Graphs in Java

Queue<Integer> queue = new LinkedList<>();
Set<Integer> visited = new HashSet<>();
queue.offer(start);
visited.add(start);

while (!queue.isEmpty()) {
    int node = queue.poll();
    for (int next : graph.get(node)) {
        if (!visited.contains(next)) {
            visited.add(next);
            queue.offer(next);
        }
    }
}