日期格式转换 yyyy-mm-dd 到 mm-dd-yyyy

我有一个格式为 yyyy-mm-dd的 java.util.Date。我希望它的格式为 mm-dd-yyyy

下面是我尝试这种转换之前的示例:

// Setting the pattern
SimpleDateFormat sm = new SimpleDateFormat("mm-dd-yyyy");
// myDate is the java.util.Date in yyyy-mm-dd format
// Converting it into String using formatter
String strDate = sm.format(myDate);
//Converting the String back to java.util.Date
Date dt = sm.parse(strDate);

我得到的输出仍然不是 mm-dd-yyyy格式。

请让我知道如何格式化 java.util.Date从 yyyy-mm-dd到 mm-dd-yyyy

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小开 
SimpleDateFormat("MM-dd-yyyy");

instead of 

SimpleDateFormat("mm-dd-yyyy");

because MM points Month, mm points minutes

SimpleDateFormat sm = new SimpleDateFormat("MM-dd-yyyy");
String strDate = sm.format(myDate);
小开
最佳答案

Date is a container for the number of milliseconds since the Unix epoch ( 00:00:00 UTC on 1 January 1970).

It has no concept of format.

Java 8+

LocalDateTime ldt = LocalDateTime.now();
System.out.println(DateTimeFormatter.ofPattern("MM-dd-yyyy", Locale.ENGLISH).format(ldt));
System.out.println(DateTimeFormatter.ofPattern("yyyy-MM-dd", Locale.ENGLISH).format(ldt));
System.out.println(ldt);

Outputs...

05-11-2018
2018-05-11
2018-05-11T17:24:42.980

Java 7-

You should be making use of the ThreeTen Backport

Original Answer

For example...

Date myDate = new Date();
System.out.println(myDate);
System.out.println(new SimpleDateFormat("MM-dd-yyyy").format(myDate));
System.out.println(new SimpleDateFormat("yyyy-MM-dd").format(myDate));
System.out.println(myDate);

Outputs...

Wed Aug 28 16:20:39 EST 2013
08-28-2013
2013-08-28
Wed Aug 28 16:20:39 EST 2013

None of the formatting has changed the underlying Date value. This is the purpose of the DateFormatters

Updated with additional example

Just in case the first example didn't make sense...

This example uses two formatters to format the same date. I then use these same formatters to parse the String values back to Dates. The resulting parse does not alter the way Date reports it's value.

Date#toString is just a dump of it's contents. You can't change this, but you can format the Date object any way you like

try {
Date myDate = new Date();
System.out.println(myDate);


SimpleDateFormat mdyFormat = new SimpleDateFormat("MM-dd-yyyy");
SimpleDateFormat dmyFormat = new SimpleDateFormat("yyyy-MM-dd");


// Format the date to Strings
String mdy = mdyFormat.format(myDate);
String dmy = dmyFormat.format(myDate);


// Results...
System.out.println(mdy);
System.out.println(dmy);
// Parse the Strings back to dates
// Note, the formats don't "stick" with the Date value
System.out.println(mdyFormat.parse(mdy));
System.out.println(dmyFormat.parse(dmy));
} catch (ParseException exp) {
exp.printStackTrace();
}

Which outputs...

Wed Aug 28 16:24:54 EST 2013
08-28-2013
2013-08-28
Wed Aug 28 00:00:00 EST 2013
Wed Aug 28 00:00:00 EST 2013

Also, be careful of the format patterns. Take a closer look at SimpleDateFormat to make sure you're not using the wrong patterns ;)

小开

'M' (Capital) represent month & 'm' (Simple) represent minutes

Some example for months

'M' -> 7  (without prefix 0 if it is single digit)
'M' -> 12


'MM' -> 07 (with prefix 0 if it is single digit)
'MM' -> 12


'MMM' -> Jul (display with 3 character)


'MMMM' -> December (display with full name)

Some example for minutes

'm' -> 3  (without prefix 0 if it is single digit)
'm' -> 19
'mm' -> 03 (with prefix 0 if it is single digit)
'mm' -> 19
小开

Please change small "mm" month to capital "MM" it will work.for reference below is the sample code.

        Date myDate = new Date();
SimpleDateFormat sm = new SimpleDateFormat("MM-dd-yyyy");
       

String strDate = sm.format(myDate);
        

Date dt = sm.parse(strDate);
System.out.println(strDate);
小开

tl;dr

LocalDate.parse(
"01-23-2017" ,
DateTimeFormatter.ofPattern( "MM-dd-uuuu" )
)

Details

I have a java.util.Date in the format yyyy-mm-dd

As other mentioned, the Date class has no format. It has a count of milliseconds since the start of 1970 in UTC. No strings attached.

java.time

The other Answers use troublesome old legacy date-time classes, now supplanted by the java.time classes.

If you have a java.util.Date, convert to a Instant object. The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction). 

Instant instant = myUtilDate.toInstant();

Time zone

The other Answers ignore the crucial issue of time zone. Determining a date requires a time zone. For any given moment, the date varies around the globe by zone. A few minutes after midnight in Paris France is a new day, while still “yesterday” in Montréal Québec.

Define the time zone by which you want context for your Instant.

ZoneId z = ZoneId.of( "America/Montreal" );

Apply the ZoneId to get a ZonedDateTime.

ZonedDateTime zdt = instant.atZone( z );

LocalDate

If you only care about the date without a time-of-day, extract a LocalDate.

LocalDate localDate = zdt.toLocalDate();

To generate a string in standard ISO 8601 format, YYYY-MM-DD, simply call toString. The java.time classes use the standard formats by default when generating/parsing strings.

String output = localDate.toString();

2017-01-23

If you want a MM-DD-YYYY format, define a formatting pattern.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "MM-dd-uuuu" );
String output = localDate.format( f );

Note that the formatting pattern codes are case-sensitive. The code in the Question incorrectly used mm (minute of hour) rather than MM (month of year).

Use the same DateTimeFormatter object for parsing. The java.time classes are thread-safe, so you can keep this object around and reuse it repeatedly even across threads.

LocalDate localDate = LocalDate.parse( "01-23-2017" , f );

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

Where to obtain the java.time classes?

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

小开

It is simple use below codes.

final Date todayDate = new Date();


System.out.println(todayDate);


System.out.println(new SimpleDateFormat("MM-dd-yyyy").format(todayDate));


System.out.println(new SimpleDateFormat("yyyy-MM-dd").format(todayDate));


System.out.println(todayDate);
小开

see these..

formatting date

DateTimeFormatter

parsing date

Instant with zone

小开

You may get day, month and year and may concatenate them or may use MM-dd-yyyy format as given below.

Date date1 = new Date();
String mmddyyyy1 = new SimpleDateFormat("MM-dd-yyyy").format(date1);
System.out.println("Formatted Date 1: " + mmddyyyy1);






Date date2 = new Date();
Calendar calendar1 = new GregorianCalendar();
calendar1.setTime(date2);
int day1   = calendar1.get(Calendar.DAY_OF_MONTH);
int month1 = calendar1.get(Calendar.MONTH) + 1; // {0 - 11}
int year1  = calendar1.get(Calendar.YEAR);
String mmddyyyy2 = ((month1<10)?"0"+month1:month1) + "-" + ((day1<10)?"0"+day1:day1) + "-" + (year1);
System.out.println("Formatted Date 2: " + mmddyyyy2);






LocalDateTime ldt1 = LocalDateTime.now();
DateTimeFormatter format1 = DateTimeFormatter.ofPattern("MM-dd-yyyy");
String mmddyyyy3 = ldt1.format(format1);
System.out.println("Formatted Date 3: " + mmddyyyy3);






LocalDateTime ldt2 = LocalDateTime.now();
int day2 = ldt2.getDayOfMonth();
int mont2= ldt2.getMonthValue();
int year2= ldt2.getYear();
String mmddyyyy4 = ((mont2<10)?"0"+mont2:mont2) + "-" + ((day2<10)?"0"+day2:day2) + "-" + (year2);
System.out.println("Formatted Date 4: " + mmddyyyy4);






LocalDateTime ldt3 = LocalDateTime.of(2020, 6, 11, 14, 30); // int year, int month, int dayOfMonth, int hour, int minute
DateTimeFormatter format2 = DateTimeFormatter.ofPattern("MM-dd-yyyy");
String mmddyyyy5 = ldt3.format(format2);
System.out.println("Formatted Date 5: " + mmddyyyy5);






Calendar calendar2 = Calendar.getInstance();
calendar2.setTime(new Date());
int day3  = calendar2.get(Calendar.DAY_OF_MONTH); // OR Calendar.DATE
int month3= calendar2.get(Calendar.MONTH) + 1;
int year3 = calendar2.get(Calendar.YEAR);
String mmddyyyy6 = ((month3<10)?"0"+month3:month3) + "-" + ((day3<10)?"0"+day3:day3) + "-" + (year3);
System.out.println("Formatted Date 6: " + mmddyyyy6);






Date date3 = new Date();
LocalDate ld1 = LocalDate.parse(new SimpleDateFormat("yyyy-MM-dd").format(date3)); // Accepts only yyyy-MM-dd
int day4  = ld1.getDayOfMonth();
int month4= ld1.getMonthValue();
int year4 = ld1.getYear();
String mmddyyyy7 = ((month4<10)?"0"+month4:month4) + "-" + ((day4<10)?"0"+day4:day4) + "-" + (year4);
System.out.println("Formatted Date 7: " + mmddyyyy7);






Date date4 = new Date();
int day5   = LocalDate.parse(new SimpleDateFormat("yyyy-MM-dd").format(date4)).getDayOfMonth();
int month5 = LocalDate.parse(new SimpleDateFormat("yyyy-MM-dd").format(date4)).getMonthValue();
int year5  = LocalDate.parse(new SimpleDateFormat("yyyy-MM-dd").format(date4)).getYear();
String mmddyyyy8 = ((month5<10)?"0"+month5:month5) + "-" + ((day5<10)?"0"+day5:day5) + "-" + (year5);
System.out.println("Formatted Date 8: " + mmddyyyy8);






Date date5 = new Date();
int day6   = Integer.parseInt(new SimpleDateFormat("dd").format(date5));
int month6 = Integer.parseInt(new SimpleDateFormat("MM").format(date5));
int year6  = Integer.parseInt(new SimpleDateFormat("yyyy").format(date5));
String mmddyyyy9 = ((month6<10)?"0"+month6:month6) + "-" + ((day6<10)?"0"+day6:day6) + "-" + (year6);`enter code here`
System.out.println("Formatted Date 9: " + mmddyyyy9);

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