I'm a software engineer student (about to start my 2nd year) and I did really enjoy last years object-oriented programming and working with Java (greenfoot and later eclipse) to make simple games. I would really like to delve deeper in to software development and can't wait for 2nd year of college to start ,so I mocked a bit in eclipse and boy writing all of the game engine is hard and takes a lot of time, so maybe anyone knows an easy game engine just to mock around and explore game development for fun ?
Have you taken a look at: jMonkey Engine it's a free Java game engine that uses OpenGL and is licensied under BSD. Since they distribute all the engine source code, you can change it up and experiment as needed.
I am not going to recommend a game engine to you. However, I think you may like my suggestion better, and it is already well suited to the skills you have developed in Java.
You may wish to develop MineCraft plugins. You will naturally need a MineCraft account, and need to learn how to set up a Bukkit server, but this task will give you accidental skills you will want as a software engineer later anyway.
Also, there is a thriving community of MC plugin makers (I'm one of them), and examples are plentiful and almost always open source.
Plus, you can then start adding or changing functionality of a well-established game, which will give you quicker results than trying to make a game from nothing.