So have you ever used Java before?

I ask because if you haven't that the logical place to start is with learning the basics, followed by the building blocks to a multiplayer game.

If I were to start building a multiplayer game (preface: the main language I write in is Java, due to it being the main language in my high school's cs classes and my current cs course), I would start with building a really simple single player game. Start with just the world and one player.

After getting a single player game working (as best you can with just one player if the game is designed solely for multiple players), start trying to decouple the world from the gui, start making the world a server and the gui a client.

After that, you've got most of the pieces you need to start adding more players.

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Additionally, I was in a class where the final project was to create a very simple multiplayer world. Our process in leading up to that project was simply working with the individual aspects of the project beforehand and adding it together for the final.

We started off the class working with graphics, making a ball bounce using relative gravity on the screen, animating sprites, creating and modifying images (taking a jpeg with the color white meant to be transparent and making it transparent on the screen, fading an image in and out, really simple sounding things that would be used to form the basis of more complex, fun things).

The second part was mostly networking. Starting with reading and writing text files (which was done before the graphics, to let us read and write images) and moving towards sending and receiving text from other machines. One of the more complex projects in the class started with our teacher building a number guessing server: his computer would pick a random number from Integer.MIN_INT to Integer.MAX_INT, and we would have to guess it. The first of the three projects with the random number server involved creating a simple client that would have users enter a number. The second part required us to automate the server. The third had us take the server and add user authentication to it.

After the random number projects, we moved on to sending and receiving objects over the network, specifically images, and having them show up on another computer's screen.

All of that lead into the final project, which had us build a server that received icons from a computer, give each of those icons a position, and track changes in position. The client for the server would pick an image, send that image to the server, and move the icon around using the arrow keys. The client would also display other icons from other clients connected to the server. Essentially a really simple multiplayer game.

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In my current class, there was just an assignment requiring us to use both the observer and proxy design patterns for a simple blogging application. Using the observer and proxy patterns would probably be beneficial to you when you get into the server/client part of the game. I don't really have any good resources for leaning about those because it was mostly a lecture coupled with an assignment and lab problem.

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I'm not sure if that answered your questions, but I hope it helped give you an idea of where to start. I also acknowledge that I put down a lot in this comment, but I personally think it's at least somewhat helpful.