Java - math : System.nanoTime() and currentTimeMilli - if we take x chars out - what is the accuracy?

Question

Java, date to long question. If we have :

 String rand = new Long(System.currentTimeMillis()).toString(); logger.info("full :" + rand); rand = rand.substring(rand.length() -9); logger.info("Significant 8 :" + rand);

Then is there a way to know for how many days / minutes is the result unique? Sample output :

full :1382519851946

Significant 8 :19851946

I guess the last 3 digits are the milli seconds, the next two 51 here are seconds, then 98 198 is hours and minutes?

Put differently if I take 10 digits will I get unique values for a full day? Assuming of course only one thread and one JVM is generating these.

Why do you think this is unique? 1 millisecond is a rather long time for any useful computer. Only if you can be sure this method is never called more often than once a millisecond, this value can be considered unique. — Matthias
– Matthias, Commented Oct 23, 2013 at 9:30
This is not a question about Java, Math nor Computer science. It is about the concept of milliseconds. — Captain Giraffe
– Captain Giraffe, Commented Oct 23, 2013 at 9:37
Yes the test case will be called with a delay of 200 milliseconds at least. and it is okay if some are not unique - the server will take care of it. I udnerstand a simple generator will do the job too (static int) just wanted to know math behind the long — tgkprog
– tgkprog, Commented Oct 23, 2013 at 10:51

Clément · Accepted Answer · 2013-10-23 09:25:36Z

currentTimeMillis is actually the number of milliseconds since January 1, 1970 (source).

You can get seconds, minutes, and hours using modulus operations (milliseconds % 1000 = seconds and so on), but the results won't be fully accurate due to leap seconds. See the discussion in http://docs.oracle.com/javase/6/docs/api/java/util/Date.html for more details.

bjedrzejewski · Accepted Answer · 2013-10-23 09:32:10Z

Miliseconds are 1/1000 of second. Form there it is quite easy:

You can take 3 digits and get the same value for a whole second (1000 milis)

You can take 4 digits and get the same value for a whole minute (60000 milis > 10000)

You can take 6 digits and get the same value for a whole hour (3600000 milis > 1000000)

You can take 7 digits and get the same value for a whole day (86400000 milis > 10000000)

If you need more the formula is quite easy from there. I assume that this is what you meant. If you take digits from the front, you will get different value every mili. If you want to create range of unique values you can reverse what I said here:

To have the unique values for a second leave 3 digts. (1000 milis)

To have unique values for a minute leave 5 digits. (60000 milis < 100000)

To have unique values for an hour leave 7 digits. (3600000 milis < 10000000)

To have unique values for a day leave 8 digits. (86400000 milis < 100000000)

The problem is not how many digits you take from the front (as the length varies), but how many digits you leave.

In the best case, this is true. In the worst case any digit can change in one millisecond.
Of course, I am just giving a window. Also- if you care about uniqueness, then the second part ofmy answer is always true.
> rand = rand.substring(rand.length() - 10); so this is right to get a value that will be unique in a given day ?
More less yes- make sure that rand.length() - 10 is not negative.

phlogratos · Accepted Answer · 2013-10-23 09:37:22Z

Assuming all 10 digits on the right are 9, then the remaining value changes the next millisecond.

Alex Suo · Accepted Answer · 2013-10-23 09:55:21Z

Apart from your ignorance (not I am mean but you apparently didn't even read the documentation), I hereby have my 2 cents about accuracy:

JVM doesn't have a say on time accuracy. The nano time and millisecond accuracy depends on the OS. E.g. if you are under Windows or normal Linux without special libraries then all digits after the 1ms point would be 0. However, special 3rd party libs supports micro and (occasionally) nano seconds.

Given the quality of your question, I don't see any reason you need to go nano, if micro.

I want a unique value not an accurate one. But i get that being mean is cool and gives you a feeling of superiority. :-)

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