Java（使用if-else语句查找未来日期）

我有个问题想不通，谢谢你： 编写一个程序，提示用户输入一周中今天的整数（星期日为0，星期一为1，…星期六为6）。还提示用户输入未来日期的后天数，并显示一周的未来日期。以下是运行示例： 输入今天：1 输入从今天起经过的天数：3 今天是星期一，明天是星期四 我的尝试是：

Scanner input = new Scanner(System.in);

System.out.print("Enter today's day (0 - 6):  ");
int day = input.nextInt();

System.out.print("Enter the number of days elapsed since today:  ");
int elapsed = input.nextInt();

if(day == 0)
{
    System.out.println("Sunday");
}
if(day   == 1)
{
    System.out.println("Monday");
}
if(day ==  2)
{
    System.out.println("Tuesday");
}
if(day  == 3)
{
    System.out.println("Wednesday");
}
if(day  ==  4)
{
    System.out.print("Thursday");
}
if(day ==  5)
{
    System.out.print("Friday");
}
if(day  == 6)
{
    System.out.print("Saturday");
}

System.out.print("Today is " + day + " and the future day is " + elapsed);

---

通过使用数组来存储日期名称，您可以做得更好

String[] dayNames = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

现在您可以使用用户输入作为索引

int nameIndex = //... get input
//validate input
//dayNames[nameIndex] is the day of the week

现在获取要添加的天数的输入

int numDays = //...get input

然后再加上那么多天来计算未来一周中某一天的指数

int futureNameIndex = nameIndex; //start with entered day of week index
for(int i=0; i<numDays; i++) {
    futureNameIndex++; //increment by 1 for numDays times
    if(futureNameIndex == dayNames.length) { //if the index reaches lenght of the array
        futureNameIndex = 0;                 //reset it to 0
    }
}

---

由于需要两次day number to day字符串，请将其放在单独的函数中。我想向你们展示几种可能的方法。版本1，基本、简单、整洁：

// isolate the daynumber --> daystring in a function, that's tidier
String dayFor (int daynumber) {
    String dayAsString = "ERROR";  // the default return value
    switch(dayNumber) {
        case 0 :
            dayAsString = "Sunday";
            break;
        case 1 :
            dayAsString = "Monday";
            break;
        // and so on, until
        case 6 :
            dayAsString = "Saturday";
            break;
     }
     return dayAsString;
}

使用数组而不是switch语句的更短版本：

String dayFor (int daynumber) {
    String dayStrings[] = new String[]{"Sunday","Monday", .... "Saturday"};
    // notice that daynumber's modulo is used here, to avoid index out of
    // bound errors caused by erroneous daynumbers:
    return dayStrings[daynumber % 7];
}

尝试以下函数的方法可能很有诱惑力，其中每个案例都会立即返回，但不鼓励使用多个return语句。只是在这里展示它，因为它在技术上是可能的，有时你会遇到它

String dayFor (int daynumber) {
    switch(dayNumber) {
        case 0 :
            return "Sunday";
        case 1 :
            return "Monday";

        // and so on, until

        case 6 :
            return "Saturday";
     }
     // normally not reached but you need it because the compiler will
     // complain otherwise anyways.
     return "ERROR";
}

在这相当长的介绍之后，主要功能变得简短。输入后，您只需要：

// present day + elapsed modulo 7 = the future day
int future = (day + elapsed) % 7;
System.out.print("Today is " + dayFor(day) + " and the future day is " + dayFor(future) );

---

别忘了添加代码来检查输入

该问题给出的天数范围为0-6天，而不是1-7天（常规）。现在，例如，如果今天是1（星期一），并且从今天开始的日期是3，那么这一天应该是星期四。由于此问题的起始日期包括在内，因此产生的日期将在1（星期一）、2、3（星期三）之后，即星期四

让我们举一个例子，并将其应用于下面的代码

日=1

daysElased=3

否则，如果（天>0&&天<7），情况如何

{

sum=1（天）+3（天选择）；//sum=4

}

如果总和在0-6范围内，则可以根据每天创建每个If案例。在上述情况下，总和小于6，因此它将有自己的if子句。如果总和更大，例如，天=1和daysElapsed=6，那么总和=1（天）+6（daysElapsed）=7

在这种情况下，它将匹配子句if（sum>6），然后sum=sum%7=7%7=0=Sunday。这意味着从1（星期一）到6（星期六）的日期已经过去，因此这一天将是星期日（0）

if（day==0）//如果输入的当前日期为零（0表示星期日）
{
sum=daysElapsed；//用户将输入daysElapsed
}
else if（天>0&&day<7）//如果当前天>0但<7（1-6天）
{
总和=天+天选择的天数；//
}
if（sum>6）//if 0据我所知，这个问题来自《Java编程入门》一书。当你问这个问题时，你对方法、循环、数组等没有任何知识，所以我只使用选择

在这里，当我试图用更好的方法求解时，我找不到任何方法，因为我们不能使用非常有用的数组或更好的方法。这就是为什么这个问题在书中有点多余

您真的不应该使用if语句，因为在这种情况下switch更好

    System.out.println("Enter today's number (0 for Sunday, 1 for Monday...) :");
    int todayNo = in.nextInt();

    System.out.println("Enter the number of days elapsed since today:");
    int elapsedDay = in.nextInt();

    int futureDay = (todayNo + elapsedDay) % 7;

    switch (todayNo) {
        case 0:
            System.out.print("Today is Sunday and");
            break;
        case 1:
            System.out.print("Today is Monday and");
            break;
        case 2:
            System.out.print("Today is Tuesday and");
            break;
        case 3:
            System.out.print("Today is Wednesday and");
            break;
        case 4:
            System.out.print("Today is Thursday and");
            break;
        case 5:
            System.out.print("Today is Friday and");
            break;
        case 6:
            System.out.print("Today is Saturday and");
            break;

    }

    switch (futureDay) {
        case 0:
            System.out.print(" the future day is Sunday.");
            break;
        case 1:
            System.out.print(" the future day is Monday.");
            break;
        case 2:
            System.out.print(" the future day is Tuesday.");
            break;
        case 3:
            System.out.print(" the future day is Wednesday.");
            break;
        case 4:
            System.out.print(" the future day is Thursday.");
            break;
        case 5:
            System.out.print(" the future day is Friday.");
            break;
        case 6:
            System.out.print(" the future day is Saturday.");
            break;

    }

在这里，您可能唯一不知道的是System.out.print（）；。与System.out.println（）的唯一区别；用这种方法，它不会打印在新行上，而是打印在同一行上，这就是我们需要的。修改它以更好地理解。
这个问题来自Y.Daniel Liang的《Java编程入门》一书。除了使用字符串类型之外，我相信这将在下一章中介绍；我为本练习编写的解决方案只使用了到目前为止所教的内容

package javaapplication2;

import java.util.Scanner;
public class JavaApplication2 {


    public static void main(String[] args) {

    int day, eday, fday;
        String str, str1;
        Scanner S = new Scanner(System.in);
        System.out.println("Enter today's day: ");
        day = S.nextInt();
        System.out.println("Enter the number of days elapsed since today: ");
        eday = S.nextInt();
        if (day == 0) {
            str = "Sunday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 1) {
            str = "Monday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 2) {
            str = "Tuesday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 3) {
            str = "Wednesday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 4) {
            str = "Thursday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 5) {
            str = "Friday";
            System.out.print("Today is " +str + " and ");
        }
        else if (day == 6) {
            str = "Saturday";
            System.out.print("Today is " +str + " and ");
        }

       fday = day + eday;

       if (fday % 7 == 0) {
            str1 = "Sunday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 1) {
            str1 = "Monday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 2) {
            str1 = "Tuesday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 3) {
            str1 = "Wednesday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 4) {
            str1 = "Thursday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 5) {
            str1 = "Friday";
            System.out.print("Future day is " +str1);
        }
        else if (fday % 7 == 6) {
            str1 = "Saturday";
            System.out.print("Future day is " +str1);
        }

    }

import java.util.Scanner;

public class Exercise_03_06 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter today's day: ");
        int todaysDay = input.nextInt();

        System.out.print("Enter the number of days elapsed since today: ");
        int elapsedDays = input.nextInt();

        int futureDay = (todaysDay + elapsedDays) % 7;
        String day_of_week = "";

        switch (todaysDay) {
            case 0: day_of_week = "Sunday"; break;
            case 1: day_of_week = "Monday"; break;
            case 2: day_of_week = "Tuesday"; break;
            case 3: day_of_week = "Wednesday"; break;
            case 4: day_of_week = "Thursday"; break;
            case 5: day_of_week = "Friday"; break;
            case 6: day_of_week = "Saturday";
        }

        switch (futureDay) {
            case 0:
                System.out.println("Today is " + day_of_week + " and the future day is Sunday."); break;
            case 1:
                System.out.println("Today is " + day_of_week + " and the future day is Monday."); break;
            case 2:
                System.out.println("Today is " + day_of_week + " and the future day is Tuesday."); break;
            case 3:
                System.out.println("Today is " + day_of_week + " and the future day is Wednesday."); break;
            case 4:
                System.out.println("Today is " + day_of_week + " and the future day is Thursday."); break;
            case 5:
                System.out.println("Today is " + day_of_week + " and the future day is Friday."); break;
            case 6:
                System.out.println("Today is " + day_of_week + " and the future day is Saturday.");
        }
    }
}

输出：

Enter today's day: 0
Enter the number of days elapsed since today: 31
Today is Sunday and the future day is Wednesday.

注意事项：

Enter today's day: 0
Enter the number of days elapsed since today: 31
Today is Sunday and the future day is Wednesday.


第一个switch语句为变量day_of_week指定一个字符串类型的day，该变量稍后用于打印“今天”

要获得未来的一天，您必须找到今天和经过的天数之和除以7的剩余部分

最后一个switch语句“匹配”一个案例编号，该编号与存储在futureDay变量中的编号相同（通过执行上述数学运算获得）

if（day==0）System.out.println（“星期日”）；如果（天==1）System.out.println（“星期一”）；如果（天==2）System.out.println（“星期二”）；如果（天==3）System.out.println（“星期三”）；如果（天==4）系统输出打印（“星期四”）；如果（天==5）系统输出打印（“星期五”）；如果（天==6）系统输出打印（“星期六”）；我认为是第%7天，但我在哪里可以使用它来编辑你的问题。如果用户在周一运行应用程序并输入其他内容（例如周五），该怎么办？@fvu：你说得对。我在你的评论中看到了这一点，所以刚刚添加了另一种（我想是天真的）方法，这肯定比你建议的
%
操作要详细得多。同意。如果你能为你的答案和逻辑提供一些解释，而不仅仅是简单的解释，那就太好了code@hawk：这些评论很有帮助。你能详细说明一下这是如何回答这个问题的吗？
Enter today's day: 0
Enter the number of days elapsed since today: 31
Today is Sunday and the future day is Wednesday.