public class CounterWindow {

    public static final CounterWindow INSTANCE = new CounterWindow();

    private final Map<ID, Tuple2<Long, Double>> lastElementMap = new ConcurrentHashMap<>();
    private final Map<ID, Queue<Tuple2<Long, Double>>> windows = new ConcurrentHashMap<>();

    public Tuple2<Long, Double> increase(String name, ImmutableMap<String, String> labels, Double value, long windowSize, long now) {
        ID id = new ID(name, labels);
        // there will return the same PriorityQueue object for concurrent operation
        Queue<Tuple2<Long, Double>> window = windows.computeIfAbsent(id, unused -> new PriorityQueue<>());
        window.offer(Tuple.of(now, value));  // some unexpected error will occur in this line when concurrent operation
        long waterLevel = now - windowSize;
        Tuple2<Long, Double> peek = window.peek();
        if (peek._1 > waterLevel) {
            return peek;
        }

        Tuple2<Long, Double> result = peek;
        while (peek._1 < waterLevel) {
            result = window.poll();  // some unexpected error will occur in this line when concurrent operation
            peek = window.element();  // some unexpected error will occur in this line when concurrent operation
        }

        // Choose the closed slot to the expected timestamp
        if (waterLevel - result._1 <= peek._1 - waterLevel) {
            return result;
        }

        return peek;
    }
    //....
}