Thanks to VERVE, a robot simulation platform running on eclipse, my knowledge of Java is increasing daily. Currently I am going through the VERVE code that deals with importing and parsing KML overlays (Google Earth File Format), and in the process I encountered the concept Scene Graph, which is how Ardor3D (the graphics library we use for VERVE) is structured.

Scene graph is a directed acyclic graph (DAG) arranged in a tree structure that is composed of nodes. The objects are stored in the leaf nodes of the tree and the non-leaf (group) nodes are there to group the leaf nodes together.The nodes in the VERVE scene graph are referred using weak references.

Weak References are references to objects that don't interfere with garbage collection. Normally, the objects are referenced using strong references:

StringBuffer buffer = new StringBuffer();

But in some cases, strong references causes memory leaks since it forces an object to remain in memory if it is reachable via chain of strong references.

Weak references don't force the object to remain in memory. They are created like this:

WeakReference<Widget> weakWidget = new WeakReference<Widget>(widget);

Sometimes garbage collector might throw out the weak referenced object before you ask for the object. And when you do ask, you might get back "null". To prevent this, you can use a weak hashmap, which has keys that are referred to with weak references. Weak Hashmap automatically removes the entry if key becomes no longer reachable.