Java is really popular or so says Google. The Java Client library for Ads API is still used by a lot of developers despite the fact that it's really old. Google tried to fix what was already there, but old dogs can't learn new tricks. That's why Google got a new dog that can learn all kinds of new tricks.
If the above canine analogy was lost on you, all it means is that Google has completely rewritten the Ads API Java Client library. It was in the experimental phase for a while, but now it's ready for showtime. It contains a number of improvements that make this the definitive version of the Java Client.
The new features contained in the new client includes product level support for AdWords, DFP and DFA while sharing a common layer code, and the library is now hosted on Maven. Other changes include support for both OAuth 1.0a and OAuth 2.0 alongside using the SLF4J logging facade. Finally, it adds more control over session building and experimental AppEngine Support.
The two big changes, however, come in the form of configuring a session and obtaining service objects. The company has provided a migration guide that explains this new code in depth. It also contains tips and strategies on how to take advantage of the new Java Client library. It's a must if you intend to start using it for future Ads API projects.
If change frightens you, worry not, Google will still offer the old Java Client library. It's being put into maintenance mode, so the new client will be the primary focus. The old client will still receive support for new releases of AdWords and DFP for the near future.
They don't say how long developers have until they fully stop support for the old Java Client, but I suggest you upgrade. It can only be a good thing especially with all the new features that came along with the client. If anything, you can pat yourself on the back for supporting progress.