HashQueueSet

public class HashQueueSet<T> extends LinkedHashSet<T> implements Queue<T> {
    @Override
    public boolean offer(T t) {
        return add(t);
    }

    @Override
    public T remove() {
        if(isEmpty()) throw new NoSuchElementException();
        return remove0();
    }

    @Override
    public T poll() {
        if(isEmpty()) return null;
        return remove0();
    }

    private T remove0() {
        Iterator<T> iterator = iterator();
        T t = iterator.next();
        iterator.remove();
        return t;
    }

    @Override
    public T element() {
        if(isEmpty()) throw new NoSuchElementException();
        return iterator().next();
    }

    @Override
    public T peek() {
        if(isEmpty()) return null;
        return iterator().next();
    }
}