mathematical graph theory library for Java

 JGraphT is a free Java graph library that provides mathematical
 graph theory objects and algorithms. JGraphT supports various types of
 graphs including:
  - directed and undirected graphs;
  - graphs with weighted, unweighted, labeled or user-defined edges;
  - various edge multiplicity options, including simple graphs,
    multigraphs and pseudographs;
  - unmodifiable graphs: allow modules to provide "read-only" access
    to internal graphs;
  - listenable graphs: allow external listeners to track modification events;
  - subgraphs: graphs that are auto-updating subgraph views on other graphs;
  - all compositions of above graphs.
 .
 JGraphT is designed to be simple and type-safe (via Java generics): graph
 vertices can be of any objects: Strings, URLs, XML documents, even graphs
 themselves.