Here you find Java implementations of tree edit distance algorithms:
http://www.inf.unibz.it/dis/projects/tree-edit-distance
In addition to Zhang&Shasha's algorithm of 1989, there are also tree edit distance implementations of more recent algorithms including Klein 1998, Demaine et al. 2009, and the Robust Tree Edit Distance (RTED) algorithm by Pawlik&Augsten, 2011.
