public HalfEdge addEdge(Coordinate orig,Coordinate dest){
  if (!isValidEdge(orig,dest))   return null;
  HalfEdge eAdj=(HalfEdge)vertexMap.get(orig);
  HalfEdge eSame=null;
  if (eAdj != null) {
    eSame=eAdj.find(dest);
  }
  if (eSame != null) {
    return eSame;
  }
  HalfEdge e=insert(orig,dest,eAdj);
  return e;
}

public HalfEdge addEdge ( Coordinate orig , Coordinate dest ) { if ( ! isValidEdge ( orig , dest ) ) return null ; HalfEdge eAdj = ( HalfEdge ) vertexMap . get ( orig ) ; HalfEdge eSame = null ; if ( eAdj != null ) { eSame = eAdj . find ( dest ) ; } if ( eSame != null ) { return eSame ; } HalfEdge e = insert ( orig , dest , eAdj ) ; return e ; }
adds an edge between the coordinates orig and dest to this graph . only valid edges can be added ( in particular , zero - length segments cannot be added )
