If you are interested in when a particular class was added to the JDK, check its javadoc - each javadoc page tells you the JDK version in which it was introduced.
Dot releases (e.g. 1.4.2) do not have any API changes over the respective base release (e.g. 1.4.0).
I don't think the documentation for old JDKs is still online, but can you download them from the
archive area.
J2SDK is the same as JDK, starting with Java 1.2 (a.k.a. Java 2).
[ May 04, 2007: Message edited by: Ulf Dittmer ]