### Question:
LightUGraph implements UGraph<L>, FastWalkable<L, UNode<L>> { @Override public Iterable<? extends ULink<L>> links() { return new LinkCollection(); } LightUGraph();  LightUGraph(int capacity); @Override int size(); @Override long numLinks(); @Override UNode<L> node(L label); @Override Collection<? extends UNode<L>> nodes(L label); @Override List<? extends UNode<L>> nodes(); @Override Iterable<? extends ULink<L>> links(); @Override UNode<L> add(L label); @Override Set<L> labels(); @Override boolean connected(L from, L to); @Override long state(); String toString(); void compact(int margin); void sort(); static LightUGraph<L> copy(Graph<L> graph); @Override int hashCode(); boolean equals(Object other); @Override UNode<L> get(int i); @SuppressWarnings("unchecked") @Override Class<? extends UGraph<L>> level(); @Override List<UNode<L>> neighborsFast(Node<L> node); static final int NEIGHBOR_CAPACITY; }### Answer:
@Test public void testLinks() { UGraph<String> graph = new LightUGraph<String>(); UNode<String> a = graph.add(null), b = graph.add(null), c = graph.add(null); a.connect(b); a.connect(c); a.connect(c); assertEquals(0, a.links(a).size()); assertEquals(1, a.links(b).size()); assertEquals(1, b.links(a).size()); assertEquals(2, a.links(c).size()); assertEquals(2, c.links(a).size()); }