public class MockGraph implements Graph<String> {

    private final Map<String, MockNode> nodes = new ConcurrentHashMap<String, MockNode>();
    private final MockNode root = new MockNode();

    public Iterable<String> getAllNodes() {
        return Collections.unmodifiableCollection(nodes.keySet());
    }

    public Iterable<String> getRootNodes() {
        return Collections.unmodifiableCollection(root.edges);
    }

    public Iterable<String> getConnectedNodesOf(String node) {
        return Collections.unmodifiableCollection(getNode(node).edges);
    }

    public void createNode(String node) {
        nodes.put(node, new MockNode());
    }

    public void createRootNode(String node) {
        createNode(node);
        root.edges.add(node);
    }

    public void removeNode(String node) {
        nodes.remove(node);
        root.edges.remove(node);
    }

    public void createDirectedEdge(@Nullable String from, String to) {
        getNode(from).edges.add(to);
    }

    public void removeDirectedEdge(@Nullable String from, String to) {
        getNode(from).edges.remove(to);
    }

    public long getStatus(String node) {
        return getNode(node).status;
    }

    public void setStatus(String node, long status) {
        getNode(node).status = status;
    }

    private MockNode getNode(@Nullable String node) {
        if (node == null) {
            return root;
        }
        MockNode n = nodes.get(node);
        if (n == null) {
            n = new MockNode();
        }
        return n;
    }

    private class MockNode {
        public long status = NULL_STATUS;
        public final List<String> edges = new CopyOnWriteArrayList<String>();
    }
}