This is a relatively simple library designed to emulate most of the features of Greenfoot in Processing. This was done for a school project and aims to enable easy 2D game creation within Processing.
First, go to the Releases tab and download the .zip file, and unzip it to wherever you want to keep the library.
To install the library, simply open Green-<version>.zip\library\ and put Green.jar in your Processing libraries folder.
Processing's libraries folder is located in <sketchbookLocation>\libraries, and it's sketchbook location can be found in the editor under File > Preferences....
From there, select Sketch > Import Library... in the editor, and Green should show up for selection.
To see the Javadoc reference, please visit https://zedseven.github.io/Green/.
The library works with two basic classes: Worlds and Actors.
The World class is where all the interaction and gameplay takes place - it's where all the Actors live, and for most games one World is enough.
The Actor class is where all the per-item code goes: you might have a player Actor, one for an item the player can pick up, and maybe a few ones for obstacles.