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.

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.

• Java >= 1.8
• Internet connection (dependencies are downloaded automatically)
• IDE: Gradle Plugin (not necessary for command line usage)

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

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

bash gradlew assemble

gradlew assemble

// 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);
}