public void addEdge(final V u, final V v, final int capacity, final int cost, final int flow) {
        Objects.requireNonNull(u);
        Objects.requireNonNull(v);
        addEdge(u, new Edge(v, capacity, cost, capacity - flow, flow));
    }
@Test
    public void testNullNode() {
        final Graph<Integer> graph1 = new Graph<>();
        assertThrows(NullPointerException.class, () -> graph1.addEdge(null, 1, 1, 1, 1));
        assertThrows(NullPointerException.class, () -> graph1.addEdge(1, null, 1, 1, 1));
    }