一个非常简单的java代码（获取一个星期的日期），但非常奇怪的结果我得到，为什么？ [英] A very simple java code (to get date of a week day), but Very Strange result I get, WHY?
问题描述
我有以下java代码来获取特定周的日期：
I have the following java code to get the date of a specific week day:
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()));
当我将这段代码放在一个 main（String [] args） / code>方法，如下所示：
When I put this code in a main(String[] args)method, like the following:
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()));
}
}
并运行它，我得到正确的结果是 date = 09/09/2010 。没有问题。
and run it, I get the correct result which is date=09/09/2010. There is no problem.
然后...
当我把这个一个类的函数中的代码，如下所示：
When I put this code in a function of a Class, like the following:
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());
}
}
在另一个类MainClass中，MainClass将调用这个服务功能，如下所示：
in another class MainClass, MainClass will invoke this service function like following:
Public Class MainClass{
MyService myService = new MyService();
myService.getDateOfWeekDay(37,2010,Calendar.THURSDAY);
}
但返回的结果始终是本周的星期四（日期= 14/10/2010），而不是我指定的星期四（第37周，2010年，星期四）。
为什么???我使用完全相同的java代码来获取特定的一周的日期，只是以不同的方式使用它，为什么结果是不同的？我不明白这个...有人可以向我解释吗？
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??
推荐答案
是的，JodaTime很棒，但我敢打赌，你宁愿知道你的东西有什么问题。
Yes, JodaTime is wonderful, but I'll bet you'd rather know what's wrong with your stuff.
添加另一个JAR依赖关系可能是移动设备的问题。
Adding another JAR dependency might be a problem for a mobile device.
该方法不好， IMO。这个名字是一个俗名，它做的太多了。您正在创建一个字符串，而不是一个日期，并且正在打印到一个日志。我建议返回一个Date，让客户端担心他们是否要将其转换成String。日志记录是一个交叉的关注。
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.
这是一个可以工作的类（和JUnit测试）。看看有什么不同：
Here's a class (and a JUnit test) that works. See what's different:
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);
}
}
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