protected int getVerticesIndexFor( Vertex<T> v ) {
    if ( v == null ) {
      throw new IllegalArgumentException( "null vertex" );
    }
    for ( int i = 0; i < vertices.length; i++ ) {
      if ( vertices[i] != null && vertices[i].equals( v ) ) {
        return i;
      }
    }
    return -1;
  }