How to Reverse Streams in Java: Specific and Generic Approaches?

Reversing Streams in Java: Generic and Specific Approaches

When dealing with Java streams, understanding how to reverse them can be crucial. Let's delve into the specific and generic approaches for reversing streams.

Specific Approach: Reversing an IntStream

To reverse an IntStream while maintaining its range, you can utilize the following approach:

<code class="java">static IntStream revRange(int from, int to) {
    return IntStream.range(from, to)
                    .map(i -> to - i + from - 1);
}</code>

This method avoids unnecessary boxing and sorting operations.

Generic Approach: Reversing a Stream of Any Type

For streams of arbitrary types, there's currently no predefined "proper" way to reverse them. However, here are two common options:

1. Reversing Using Array:

This method stores the stream elements in an array and retrieves them in reverse order:

<code class="java">@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]);
}</code>

2. Reversing Using Collector:

This method uses a collector to accumulate elements in a reversed list:

<code class="java">Stream<T> input = ... ;
List<T> output =
    input.collect(ArrayList::new,
                  (list, e) -> list.add(0, e),
                  (list1, list2) -> list1.addAll(0, list2));</code>

Efficient Reversal Using ArrayDeque

Due to its O(N^2) copying inefficiency, we update our previous code using an ArrayDeque for more efficient insertion at the front:

<code class="java">Deque<String> output =
    input.collect(Collector.of(
        ArrayDeque::new,
        (deq, t) -> deq.addFirst(t),
        (d1, d2) -> { d2.addAll(d1); return d2; }));</code>

This method produces a reversed Deque, which can be easily converted back into a Stream or iterated directly.

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