JCSG

Java implementation of BSP based CSG (Constructive Solid Geometry). It is the only simple and free Java implementation I am aware of. This implementation uses an optimized CSG algorithm based on csg.js (see CSG and Node classes). Thanks to the author for creating the csg.js library.

There's also the related VCSG library, a plugin for VRL-Studio and two extension libraries: JCSG-MeshExtension and JCSG-PathExtension.

In addition to CSG this library provides the following features:

optimized difference() and union() operations (many thanks to Sebastian Reiter)
extrusion of concave, non-intersecting polygons (uses Poly2Tri for triangulation)
convex hull (uses QuickHull3D)
weighted transformations (Scale, Rotation, Translation and Mirror)
STL import and export (STLLoader from Fiji)
OBJ export including material information (see screenshot below)
supports conversion of CSG's to JavaFX 3D nodes
3d text support (using FXyz)

JCSG on stackoverflow.

To see what's possible with JCSG try JFXScad.

How to Build JCSG

Requirements

Java >= 11
Internet connection (dependencies are downloaded automatically)
IDE: Gradle Plugin (not necessary for command line usage)

IDE

Open the JCSG Gradle project in your favourite IDE (tested with NetBeans 7.4) and build it by calling the assemble task.

Command Line

Navigate to the Gradle project (e.g., path/to/JCSG) and enter the following command

Bash (Linux/OS X/Cygwin/other Unix-like shell)

bash gradlew assemble

Windows (CMD)

gradlew assemble

Code Sample:

// we use cube and sphere as base geometries
CSG cube = new Cube(2).toCSG();
CSG sphere = new Sphere(1.25).toCSG();

// perform union, difference and intersection
CSG cubePlusSphere = cube.union(sphere);
CSG cubeMinusSphere = cube.difference(sphere);
CSG cubeIntersectSphere = cube.intersect(sphere);
        
// translate geometries to prevent overlapping 
CSG union = cube.
        union(sphere.transformed(Transform.unity().translateX(3))).
        union(cubePlusSphere.transformed(Transform.unity().translateX(6))).
        union(cubeMinusSphere.transformed(Transform.unity().translateX(9))).
        union(cubeIntersectSphere.transformed(Transform.unity().translateX(12)));
        
// save union as stl
try {
    FileUtil.write(
            Paths.get("sample.stl"),
            union.toStlString()
    );
} catch (IOException ex) {
    Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}

Thanks to

JetBrains for their IntelliJ IDEA license(s)
Evan Wallace for creating the JavaScript library csg.js

mwturvey/JCSG