how can i fetch current date and time of my system in java without using any predefined classes or libraries?

Question

how can i fetch current date and time of my system in java without using any predefined classes or libraries? i am actually making a Date ADT of my own from scratch as java.util.Date or Java.util.Calendar. but i am not able to find out a way to fetch system date and time, say to calculate age of somebody if he enters his current age. in a way i want to learn how they developers of Date or Calendar class actually implemented it. is there a way that i can see the code of Date and Calendar classes defined in java.util package.

Answer 1

Using System.currentTimeMillis():

Returns the difference, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.

As for the source code of java.util.Date and java.util.Calendar, you can either download the source directly from http://jdk7.java.net/source.html, or view it online in docjar.com or here.

Answer 2

Well System.currentTimeMillis() isn't in java.util - is that what you were looking for? You'd then have to write the code to work out what that value (milliseconds since the unix epoch) means in terms of years etc - and you should consider the time zone and calendar system.

I would personally avoid trying to write your own date/time package unless it's just for fun. java.util.Date / Calendar are awful, but Joda Time is much better, and there's the upcoming JSR 310.

Is there a way that i can see the code of Date and Calendar classes defined in java.util package.

Sure - when you installed the JDK, you should have had the option of also installing a copy of the source code for the standard libraries, typically in src.zip. Alternatively, you could go to openjdk.java.net and look there.

actually i don't want to use any predefined function at all.

Then you'd have to use JNI to get to an underlying system call, which seems pretty pointless to me - as it's basically what System.currentTimeMillis will do. That's fine if you're trying to learn JNI, but it's not really got much to do with date/time.

Of course if you're interested in an accurate time instead of the *system time, you could implement an NTP client.