(Again) java.util.Date vs java.sql.Date

Question

Hi there,
(Again) Im trying to understand the EXACT difference between
java.util.Date vs java.sql.Date.
Googling, I can see that this is a very "popular" subject, but I still
cannot figure out it exactly.
Many writers claim that java.sql.Date only stores the DATE part (yyyy-
mm-dd) but not the TIME part (hh:MM:ss) of a Date/Time value, but that
I can easily disprove:
                java.util.Date ud = new java.util.Date();                 java.sql.Date sd = new java.sql.Date(ud.getTime());                 System.out.println(DateFormatUtils.                                 format(ud, "yyyy-MM-dd HH:mm:ss.SSS"));                 System.out.println(DateFormatUtils.                                 format(sd, "yyyy-MM-dd HH:mm:ss.SSS"));
Output:
                2009-09-18 15:17:36.635                 2009-09-18 15:17:36.635
So, apparently, java.sql.Date and java.util.Date have THE SAME
precision (at least down to the millisecs...).
And the official API documentation, really looks more confusing than
helpful to me::
*"java.sql.Date:*
*A thin wrapper around a millisecond value that allows JDBC to identify*
*this as an SQL DATE value. A milliseconds value represents the*
*number of milliseconds that have passed since January 1, 1970*
*00:00:00.000 GMT.*
*To conform with the definition of SQL DATE, the millisecond values*
*wrapped by a java.sql.Date instance must be 'normalized' by setting*
*the hours, minutes, seconds, and milliseconds to zero in the*
*particular time zone with which the instance is associated. "*
Exactly what means "an SQL DATE value" ? How EXACTLY does it differ
from a java.util.Date value?
Most importantly: WHY does JDBC need to distinguish between them?
And, here again: *"a java.sql.Date instance must be 'normalized' by*
*setting the hours, minutes, seconds, and milliseconds to zero in the*
*particular time zone..."*
What does that mean exactly? Apparently, the constructor doesnt
enforce this restriction, per the example above. So what's the REAL
point with this type, java.sql.Date?
Very greatful, if you can help me clarify this, once and for all.
TIA,

Answer 1

And the official API documentation, really looks more confusing than helpful to me:The problem is that you need to understand SQL as well as Java for this to make sense. It's not the Java API's job to tell you how your SQL database works - there's a myriad of subtle differences even when the DB is compliant with the SQL spec.
Most compliant databases support DATE, TIME, and TIMESTAMP values. DATE represents only a date. TIME represents only a time. TIMESTAMP represents both. There are further complicating factors, but that's roughly how it stands.
In Java the normal type for representing time is (or was when the API was created) the java.util.Date but this is a close approximation only to the TIMESTAMP value. In order to bring the two together the java.sql.Date, java.sql.Time and java.sql.Timestamp classes were created. Making them derive from java.util.Date was probably not a good idea.
java.util.Date suffers from a number of deficiencies. java.util.Calendar was supposed to address them but didn't really succeed. The JodaTime library is rather better, but it's all a lot more complicated than you might expect - partly because time management really is a much harder problem than it appears at first glance - there are timezones, leap years, leap seconds, the difference between astronomical and atomic time, and so on and so forth.