public void add(N node){
  if (theNeighbors.containsKey(node))   return;
  theNeighbors.put(node,new ArraySet<N>());
}

public void add ( N node ) { if ( theNeighbors . containsKey ( node ) ) return ; theNeighbors . put ( node , new ArraySet < N > ( ) ) ; }
add a node . if node is already in graph then no change .
