Java 流“forEach"但不消耗流 [英] Java stream "forEach" but not consuming stream

问题描述

有时在处理流的步骤之间对流中的每个元素执行某些操作"(例如打印)会很方便，例如用于调试.

Sometimes it would be handy do "something" (e.g. print) with every element in a stream in between steps of processing the stream, e.g. for debugging.

一个简单的例子可能看起来像这样，不幸的是这不起作用，因为 forEach 消耗流:

A simple example could look like this, unfortunately this does not work as forEach consumes the stream:

List<String> list = new ArrayList<>();
list.add("one");
list.add("two");
list.add("three");
list.add("four");

List<String> filteredList = 
        list.stream()
        .filter(s -> s.startsWith("t"))
        .forEach(System.out::println)
        .collect(Collectors.toList());

如何做到这一点?

推荐答案

您正在寻找 peek 操作:

You are looking for the peek operation:

此方法的存在主要是为了支持调试，您希望在元素流过管道中的某个点时查看它们

This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline

此方法将在 Stream 管道的所有元素被消耗时执行给定的操作.因此，它允许获取 peek 元素.

This method will execute the given action on all elements of the Stream pipeline as they are consumed. As such, it allows to take a peek of the elements.

List<String> filteredList = 
    list.stream()
        .filter(s -> s.startsWith("t"))
        .peek(System.out::println)
        .collect(Collectors.toList());

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