How to do something like listener of creating instances in other module?

Question

Hello, I have module GUI, which has JTRee. And 2nd module creates ArrayLists of my own defined objects, which have some structure. For example car = new Car(); car.addEngine(new Engine("eng")); car.getEngine("eng").addWheel(new Wheel());

How to do my JTree in GUI module listens creating of objects in 2nd module and adding it to JTree?