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]);
}</t></t></t></t></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));</t></t></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; }));</string></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.

The above is the detailed content of How to Reverse Streams in Java 8: A Comprehensive Guide. For more information, please follow other related articles on the PHP Chinese website!