Ykhjuy

The Collections.reverse implies only to List which is just one type of Collection, you cannot cast a Queue to a List. But you can try casting it to a LinkedList as:

Collections.reverse((LinkedList)queue);

Details:

I doubt that there is a built-in API for reversing the queue. You could still follow a conventional way of doing that using a Stack as :

Stack<Integer> stack = new Stack<>();

while (!queue.isEmpty()) {

    stack.add(queue.remove());

}

while (!stack.isEmpty()) {

    queue.add(stack.pop());

}

and then convert to an array as you will

int res = queue.stream().mapToInt(Integer::intValue).toArray();

On the other hand, if a Deque satisfies your needs currently, you can simply rely on the LinkedList itself since it implements a Deque as well. Then your current implementation would be as simple as :

LinkedList<Integer> dequeue = new LinkedList<>();

Collections.reverse(dequeue);

int res = dequeue.stream().mapToInt(Integer::intValue).toArray();

whether the queue is reversed is not important. An int array of the
reversed elements is what I need.

Another solution from what others have already suggested is to reverse the Stream of the queue and then mapToInt to convert to an array as :

Queue<Integer> queue = new LinkedList<>();

int res = reverse(queue.stream()).mapToInt(Integer::intValue).toArray();

This uses a utility reverse suggested by Stuart Marks in this answer such that:

@SuppressWarnings("unchecked")

static <T> Stream<T> reverse(Stream<T> input) {

    Object temp = input.toArray();

    return (Stream<T>) IntStream.range(0, temp.length)

            .mapToObj(i -> temp[temp.length - i - 1]);

}

No need to get fancy here.

static int toReversedArray(Queue<Integer> queue) {

    int i = queue.size();

    int array = new int[i];

    for (int element : queue) {

        array[--i] = element;

    }

    return array;

}

Not a one-liner, but easy to read and fast.

First, please don't use raw types (do use the diamond operator). Not quite a one liner, but you could first convert to an int and then use commons lang ArrayUtils.reverse(int) like

Queue<Integer> queue = new LinkedList<>();

// ...

int arr = queue.stream().mapToInt(Integer::intValue).toArray();

ArrayUtils.reverse(arr);

You could also write your own int reverse method that allowed for a fluent interface (e.g. return the int) then you could make it a one liner. Like,

public static int reverse(int arr) {

    for (int i = 0; i < arr.length / 2; i++) {

        int temp = arr[i];

        arr[i] = arr[arr.length - i - 1];

        arr[arr.length - i - 1] = temp;

    }

    return arr;

}

And then

int arr = reverse(queue.stream().mapToInt(Integer::intValue).toArray());

In Java8 version you can use Stream API to help you.

The skeleton of code like this：

int reversedQueue = queue.stream()

    .collect(Collector.of(() -> new ArrayDeque<Integer>(), ArrayDeque::addFirst, (a,b)->a))

    .stream().mapToInt(Integer::intValue).toArray();

You can use the LazyIterate utility from Eclipse Collections as follows.

int res = LazyIterate.adapt(queue)

        .collectInt(i -> i)

        .toList()

        .asReversed()

        .toArray();

You can also use the Collectors2 class with a Java Stream.

int ints = queue.stream()

        .collect(Collectors2.collectInt(i -> i, IntLists.mutable::empty))

        .asReversed()

        .toArray();

You can stream the int values directly into a MutableIntList, reverse it, and then convert it to an int array.

int ints =

    IntLists.mutable.ofAll(queue.stream().mapToInt(i -> i)).asReversed().toArray();

Finally, you can stream the int values directly into a MutableIntStack and convert it to an int array.

int ints =

    IntStacks.mutable.ofAll(queue.stream().mapToInt(i -> i)).toArray();

Note: I am a committer for Eclipse Collections.

This is one line, but it may not be very efficient:

int res = queue.stream()

                 .collect(LinkedList<Integer>::new, (l, e) -> l.addFirst(e), (l1, l2) -> l1.addAll(l2))

                 .stream()

                 .mapToInt(Integer::intValue)

                 .toArray();

If you want to be efficient and readable, you should continue using what you have now.

Finally, I figure out this one line solution.

Integer intArray = queue.stream()

            .collect(LinkedList::new, LinkedList::addFirst, LinkedList::addAll)

            .toArray(new Integer[queue.size()]);

the int version should like

int intArray = queue.stream()

            .collect(LinkedList<Integer>::new, LinkedList::addFirst, LinkedList::addAll)

            .stream()

            .mapToInt(Integer::intValue)

            .toArray();

Here is a different solution using Stream and Collections.reverse() in one line of code:

Integer reversedArray = queue.stream()

        .collect(Collectors.collectingAndThen(Collectors.toList(),

                list -> {

                    Collections.reverse(list);

                    return list.toArray(new Integer[0]);

                }

        ));

OR

int reversedArray = queue.stream()

        .collect(Collectors.collectingAndThen(Collectors.toList(),

                list -> {

                    Collections.reverse(list);

                    return list.stream()

                            .mapToInt(Integer::intValue)

                            .toArray();

                }

        ));