(int i=0;i<month;i++)
		  n += month_date[i];
		return n+date;
	}
}

输出：