How to Reverse Streams in Java 8: A Comprehensive Guide

How to Reverse Java Streams: A Comprehensive Guide

Introduction

Reversing streams in Java 8 can be a common requirement in various scenarios. This article delves into effective techniques for achieving stream reversals, addressing both specific and general cases.

Specific Case: Reversing an IntStream

To reverse an IntStream, a customized approach is necessary. The following revRange method generates a reverse IntStream within a given range:

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

General Case: Reversing Streams of Any Type

For general streams of any type, there are several approaches:

1. Store Elements and Reverse:

Store stream elements in an array and then read them in reverse order:

<code class="java">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. Collect into Reversed List:

Accumulate elements into a reversed list using collectors:

<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>

3. Efficient List Reversal (Update 2016-01-29):

Use an ArrayDeque, which efficiently supports insertion at the front, to avoid inefficiency:

<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>

Summary

These techniques provide effective ways to reverse Java 8 streams. For specific IntStreams, revRange is a dedicated solution. For general streams, storing elements or collecting into lists offers practical options. For optimal efficiency when collecting into lists, ArrayDeque should be used.

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