blocks|key|329942|text|使用日历进行日期计算|type|unstyled|depth|inlineStyleRanges|entityRanges|data|329943|++++Calendar+c+=+Calendar.getInstance();
++++c.set(Calendar.YEAR,+2013);
++++c.set(Calendar.MONTH,+Calendar.JANUARY);
++++c.set(Calendar.DATE,+1);
++++c.set(Calendar.HOUR,+0);
++++c.set(Calendar.MINUTE,+0);
++++c.set(Calendar.SECOND,+0);
++++c.set(Calendar.MILLISECOND,+0);
++++c.add(Calendar.DATE,+numberOfDays);
++++Date+date+=+c.getTime();|code-block|syntax|javascript|329944|请注意，由于DST+(夏季时间)的不同，不同地区的结果可能有所不同。上面的示例使用默认区域设置。|329945|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Use Calendar for date arithmetic

<pre><code> Calendar c = Calendar.getInstance();
 c.set(Calendar.YEAR, 2013);
 c.set(Calendar.MONTH, Calendar.JANUARY);
 c.set(Calendar.DATE, 1);
 c.set(Calendar.HOUR, 0);
 c.set(Calendar.MINUTE, 0);
 c.set(Calendar.SECOND, 0);
 c.set(Calendar.MILLISECOND, 0);
 c.add(Calendar.DATE, numberOfDays);
 Date date = c.getTime();
</code></pre>

Note that the result may be different for different locales because of DST (summer time). The above example uses default locale.

blocks|key|112517|text|您可以简单地使用SimpleDateFormat将String转换为Date。模式D可以用来表示一年中的天数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|112518|假设你有一个|112519|int+numberOfDays+=+42;+//+Arbitrary+number.|code-block|syntax|javascript|112520|然后计算自1970年1月1日以来的天数(the+Epoch)|112521|Date+date+=+new+SimpleDateFormat("D").parse(String.valueOf(numberOfDays));|112522|或者，此参数计算从当前年的1月1日起的天数。|112523|Date+date+=+new+SimpleDateFormat("D+yyyy").parse(numberOfDays+%2B+"+"+%2B+Calendar.getInstance().get(Calendar.YEAR));|112524|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html|1|http://en.wikipedia.org/wiki/Epoch_%2528reference_date%2529^0|8|G|P|6|Y|4|15|1|8|G|0|0|0|0|K|9|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|14|8|@$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]]|D|@$9|1D|A|1E|1|1F]]|E|$]]|$1|F|3|G|5|6|7|1G|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1H|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1I|8|@]|D|@$9|1J|A|1K|1|1L]]|E|$]]|$1|O|3|P|5|J|7|1M|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1N|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|1O|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|1P|8|@]|D|@]|E|$]]]|V|$W|$5|X|Y|Z|E|$10|11]]|12|$5|X|Y|Z|E|$10|13]]]]

You can simply use <a href="http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html" rel="noreferrer"><code>SimpleDateFormat</code></a> to convert <code>String</code> to <code>Date</code>. The pattern <code>D</code> can be used to represent the day number of year.

Provided that you've an

<pre><code>int numberOfDays = 42; // Arbitrary number.
</code></pre>

then this one counts the number of days since 1 Jan 1970 (<a href="http://en.wikipedia.org/wiki/Epoch_%28reference_date%29" rel="noreferrer">the Epoch</a>)

<pre><code>Date date = new SimpleDateFormat("D").parse(String.valueOf(numberOfDays));
</code></pre>

alternatively, this one counts the number of days since 1 Jan of current year.

<pre><code>Date date = new SimpleDateFormat("D yyyy").parse(numberOfDays + " " + Calendar.getInstance().get(Calendar.YEAR));
</code></pre>

blocks|key|112545|text|当涉及到日期处理时，我发现JodaTime使用起来非常优雅。有了它，你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|112546|DateTime+date+=+new+DateTime().withDayOfYear(dayOfYear);|code-block|syntax|javascript|112547|entityMap|0|LINK|mutability|MUTABLE|url|http://joda-time.sourceforge.net/^0|D|8|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

I find <a href="http://joda-time.sourceforge.net/" rel="nofollow">JodaTime</a> very elegant to use when it comes to date handling. With it, you could do it like this:

<pre><code>DateTime date = new DateTime().withDayOfYear(dayOfYear);
</code></pre>

blocks|key|112585|text|使用GregorianCalendar和simpleDateFormat|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|112586|int+days+=+20;
GregorianCalendar+dayCountofYear+=+new+GregorianCalendar(2020,Calendar.JANUARY,+1);
dayCountofYear.add(Calendar.DAY_OF_YEAR,+days);
System.out.println("GregorianCalendar:+"%2B+dateFormat.format(dayCountofYear.getTime()));|code-block|syntax|javascript|112587|上述逻辑可以在函数调用中简化如下：|112588|Date+nextDate+=+addDaysToDate("01/01/2020",+20);
System.out.println("After+Adding+Number+of+Days:"%2BnextDate);+//+O/P:21/01/2020|112589|完整示例：|112590|public+class+DateHelper+{

++++public+static+void+main(String[]+args)+throws+ParseException+{
++++++++int+days+=+20;
++++++++String+initialDay+=+"01/01/2020";
++++++++Date+nextDate+=+addDaysToDate(initialDay,+days);
++++++++System.out.println("After+Adding+Number+of+Days:"%2BnextDate);

++++++++/*Date+firstDate+=+DateHelper.parseDate(initialDay,+formatStr);
++++++++int+daysBetween+=+getDaysBetween(firstDate,+nextDate);
++++++++System.out.println(days+%2B+"+-+Days+Between+-+"%2BdaysBetween);*/
++++}

++++static+String+formatStr+=+"dd/MM/yyyy";
++++static+DateFormat+dateFormat+=+new+SimpleDateFormat(formatStr);
++++static+DateFormat+reverseDateFormat+=+new+SimpleDateFormat("yyyy/MM/dd");

++++public+static+Date+addDaysToDate(String+specificDate,+int+daysCount)+throws+ParseException+{
++++++++Date+date+=+dateFormat.parse(specificDate);
++++++++GregorianCalendar+gregCal+=+(GregorianCalendar)+GregorianCalendar.getInstance();+//+Current+Date
++++++++gregCal.setTime(date);+//+Change+the+Date+to+Provided+One
++++++++gregCal.add(Calendar.DAY_OF_YEAR,+daysCount);
++++++++Date+time+=+gregCal.getTime();

++++++++ZonedDateTime+zdt+=+gregCal.toZonedDateTime();
++++++++System.out.println("GregorianCalendar+DayofWeek:+"%2B+zdt.getDayOfWeek());
++++++++return+time;
++++}
++++public+static+String+addDaysToDateStr(String+specificDate,+int+daysCount)+throws+ParseException+{
++++++++String+format+=+dateFormat.format(+addDaysToDate(specificDate,+daysCount)+);
++++++++System.out.println("GregorianCalendar+[To+a+Date+added+Number+of+days+leads+to+New+Date]:+"%2B+format);
++++++++return+format;
++++}
++++public+static+Date+parseDate(String+aSource,+String+formatStr)+throws+ParseException+{
++++++++DateFormat+dateFormat+=+new+SimpleDateFormat(formatStr);
++++++++dateFormat.setLenient(false);
++++++++return+dateFormat.parse(aSource);
++++}
++++public+static+String+formatDate(Date+aDate,+String+formatStr,+Calendar+calendar)+{
++++++++DateFormat+dateFormat+=+new+SimpleDateFormat(formatStr);
++++++++dateFormat.setLenient(false);
++++++++dateFormat.setCalendar(calendar);
++++++++return+dateFormat.format(aDate);
++++}
++++public+static+String+formatDate(Date+aDate,+String+formatStr)+{
++++++++DateFormat+dateFormat+=+new+SimpleDateFormat(+formatStr+);
++++++++//dateFormat.setCalendar(Calendar.getInstance(TimeZone.getTimeZone("UTF")));
++++++++return+dateFormat.format(aDate);
++++}
}|112591|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/7/docs/api/java/util/GregorianCalendar.html|1|https://docs.oracle.com/javase/tutorial/i18n/format/simpleDateFormat.html^0|2|H|2|H|0|K|G|1|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]|$9|18|A|19|1|1A]]|E|$]]|$1|F|3|G|5|H|7|1B|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1C|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1D|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1E|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1F|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]|10|$5|V|W|X|E|$Y|11]]]]

<h3>Using <a href="https://docs.oracle.com/javase/7/docs/api/java/util/GregorianCalendar.html" rel="nofollow noreferrer"><code>GregorianCalendar</code></a> and <a href="https://docs.oracle.com/javase/tutorial/i18n/format/simpleDateFormat.html" rel="nofollow noreferrer">simpleDateFormat</a>:</h3>

<pre class="lang-java prettyprint-override"><code>int days = 20;
GregorianCalendar dayCountofYear = new GregorianCalendar(2020,Calendar.JANUARY, 1);
dayCountofYear.add(Calendar.DAY_OF_YEAR, days);
System.out.println("GregorianCalendar: "+ dateFormat.format(dayCountofYear.getTime()));
</code></pre>

Above logic can be simplified in a function call as below:

<pre class="lang-java prettyprint-override"><code>Date nextDate = addDaysToDate("01/01/2020", 20);
System.out.println("After Adding Number of Days:"+nextDate); // O/P:21/01/2020
</code></pre>

Full Example:

<pre class="lang-java prettyprint-override"><code>public class DateHelper {

 public static void main(String[] args) throws ParseException {
 int days = 20;
 String initialDay = "01/01/2020";
 Date nextDate = addDaysToDate(initialDay, days);
 System.out.println("After Adding Number of Days:"+nextDate);

 /*Date firstDate = DateHelper.parseDate(initialDay, formatStr);
 int daysBetween = getDaysBetween(firstDate, nextDate);
 System.out.println(days + " - Days Between - "+daysBetween);*/
 }

 static String formatStr = "dd/MM/yyyy";
 static DateFormat dateFormat = new SimpleDateFormat(formatStr);
 static DateFormat reverseDateFormat = new SimpleDateFormat("yyyy/MM/dd");

 public static Date addDaysToDate(String specificDate, int daysCount) throws ParseException {
 Date date = dateFormat.parse(specificDate);
 GregorianCalendar gregCal = (GregorianCalendar) GregorianCalendar.getInstance(); // Current Date
 gregCal.setTime(date); // Change the Date to Provided One
 gregCal.add(Calendar.DAY_OF_YEAR, daysCount);
 Date time = gregCal.getTime();

 ZonedDateTime zdt = gregCal.toZonedDateTime();
 System.out.println("GregorianCalendar DayofWeek: "+ zdt.getDayOfWeek());
 return time;
 }
 public static String addDaysToDateStr(String specificDate, int daysCount) throws ParseException {
 String format = dateFormat.format( addDaysToDate(specificDate, daysCount) );
 System.out.println("GregorianCalendar [To a Date added Number of days leads to New Date]: "+ format);
 return format;
 }
 public static Date parseDate(String aSource, String formatStr) throws ParseException {
 DateFormat dateFormat = new SimpleDateFormat(formatStr);
 dateFormat.setLenient(false);
 return dateFormat.parse(aSource);
 }
 public static String formatDate(Date aDate, String formatStr, Calendar calendar) {
 DateFormat dateFormat = new SimpleDateFormat(formatStr);
 dateFormat.setLenient(false);
 dateFormat.setCalendar(calendar);
 return dateFormat.format(aDate);
 }
 public static String formatDate(Date aDate, String formatStr) {
 DateFormat dateFormat = new SimpleDateFormat( formatStr );
 //dateFormat.setCalendar(Calendar.getInstance(TimeZone.getTimeZone("UTF")));
 return dateFormat.format(aDate);
 }
}
</code></pre>

blocks|key|330006|text|Calendar+c+=+Calendar.getInstance();
+++++++++c.set(Calendar.YEAR,+2016);
+++++++++c.set(Calendar.MONTH,+8);
+++++++++c.set(Calendar.DAY_OF_YEAR,+244);
+++++++++c.set(Calendar.HOUR,+0);
+++++++++c.set(Calendar.MINUTE,+0);
+++++++++c.set(Calendar.SECOND,+0);
+++++++++c.set(Calendar.MILLISECOND,+0);
++++++++Date+date+=+c.getTime();|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|330007|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>Calendar c = Calendar.getInstance();
 c.set(Calendar.YEAR, 2016);
 c.set(Calendar.MONTH, 8);
 c.set(Calendar.DAY_OF_YEAR, 244);
 c.set(Calendar.HOUR, 0);
 c.set(Calendar.MINUTE, 0);
 c.set(Calendar.SECOND, 0);
 c.set(Calendar.MILLISECOND, 0);
 Date date = c.getTime();
</code></pre>

Simple Question, but Google surprisingly had little on this. I have the <code>number of days</code> from Jan 1st of the year. How can I convert that to a <code>date</code> in Java?

Get Date based on day count in Java

问题很简单，但令人惊讶的是，谷歌对此几乎一无所知。我有今年1月1日的number of days。在Java中如何将其转换为date？

问在Java中根据天数获取日期
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回答 5

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