What about the new Java?
Are you looking for documentation on the WPI Library APIs that we use in FRC? If so, all of the previous documentation is more or less applicable- those haven't changed much at all.
If you are looking for documentation on new features that we get by switching to Java 8 SE, then I would recommend starting with the
Java 8 release notes from Oracle. You will also want to search for the Java 7, Java 6, and Java 5 release notes. You can also find an countless number of tutorials, guides, and tips for Java 5-8 online.