public void addEdge(final EdgeType edge){
  Preconditions.checkNotNull(edge,"Edge argument can not be null");
  m_edges.add(edge);
  updateNodeToEdgeMapping(edge);
}

public void addEdge ( final EdgeType edge ) { Preconditions . checkNotNull ( edge , "Edge argument can not be null" ) ; m_edges . add ( edge ) ; updateNodeToEdgeMapping ( edge ) ; }
adds an edge to the graph .
