JVoroTreemap is a fast standalone java library which computes Voronoi treemaps. The following article contains most important references related to this implementation. Arlind Nocaj, Ulrik Brandes, ...

The TreeMap in Java is a powerful data structure that is used for sorting and associating values with keys. It’s easy to create, modify, and iterate over a TreeMap in Java.

My best guess to what was causing this is that your TreeMap contained non-static inner class instances and that your outer class wasn't serializable or it contained a member that wasn't.