如何在不使用任何预定义类或库的情况下用Java获取系统的当前日期和时间？

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

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.

Answer 2

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 .