@Test
  public void itShouldBeAdjacentToEachOther() throws Exception {
    WeightedEdge edge = new WeightedEdge(1, 2, 1.0);
    graph.addEdge(edge);

    assertThat(graph.adjacentTo(1), is(Collections.singletonList(edge)));
    assertThat(graph.adjacentTo(2), is(Collections.singletonList(edge)));
  }