We have a java code snippet here
import java.text.SimpleDateFormat;
import java.util.Date;
public class SimpleDateFormatExample {
public static void main(String[] args) {
Date date = new Date();
int days = 5;
SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy");
String strDate= formatter.format(date.getTime() + (days*86400000));
System.out.println(strDate);
}
}
to add n no. of days to today's date. The result will be correct upto n=24
but gives previous month' after n=24
. Why it is so?
The problem is the the int is overflowing
consider
int days = 25;
int d = days*86400000;
System.out.println(d);
try
int days = 25;
long d = days*86400000L;
System.out.println(d);
LocalDate // Represent a date-only, without a time-of-day and without a time zone.
.now() // Capture the current date, as seen through your JVM’s current default time zone. Better to pass a `ZoneId` as the optional argument.
.plusDays( 5 ) // Add five days, returning a new `LocalDate` object. Per the Immutable Objects pattern, a new object is produced rather than changing (“mutating”) the original.
.format( // Generate text representing the date value of our `LocalDate` object.
DateTimeFormatter.ofPattern( "dd/MM/uuuu" ) // Define a formatting pattern to suit your taste. Or call the `.ofLocalized…` methods to localize automatically.
) // Returns a `String`.
Date
class represents a moment in UTC, a date with a time-of-day, and an offset-from-UTC of zero. Wrong class to use when working with date-only values.
Avoid using the terrible old legacy date-time classes such as Calendar
, Date
, and SimpleDateFormat
. These classes were supplanted years ago by the java.time classes.
Do not track days as a count of seconds or milliseconds. Days are not always 24 hours long, and years are not always 365 days long.
LocalDate
Instead, use LocalDate
class.
LocalDate today = LocalDate.now() ;
LocalDate later = today.plusDays( 5 ) ;
Best to avoid the legacy classes altogether. But if you must interoperate with old code not yet updated to java.time classes, you can convert back-and-forth. Call new methods added to the old classes.
For Date
you need to add a time-of-day. I expect you will want to go with the first moment of the day. And I'll assume you want to frame the date as UTC rather than a time zone. We must go through a OffsetDateTime
object to add the time-of-day and offset. For the offset, we use the constant ZoneOffset.UTC
. Then we extract the more basic Instant
class object to convert to a java.util.Date
.
OffsetDateTime odt = OffsetDateTime.of( later , LocalTime.MIN , ZoneOffset.UTC ) ; // Combine the date with time-of-day and with an offset-from-UTC.
Instant instant = odt.toInstant() ; // Convert to the more basic `Instant` class, a moment in UTC, always UTC by definition.
java.util.Date d = java.util.Date.from( instant ) ; // Convert from modern class to legacy class.
Going the other direction:
Instant instant = d.toInstant() ; // Convert from legacy class to modern class.
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 .
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.*
classes.
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 .
使用days*86400000L
进行long
计算,否则int
值将溢出。
Try this one in your code:
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.add(Calendar.DATE, 5);
strDate = formatter.format(cal.getTime());
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