A super-simple platformer game I made in Java (no external gaming libraries) for an Android phone I had at the time, must have been a Galaxy SII. As far as I recall this is my first more software-focused project where I tried to engineer a simple 2D platformer game engine.
To generate new worlds there is also a "World Maker" application I made in Processing, which runs on Windows (and since Java's "Write once, run anywhere" principle applies, theoretically it should also work on Linux and MacOS too). It allows to create different worlds quickly.
Looking back at how I made it and how the code was written it makes me cringe a little, but on the other hand it's always good to remember where it all began. Available on my GitHub.