Mengine is a project mainly written in Java, it's free.
Simple Java game engine for educational purposes
MooseEngine is a small, simple game engine created in Java as a learning tool. It's meant to be a model for introducing novice Java programmers to game engine concepts like the game loop, entities in a world, drawing sprites, etc.
Included is a sample demo featuring MegaShoaff, the most amazing department head in the world. It's over-commented as a sort've walkthrough, so feel free to start there.
The MIT License
Copyright (c) 2011 Michael Kelly
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.