Java Built In Date Manipulation and Comparison

Question

I'm looking to create a datetime stamp, then add 10 hours to it, then have a thread check to see if the time has elapsed.

I read through, Time comparison but it seems a bit complicated/convoluted for something so simple. Especially if your time comparison goes across midnight.

My understanding is that java's underlying datetime, is suppose to be a long, if this is true, is there a simple way to add another long to it, such as the number equivalent of 10 hours? Or some other means such as adding two dates?

Note: The solution needs to be part of core java, can't be part of a 3rd party lib.

Answer 1

You can use a Calendar to perform that math,

Calendar cal = Calendar.getInstance();
cal.add(Calendar.HOUR, 10); // Add 10 hours.
Date date2 = cal.getTime(); // Now plus 10 hours.
Date date = new Date(); // Now.

Answer 2

You can use the Date.getTime() method to obtain the underlying timestamp, the timestamp is basically the number of milliseconds elapsed since a defined base instant (1970-01-01 00:00:00 IIRC).

System.currentTimeMillis() allows you the get the "now" instant directly, without any detours using Date , Calendar and the like.

The timestamp can then be manipulated basic math:

 timestamp += TimeUnit.MILLISECONDS.convert(10, TimeUnit.HOURS);

Example of adding 10 hours:

long nowInMilliSince1970 = System.currentTimeMillis();
long tenHoursAsMilli = TimeUnit.MILLISECONDS.convert(10L, TimeUnit.MINUTES);
long tenHoursLater = nowInMilliSince1970 + tenHoursAsMilli;
System.out.println("now in milliseconds: \t\t" + nowInMilliSince1970);
System.out.println("10 hours in milliseconds: \t" + tenHoursAsMilli);
System.out.println("10 hours from now: \t\t" + tenHoursLater);

Checking if the timestamp is in the past is as easy as:

 if (timestamp < System.currentTimeMillis()) {
     System.out.println("timestamp is in the past");
 }

Do note that direct timestamp math has no concept of daylight saving and time zones. If you want that, use a Calendar for math - Calendar implements the dirty exceptional rules for that.

Answer 3

Another way of achieving it using just JDK built in stuff is:

long tenHoursFromNow = System.currentTimeMillis() + TimeUnit.HOURS.toMillis(10);

and then in your Thread you would check:

if(System.currentTimeMillis() > tenHoursFromNow)
{
        //Do something as the time has elapsed 
}

Although I would argue that the use of Calendar and Date is clearer as to what the intention of your code is trying to achieve.

Answer 4

The bundled java.util.Date and .Calendar are notoriously troublesome. They really should be avoided.

You stated a requirement of no added libraries. So see the java.time part of my answer, using the new package newly added to Java 8. But I urge you to reconsider your reluctance to add a library, especially if you cannot move to Java 8 yet; juDate/Calendar really are that bad.

Both libraries handle anomalies such as Daylight Saving Time.

Consider specifying a time zone rather than rely on the JVM's default. Generally best to work in UTC , and then translate to a local time zone for presentation to the user.

java.time

The java.time package is newly added to Java 8 . Inspired by Joda-Time but re-architected. Defined by JSR 310 . Extended by the threeten-extra project.

ZonedDateTime tenHoursLater = ZonedDateTime.now().plusHours( 10 );

Joda-Time

Using the Joda-Time 2.3 library.

DateTime tenHoursLater = DateTime.now().plusHours( 10 );

For more info on this kind of use of Joda-Time, see my answer to a similar question.