Create a Java Date from a String and vise versa

Question

I have a date in Integer format(YYYYMMDD). And a start_time as a String (HH:mm 24 hour system)

Answer 1

Strange Data Types For Date-Time

Using:

• An int to represent the digits of a calendar date
• A string to represent time-of-day digits
• A double to represent a duration of fractional hours

…are all unusual approaches. Probably not the wisest choices in handling date-time values.

Avoid java.util.Date/Calendar

Know that the bundled classes java.util.Date and .Calendar are notoriously troublesome and should be avoided. Use either Joda-Time or the new java.time.* package (Tutorial) in Java 8. And get familiar with the handy ISO 8601 standard.

Time Zone

Your question and example ignore the crucial issue of time zone. Handling date-time data without time zone is like handling text files without knowing their character encoding. Not good.

Use proper time zone names to create time zone object. Avoid the non-standard 3-letter codes.

Joda-Time

In Joda-Time, a DateTime object is similar to a java.util.Date object but actually knows its own assigned time zone.

Joda-Time offers three classes for representing spans of time: Period, Duration, and Interval.

The Interval class uses the "Half-Open" approach, where the beginning is inclusive and the ending is exclusive. This approach works well for handling spans of time and comparisons. Look for the handy contains, abuts, overlap, and gap methods.

int dateInput = 20140214;
String start_timeInput = "14:30";
double durationInput = 50.30;

// Clean up these inputs.
String datePortion = Integer.toString( dateInput );
String input = datePortion + " " + start_timeInput; 
DateTimeFormatter formatterInput = DateTimeFormat.forPattern( "yyyyMMdd HH:mm");

// Specify the time zone this date-time represents.
DateTimeZone timeZone = DateTimeZone.forID( "Europe/Paris" ); // Or, DateTimeZone.UTC
DateTime dateTime = formatterInput.withZone( timeZone ).parseDateTime( input );

// Convert fractional hours to milliseconds, then milliseconds to a Duration object.
long millis = ( 60L * 60L * (long)(1000L * durationInput) ); // 1 hour = 60 minutes * 60 seconds * 1000 milliseconds.
Duration duration = new Duration( millis );

Interval interval = new Interval( dateTime, duration );

DateTimeFormatter formatterOutput = DateTimeFormat.forStyle( "MM" ).withLocale( Locale.FRANCE );
String description = "De " + formatterOutput.print( interval.getStart() ) + " à " + formatterOutput.print( interval.getEnd() );

Dump to console…

System.out.println( "input: " + input );
System.out.println( "dateTime: " + dateTime );
System.out.println( "duration: " + duration ); // Format: PnYnMnDTnHnMnS (from ISO 8601)
System.out.println( "interval: " + interval ); // Format: <start>/<end> (from ISO 8601)
System.out.println( "description: " + description );   

When run…

input: 20140214 14:30
dateTime: 2014-02-14T14:30:00.000+01:00
duration: PT181080S
interval: 2014-02-14T14:30:00.000+01:00/2014-02-16T16:48:00.000+01:00
description: De 14 févr. 2014 14:30:00 à 16 févr. 2014 16:48:00

Answer 2

 you can write like
 int date = 20140214;
 String s=""+date;  
 Date date = new SimpleDateFormat("yyyyMMdd").parse(s);

Answer 3

You have very many representations of date.

When in doubt, I usually head for getting to unix standard time (milliseconds since 1970) as soon as possible.

In this case it would be to convert the Integer date to a String, read out the four first as a year, two digits as month and the last two as day day, and then do the similar thing for the 24h time, and create a java.util.Date from this like so:

SimpleDateFormat dateParser=new SimpleDateFormat("yyyyMMdd HH:mm"); //please double check the syntax for this guy...
String yyyyMmDd = date.toString();
String fullDate = yyyyMmDd + " " + start_time;
java.util.Date startDate = dateParser.parse(fullDate);
long startTimeInMillis = startDate.getTime();
final long MILLISECONDS_PER_HOUR = 1000*60*60;
long durationInMillis = (long)duration*MILLISECONDS_PER_HOUR;
java.util.Date endDate = new java.util.Date(startTimeInMillis + durationInMillis);

Don't miss Joda time or Java 8 new, finally improved date handling named java.time.