public void removeEdge( Vertex<T> v1, Vertex<T> v2 ) {
    v1Pos = getVerticesIndexFor( v1 );
    v2Pos = getVerticesIndexFor( v2 );

    if ( v1Pos == -1 || v2Pos == -1 ) {
      throw new IllegalArgumentException( "vertex not found" );
    }
    if ( this.adjMatrix[v1Pos][v2Pos] == 1 ) {
      this.adjMatrix[v1Pos][v2Pos] = 0;
      this.numberOfEdges--;
    }
    else {
      throw new IllegalArgumentException( "edge not found" );
    }
  }