public void setCollisionGroup(int collisionGroup) {
        this.collisionGroup = collisionGroup;
    }
Sets the collision group number for this physics object. <br>
The groups are integer bit masks and some pre-made variables are available in CollisionObject.
All physics objects are by default in COLLISION_GROUP_01.<br>
Two object will collide when <b>one</b> of the parties has the
collisionGroup of the other in its collideWithGroups set.
@param collisionGroup the collisionGroup to set
jme3-jbullet/src/main/java/com/jme3/bullet/collision/PhysicsCollisionObject.java