I can't see Java's performance being an issue in a 2D game. May as well stick with what you're comfortable with. Unless you're really interested in learning other languages/libraries purely for the sake of learning, of course, in which case both C#/XNA and C++/DirectX would be good.

Learning OpenGL over DirectX would be better, in my opinion. Especially if the OP is wanting to port his games to other platforms other than Windows. I'm not trying to say there's a harm in learning DirectX, just might be better for the OP to learn OpenGL first.

Java is absolutely an option. There are loads of great titles out there written with it- Ever heard of MineCraft?

That said, coming off of 5 years playing around in XNA, I can say that the library and tooling (Visual Studio, XNA's Content Pipeline) are just fantastic and since Java and C# are very similar the learning curve would not be drastic. If I wasn't concerned about portability (XNA is stuck to Windows, 360, and Windows Phone) I'd be using it for my current project.

I'm presently working in C++ but it's hard for me to recommend that to fans of C# or Java. My suggestion though, is stick with what you know and love. There's no reason to not use Java if you're already very comfortable with it. You can get to OpenGL from Java using libraries like LWJGL, so there's no reason to dive into another language for it.

If you're "incredibly comfortable" with Java then stick with it. Java is a capable language that has successfully been used in numerous commercial games (I listed some in this post) and which shouldn't present any barriers to a 2d platform game. If you want to create the best possible product and you already know a capable language then that language is the best choice for you.

You might also consider packages such as Construct 2 or Game Maker if your main interest is simply getting the game made rather than the development experience -- either would easily be capable of a 2d side-scrolling game with good performance, and would likely be quicker and easier than writing your own code.

C# and XNA are also very good, and probably wouldn't be overly difficult to pick up given your Java experience. These might make a good choice if you really wanted to take the opportunity to learn a new language.

Honestly though, it sounds like Java would be a great choice for you, and you can feel comfortable knowing that there shouldn't be any unexpected problems which stop you from making your game.

Hope that helps!

I just wanted to point out a bit of something that hasn't really been touched on yet. (And seems to be a hot topic lately as this is about the fourth time in the past week I've commented on it lol). Java and C# are both intermediary languages meaning that they compile to a byte code and then are executed through a virtual machine which acts as a final compilation process (at least this is the best way I can describe it). That means that your code in either of these languages is partially compiled then distributed, your players run time's will then execute the byte code converting it to native code to operate.

In older computers this could lead to some sever performance issues because the computer had to "translate" the byte code into the native code and then execute against OS specific libraries. Although this is still the case we are in the 21st century now and most if not all gamer's tend to have fairly powerful machines. It is normally a safe bet to assume that you end user has at the least a dual core processor, 3+ gigs of ram and at least a 1gb video card. All that power tends to muscle it's way through these "translations" pretty quickly.

It is also worth noting that Microsoft is pushing their "managed sdk" or .NET frameworks pretty hard and it seems that they really want everyone to give up on lower level languages such as C++ in favor of the "more correct" way of accessing their core kernels and libraries. That is to say that Microsoft is providing a much more stable and performance oriented managed system through C# and .NET than most people create on their own (and why not, they wrote the base libraries they would most likely know the best way to use them). So, C++ is still faster at the core level BUT after you take into account improper methods of doing things performance can actually turn out worse unless you are really experienced and know what you are doing.

So now lets compare Java to C# and the biggest question here is what platforms do you care about? If you care that your game is cross platform forget C# all together. There are ports through the Mono libraries that offer the .Net like functions on other operating systems but they are third party, open source and in my opinion shouldn't really be trusted. To new, a lot of it seems slapped together and with no monetary backing I just don't see hobbyist coders exposing the true potential of the .Net framework. With Java on the other hand it's been around a LOT longer, it's managed primarily by a for profit company (Oracle) and is maintained better for all operating systems than C# is.

The next thing to think about (and many may disagree here) is what operating systems gamer's use. 9 times out of 10 this turns out to be the latest windows (at least 7 right now). With that being said and considering that Microsoft wrote windows and all it's libraries, Microsoft wrote the .Net framework specifically to use these libraries to the best of their performance and potential and add in the fact that C# and .Net adapt themselves to the hardware they are running on.. C# on Windows tends to be a bit faster than Java on Windows. So if you're like me you aim to mainly (or only) target the windows audience. In this case C#, .NET and XNA are the quickest and easiest way's to get a game up and running with good performance. Also using C# and XNA you also gain the ability to submit your game to XBox Indie market (and maybe even the arcade market if you win a Dream Build Play or get a publisher). Considering that Windows game sales always surpass Mac and Linux game sales and XBox game sales normally surpass PS3 and Wii sales this is a pretty good path to take from a business / marketing and sales stand point.

So in conclusion, if you want to try to sell your game think long and hard about how important non windows based platforms matter to you. If you come up with the idea that they really don't I HIGHLY recommend starting to migrate to C# and XNA. If they do matter I suggest sticking to Java. If you are not aiming to try and sell your product just stick with Java as you are already comfortable with it the performance of 2D and not so intensive games won't be all that noticable.