一个非常简单的 java 代码（获取工作日的日期），但是我得到的结果很奇怪，为什么？

Question

提问by Mellon

I have the following java code to get the date of a specific week day:

我有以下 java 代码来获取特定工作日的日期：

SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.set(Calendar.YEAR, 2010);
cal.set(Calendar.WEEK_OF_YEAR, 37); //week 37 of year 2010
cal.set(Calendar.DAY_OF_WEEK, Calendar.THURSDAY);
System.out.println("date="+sdf.format(cal.getTime()));

When I put this code in a main(String[] args)method, like the following:

当我将此代码放入main(String[] args)方法中时，如下所示：

import java.util.*;
import java.lang.*;
import java.text.SimpleDateFormat;

public class test{
    public static void main(String[] args){

    /** get dates from a known week ID **/
    SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
    Calendar cal = Calendar.getInstance();
    cal.setTime(new Date());
    cal.set(Calendar.YEAR, 2010);
    cal.set(Calendar.WEEK_OF_YEAR, 37);
    cal.set(Calendar.DAY_OF_WEEK, Calendar.THURSDAY);
    System.out.println("date="+sdf.format(cal.getTime()));
    }
}

and run it, I get the correct result which is date=09/09/2010. There is no problem.

并运行它，我得到正确的结果，即date=09/09/2010. 没有问题。

HOWEVER...

然而...

When I put this code in a function of a Class, like the following:

当我将此代码放在类的函数中时，如下所示：

Public Class MyService{

  MyService(){}
  ...
  ...
  public String getDateOfWeekDay(int weekId, int year, int weekDay){
      //weekId = 37; year=2010; weekDay = Calendar.THURSDAY

      SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
      Calendar cal = Calendar.getInstance();
      cal.setTime(new Date());
      cal.set(Calendar.YEAR, year);
      cal.set(Calendar.WEEK_OF_YEAR, weekId);
      cal.set(Calendar.DAY_OF_WEEK, weekDay);

      //I am developing android app. so I use Log to printout
      logPrinter.println("date="+sdf.format(cal.getTime())); 

      return  sdf.format(cal.getTime());    
  }

}

in another class MainClass, MainClass will invoke this service function like following:

在另一个类 MainClass 中，MainClass 将调用此服务函数，如下所示：

Public Class MainClass{

MyService myService = new MyService();
myService.getDateOfWeekDay(37,2010,Calendar.THURSDAY);

}

But the result returned is always the date of the current week's thursday (date=14/10/2010), not the Thursday of the week which I specified (week 37, year 2010, Thursday). WHY???? I use exactly the same java code to get the date of the specific week day, only used it in different ways, why the result is different???? I can not understand this...Anybody can explain to me??

但返回的结果始终是当周的星期四 (date=14/10/2010) 的日期，而不是我指定的那一周的星期四（2010 年第 37 周，星期四）。为什么？？？？我使用完全相同的java代码获取特定工作日的日期，只是使用方式不同，为什么结果不同？？？？我无法理解这一点……谁能给我解释一下？？

Answer 1

回答by duffymo

Yes, JodaTime is wonderful, but I'll bet you'd rather know what's wrong with your stuff.

是的，JodaTime 很棒，但我敢打赌你更想知道你的东西有什么问题。

Adding another JAR dependency might be a problem for a mobile device.

添加另一个 JAR 依赖项对于移动设备来说可能是个问题。

That method isn't good, IMO. The name is a misnomer, and it's doing too many things. You're creating a String, not a Date, and you're printing to a log. I'd recommend returning a Date and let clients worry about whether they want to turn it into a String. Logging is a cross-cutting concern.

那个方法不好，IMO。这个名字用词不当，它做的事情太多了。您正在创建一个字符串，而不是一个日期，并且您正在打印到日志。我建议返回一个 Date 并让客户担心他们是否要将其转换为 String。日志记录是一个跨领域的问题。

Here's a class (and a JUnit test) that works. See what's different:

这是一个有效的类（和一个 JUnit 测试）。看看有什么不同：

package util;

import java.util.Calendar;
import java.util.Date;

public class DateUtils
{
    public static Date getDateOfWeekDay(int weekId, int year, int weekDay)
    {
        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.WEEK_OF_YEAR, weekId);
        cal.set(Calendar.DAY_OF_WEEK, weekDay);
        cal.set(Calendar.HOUR, 0);
        cal.set(Calendar.MINUTE, 0);
        cal.set(Calendar.SECOND, 0);
        cal.set(Calendar.MILLISECOND, 0);

        return cal.getTime();
    }

}

JUnit test:

JUnit 测试：

package util;

import org.junit.Assert;
import org.junit.Test;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

/**
 * DateUtilsTest
 * User: Michael
 * Date: 10/16/10
 * Time: 10:40 AM
 */
public class DateUtilsTest
{
    public static final DateFormat DEFAULT_FORMAT = new SimpleDateFormat("dd/MM/yyyy");

    @Test
    public void testNewYearsDayLastYear() throws ParseException
    {
        Date expected = DEFAULT_FORMAT.parse("1/1/2009");
        Date actual = DateUtils.getDateOfWeekDay(1, 2009, Calendar.THURSDAY);

        Assert.assertEquals(expected, actual);
    }

    @Test
    public void testTaxDay() throws ParseException
    {
        Date expected = DEFAULT_FORMAT.parse("15/4/2010");
        Date actual = DateUtils.getDateOfWeekDay(16, 2010, Calendar.THURSDAY);

        Assert.assertEquals(expected, actual);
    }

    @Test
    public void testGetDateOfWeekDay() throws ParseException
    {
        Date expected = DEFAULT_FORMAT.parse("16/10/2010");
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(expected);
        int week = calendar.get(Calendar.WEEK_OF_YEAR);
        int year = calendar.get(Calendar.YEAR);
        int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);

        Date actual = DateUtils.getDateOfWeekDay(week, year, dayOfWeek);

        Assert.assertEquals(expected, actual);
    }
}

Answer 2

回答by andersoj

It's becoming a rote answer around here, but: Use JodaTimeor one of the alternatives. Investing precious cycles into understanding the JDK calendar API is throwing good money after bad.

它在这里成为一个死记硬背的答案，但是：使用JodaTime或其中一种替代方法。在理解 JDK 日历 API 上投入宝贵的周期是在赔钱。

Related on SO: Should I use native date/time; I see (now) you're doing Android development, which I can't speak to, but on SO: JodaTime on Android?

与 SO 相关：我是否应该使用本机日期/时间；我看到（现在）你正在做 Android 开发，我不能说，但是在 SO：Android上的JodaTime？

If you're insisting on JDK, I did try your code, with a slight modification. First, I'd eliminate the cal.setTime(new Date())-- it's not doing anything, and it's confusing since it might (seem to) correlate to always getting a date around "now," since new Date()gives you now. However, in your code, it still should be overridden by the following setX calls.

如果您坚持使用 JDK，我确实尝试了您的代码，并稍作修改。首先，我会消除cal.setTime(new Date())- 它没有做任何事情，并且令人困惑，因为它可能（似乎）与总是在“现在”附近约会，因为new Date()现在给了你。但是，在您的代码中，它仍应被以下 setX 调用覆盖。

class DateTest
{
    DateTest(){}

    public String getDateOfWeekDay(int weekId, int year, int weekDay){

        Calendar cal = Calendar.getInstance();
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.WEEK_OF_YEAR, weekId);
        cal.set(Calendar.DAY_OF_WEEK, weekDay);

        SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
        System.out.println("date="+sdf.format(cal.getTime())); 

        return  sdf.format(cal.getTime());    
    }

    public static void main(String args[])
    {
      DateTest dt = new DateTest();
      for (int weekCount = 1; weekCount <= 53; weekCount++) {
        dt.getDateOfWeekDay(weekCount,2010,Calendar.THURSDAY);
      }
    }

}

And I get exactly what I expect:

我得到了我所期望的：

date=31/12/2009 
date=07/01/2010 
...
date=23/12/2010 
date=30/12/2010

Do you see different?

你看不一样？

Answer 3

回答by RedGrittyBrick

public class CalendarTest {

    public static String getDateOfWeekDay(int weekId, int year, int weekDay) {
        SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
        Calendar cal = Calendar.getInstance();
        cal.setTime(new Date());
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.WEEK_OF_YEAR, weekId);
        cal.set(Calendar.DAY_OF_WEEK, weekDay);
        return sdf.format(cal.getTime());
    }

    public static void main(String[] args) {
        String someDay = getDateOfWeekDay(37,2010,Calendar.THURSDAY);
        System.out.println(someDay);
    }

}

Outputs 16/09/2010 As does

产出 16/09/2010 一样

public class CalendarTest {
    public static void main(String[] args) {
        MyService s = new MyService();
        String someDay = s.getDateOfWeekDay(37, 2010, Calendar.THURSDAY);
        System.out.println(someDay);
    }
}

class MyService {
    public String getDateOfWeekDay(int weekId, int year, int weekDay) {
        SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
        Calendar cal = Calendar.getInstance();
        cal.setTime(new Date());
        cal.set(Calendar.YEAR, year);
        cal.set(Calendar.WEEK_OF_YEAR, weekId);
        cal.set(Calendar.DAY_OF_WEEK, weekDay);
        return sdf.format(cal.getTime());
    }
}

Answer 4

回答by soc

Well, because these answers are less about answering your actual question than recommending to use JodaTime, I'll throw in my recommendation againstJodaTime.

好吧，因为这些答案与其说是回答您的实际问题，不如说是建议使用 JodaTime，所以我将提出反对JodaTime 的建议。

Date/Calendar as well as JodaTime have significant design flaws (quoting the authors) and if you plan to do work with time and date, use JSR-310.

Date/Calendar 和 JodaTime 都有明显的设计缺陷（引用作者的话），如果您打算使用时间和日期，请使用JSR-310。

JSR-310 is the replacement for both Date/Calendar and JodaTime which is more consistent and works better than both. There are considerations to include JSR-310 in Java 7.

JSR-310 是 Date/Calendar 和 JodaTime 的替代品，它比两者更一致且效果更好。在 Java 7 中包含JSR-310需要考虑。

Regardless if that works out or not, it is possible to use it today. It is just a jar file like JodaTime.

不管这是否有效，今天都可以使用它。它只是一个类似于 JodaTime 的 jar 文件。

The benefits are:

好处是：

Fixes the remaining problems of JodaTime
Consistent library design
Better naming conventions
Easier to read API
Everything immutable
Better usability from other programming languages

修复 JodaTime 剩余问题
一致的库设计
更好的命名约定
更易于阅读的 API
一切不变
来自其他编程语言的更好的可用性

If you are interested in this topic:

如果您对此主题感兴趣：

Video about JSR-310.
Slightly related answerabout the design decisions of Date/Time APIs in different languages.

关于 JSR-310 的视频。
关于不同语言的日期/时间 API 的设计决策的稍微相关的答案。

一个非常简单的 java 代码（获取工作日的日期），但是我得到的结果很奇怪，为什么？

提问by Mellon

回答by duffymo

回答by andersoj

回答by RedGrittyBrick

回答by soc

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一个非常简单的 java 代码（获取工作日的日期），但是我得到的结果很奇怪，为什么？

提问by Mellon

回答by duffymo

回答by andersoj

回答by RedGrittyBrick

回答by soc

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