You could download the javadocs for each version, and run a UNIX diff on
the relevant files. For Sun's standard API, they tend to only add new API,
or deprecate old API. They *usually* don't change existing API, nor do they
completely remove old API.

   - Oliver

...

Theoretically, the /current/ JavaDocs should tell you the differences
through use of the @since tag in the source, but that information
is patch.

For example..
http://java.sun.com/j2se/1.5.0/docs/api/javax/swing/JSpinner.html
..reports that JSpinner is 'since 1.4', but beware!

http://java.sun.com/j2se/1.5.0/docs/api/java/awt/Color.html#WHITE
(Color.WHITE, as opposed to Color.white) also introduced in 1.4,
but /not/ documented.  ..I also quickly checked the javax.swing
package summary and another swing class to see that neither
had an @since tag.  Uggghhh!

OTOH, all is not lost, if you can obtain a JRE of the variety
you are interested in, you can find out quite definitely if it
has a particular class, field of method - by compiling
Java source using it as the -bootclasspath.

HTH

Andrew T.