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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