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<!DOCTYPE html>

<head>

var _hmt = _hmt || [];

(function() {

var hm = document.createElement("script");

hm.src = "https://hm.baidu.com/hm.js?5230ac143650bf5eb3c14f3fb9b1d3ec";

var s = document.getElementsByTagName("script")[0];

s.parentNode.insertBefore(hm, s);

})();

</script><link rel="stylesheet" href="//at.alicdn.com/t/font_3180624_7cy10l7jqqh.css"><link rel="icon" href="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/favicon.ico"><link rel="icon" href="/assets/icon/chrome-mask-512.png" type="image/png" sizes="512x512"><link rel="icon" href="/assets/icon/chrome-mask-192.png" type="image/png" sizes="192x192"><link rel="icon" href="/assets/icon/chrome-512.png" type="image/png" sizes="512x512"><link rel="icon" href="/assets/icon/chrome-192.png" type="image/png" sizes="192x192"><link rel="manifest" href="/manifest.webmanifest" crossorigin="use-credentials"><meta name="theme-color" content="#096dd9"><link rel="apple-touch-icon" href="/assets/icon/apple-icon-152.png"><meta name="apple-mobile-web-app-status-bar-style" content="black"><meta name="msapplication-TileImage" content="/assets/icon/ms-icon-144.png"><meta name="msapplication-TileColor" content="#ffffff"><meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, viewport-fit=cover"><title>Java 8 Stream流详细用法 | Java程序员进阶之路</title><meta name="description" content="Java程序员进阶之路，小白的零基础Java教程，从入门到进阶，Java 8 Stream流详细用法">

<style>

:root {

--bg-color: #fff;

}

html[data-theme="dark"] {

--bg-color: #1d2025;

}

html,

body {

background: var(--bg-color);

}

</style>

const userMode = localStorage.getItem("vuepress-theme-hope-scheme");

const systemDarkMode =

window.matchMedia &&

window.matchMedia("(prefers-color-scheme: dark)").matches;

if (userMode === "dark" || (userMode !== "light" && systemDarkMode)) {

document.querySelector("html").setAttribute("data-theme", "dark");

}

</script>

</head>

<body>

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class="button-middle"></span><span class="button-bottom"></span></span></button></div></header><div class="toggle-sidebar-wrapper"><span class="arrow left"></span></div><aside class="sidebar"><ul class="sidebar-links"><li><a href="/home.html" class="nav-link sidebar-link sidebar-page" aria-label="一、前言">一、前言</a><ul class="sidebar-sub-headers"></ul></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><span class="title">二、Java核心</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.1 Java概述</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.2 Java语法基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.3 数组&字符串</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.4 面向对象编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.5 集合框架（容器）</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.6 IO</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.7 异常处理</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><span class="title">2.9 Java新特性</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><a aria-current="page" href="/java8/stream.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="Stream流详细用法">Stream流详细用法</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_01、创建流" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="01、创建流">01、创建流</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_02、操作流" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="02、操作流">02、操作流</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_1-过滤" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="1）过滤">1）过滤</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_2-映射" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="2）映射">2）映射</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_3-匹配" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="3）匹配">3）匹配</a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_4-组合" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="4）组合">4）组合</a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/stream.html#_03、转换流" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="03、转换流">03、转换流</a><ul class="sidebar-sub-headers"></ul></li></ul></li><li><a href="/java8/optional.html" class="nav-link sidebar-link sidebar-page" aria-label="Optional最佳指南">Optional最佳指南</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/java8/Lambda.html" class="nav-link sidebar-link sidebar-page" aria-label="Lambda表达式">Lambda表达式</a><ul class="sidebar-sub-headers"></ul></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.11 网络编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.12 NIO</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.13 并发编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.14 JVM</span><span class="arrow right"></span></button></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>Java 8 Stream流详细用法</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span>2022年3月15日</span><meta property="datePublished" content="2022-03-15T14:42:30.000Z"></span><span class="category-info" aria-label="分类🌈" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 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0173.143 85.577A457.143 457.143 0 11371.566 117.76a73.143 73.143 0 0185.577 73.143v339.383a36.571 36.571 0 0036.571 36.571z"></path></svg><span>约 2455 字</span><meta property="wordCount" content="2455"></span><span class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 8 分钟</span><meta property="timeRequired" content="PT8M"></span></div><hr></div><div class="toc-place-holder"><aside id="toc"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_01、创建流" class="router-link-active router-link-exact-active toc-link level2">01、创建流</a></li><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_02、操作流" class="router-link-active router-link-exact-active toc-link level2">02、操作流</a></li><ul class="toc-list"><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_1-过滤" class="router-link-active router-link-exact-active toc-link level3">1）过滤</a></li><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_2-映射" class="router-link-active router-link-exact-active toc-link level3">2）映射</a></li><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_3-匹配" class="router-link-active router-link-exact-active toc-link level3">3）匹配</a></li><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_4-组合" class="router-link-active router-link-exact-active toc-link level3">4）组合</a></li></ul><li class="toc-item"><a aria-current="page" href="/java8/stream.html#_03、转换流" class="router-link-active router-link-exact-active toc-link level2">03、转换流</a></li></ul></div></aside></div><div class="theme-hope-content"><p>两个星期以前，就有读者强烈要求我写一篇 Java Stream 流的文章，我说市面上不是已经有很多了吗，结果你猜他怎么说：“就想看你写的啊！”你看你看，多么苍白的喜欢啊。那就“勉为其难”写一篇吧，嘻嘻。</p><p><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/stream-1.jpg" alt="" loading="lazy"></p><p>单从“Stream”这个单词上来看，它似乎和 <a href="http://java.io" target="_blank" rel="noopener noreferrer">java.io<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 包下的 InputStream 和 OutputStream 有些关系。实际上呢，没毛关系。Java 8 新增的 Stream 是为了解放程序员操作集合（Collection）时的生产力，之所以能解放，很大一部分原因可以归功于同时出现的 Lambda 表达式——极大的提高了编程效率和程序可读性。</p><p>Stream 究竟是什么呢？</p><blockquote><p>Stream 就好像一个高级的迭代器，但只能遍历一次，就好像一江春水向东流；在流的过程中，对流中的元素执行一些操作，比如“过滤掉长度大于 10 的字符串”、“获取每个字符串的首字母”等。</p></blockquote><p>要想操作流，首先需要有一个数据源，可以是数组或者集合。每次操作都会返回一个新的流对象，方便进行链式操作，但原有的流对象会保持不变。</p><p>流的操作可以分为两种类型：</p><p>1）中间操作，可以有多个，每次返回一个新的流，可进行链式操作。</p><p>2）终端操作，只能有一个，每次执行完，这个流也就用光光了，无法执行下一个操作，因此只能放在最后。</p><p>来举个例子。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"武汉加油"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"中国加油"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"世界加油"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">long</span> count <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">count</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>distinct()</code> 方法是一个中间操作（去重），它会返回一个新的流（没有共同元素）。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Stream</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token function">distinct</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p><code>count()</code> 方法是一个终端操作，返回流中的元素个数。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">long</span> <span class="token function">count</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>中间操作不会立即执行，只有等到终端操作的时候，流才开始真正地遍历，用于映射、过滤等。通俗点说，就是一次遍历执行多个操作，性能就大大提高了。</p><p>理论部分就扯这么多，下面直接进入实战部分。</p><h2 id="_01、创建流" tabindex="-1"><a class="header-anchor" href="#_01、创建流" aria-hidden="true">#</a> 01、创建流</h2><p>如果是数组的话，可以使用 <code>Arrays.stream()</code> 或者 <code>Stream.of()</code> 创建流；如果是集合的话，可以直接使用 <code>stream()</code> 方法创建流，因为该方法已经添加到 Collection 接口中。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CreateStreamDemo</span> <span class="token punctuation">{</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> arr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span><span class="token string">"武汉加油"</span><span class="token punctuation">,</span> <span class="token string">"中国加油"</span><span class="token punctuation">,</span> <span class="token string">"世界加油"</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

stream <span class="token operator">=</span> <span class="token class-name">Stream</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">"武汉加油"</span><span class="token punctuation">,</span> <span class="token string">"中国加油"</span><span class="token punctuation">,</span> <span class="token string">"世界加油"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

stream <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>查看 Stream 源码的话，你会发现 <code>of()</code> 方法内部其实调� 10BC0 �了 <code>Arrays.stream()</code> 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token class-name">Stream</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token function">of</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> values<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">return</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span>values<span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>另外，集合还可以调用 <code>parallelStream()</code> 方法创建并发流，默认使用的是 <code>ForkJoinPool.commonPool()</code>线程池。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Long</span><span class="token punctuation">></span></span> aList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Stream</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Long</span><span class="token punctuation">></span></span> parallelStream <span class="token operator">=</span> aList<span class="token punctuation">.</span><span class="token function">parallelStream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><h2 id="_02、操作流" tabindex="-1"><a class="header-anchor" href="#_02、操作流" aria-hidden="true">#</a> 02、操作流</h2><p>Stream 类提供了很多有用的操作流的方法，我来挑一些常用的给你介绍一下。</p><h3 id="_1-过滤" tabindex="-1"><a class="header-anchor" href="#_1-过滤" aria-hidden="true">#</a> 1）过滤</h3><p>通过 <code>filter()</code> 方法可以从流中筛选出我们想要的元素。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">FilterStreamDemo</span> <span class="token punctuation">{</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"周杰伦"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"王力宏"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"陶喆"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"林俊杰"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Stream</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> stream <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>element <span class="token operator">-></span> element<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span><span class="token string">"王"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

stream<span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token operator">::</span><span class="token function">println</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>filter()</code> 方法接收的是一个 Predicate（Java 8 新增的一个函数式接口，接受一个输入参数返回一个布尔值结果）类型的参数，因此，我们可以直接将一个 Lambda 表达式传递给该方法，比如说 <code>element -> element.contains("王")</code> 就是筛选出带有“王”的字符串。</p><p><code>forEach()</code> 方法接收的是一个 Consumer（Java 8 新增的一个函数式接口，接受一个输入参数并且无返回的操作）类型的参数，<code>类名 :: 方法名</code>是 Java 8 引入的新语法，<code>System.out</code> 返回 PrintStream 类，println 方法你应该知道是打印的。</p><p><code>stream.forEach(System.out::println);</code> 相当于在 for 循环中打印，类似于下面的代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">String</span> s <span class="token operator">:</span> strs<span class="token punctuation">)</span> <span class="token punctuation">{</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>很明显，一行代码看起来更简洁一些。来看一下程序的输出结果：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>王力宏

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><h3 id="_2-映射" tabindex="-1"><a class="header-anchor" href="#_2-映射" aria-hidden="true">#</a> 2）映射</h3><p>如果想通过某种操作把一个流中的元素转化成新的流中的元素，可以使用 <code>map()</code> 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MapStreamDemo</span> <span class="token punctuation">{</span>

<span class="token class-name">Stream</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> stream <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token operator">::</span><span class="token function">length</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>map()</code> 方法接收的是一个 Function（Java 8 新增的一个函数式接口，接受一个输入参数 T，返回一个结果 R）类型的参数，此时参数为 String 类的 length 方法，也就是把 <code>Stream<String></code> 的流转成一个 <code>Stream<Integer></code> 的流。</p><p>程序输出的结果如下所示：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>3

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="_3-匹配" tabindex="-1"><a class="header-anchor" href="#_3-匹配" aria-hidden="true">#</a> 3）匹配</h3><p>Stream 类提供了三个方法可供进行元素匹配，它们分别是：</p><ul><li><p><code>anyMatch()</code>，只要有一个元素匹配传入的条件，就返回 true。</p></li><li><p><code>allMatch()</code>，只有有一个元素不匹配传入的条件，就返回 false；如果全部匹配，则返回 true。</p></li><li><p><code>noneMatch()</code>，只要有一个元素匹配传入的条件，就返回 false；如果全部匹配，则返回 true。</p></li></ul><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">MatchStreamDemo</span> <span class="token punctuation">{</span>

<span class="token keyword">boolean</span> anyMatchFlag <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">anyMatch</span><span class="token punctuation">(</span>element <span class="token operator">-></span> element<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span><span class="token string">"王"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">boolean</span> allMatchFlag <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">allMatch</span><span class="token punctuation">(</span>element <span class="token operator">-></span> element<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">boolean</span> noneMatchFlag <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">noneMatch</span><span class="token punctuation">(</span>element <span class="token operator">-></span> element<span class="token punctuation">.</span><span class="token function">endsWith</span><span class="token punctuation">(</span><span class="token string">"沉"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>因为“王力宏”以“王”字开头，所以 anyMatchFlag 应该为 true；因为“周杰伦”、“王力宏”、“陶喆”、“林俊杰”的字符串长度都大于 1，所以 allMatchFlag 为 true；因为 4 个字符串结尾都不是“沉”，所以 noneMatchFlag 为 true。</p><p>程序输出的结果如下所示：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>true

true

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="_4-组合" tabindex="-1"><a class="header-anchor" href="#_4-组合" aria-hidden="true">#</a> 4）组合</h3><p><code>reduce()</code> 方法的主要作用是把 Stream 中的元素组合起来，它有两种用法：</p><ul><li><code>Optional<T> reduce(BinaryOperator<T> accumulator)</code></li></ul><p>没有起始值，只有一个参数，就是运算规则，此时返回 <a href="https://mp.weixin.qq.com/s/PqK0KNVHyoEtZDtp5odocA" target="_blank" rel="noopener noreferrer">Optional<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>。</p><ul><li><code>T reduce(T identity, BinaryOperator<T> accumulator)</code></li></ul><p>有起始值，有运算规则，两个参数，此时返回的类型和起始值类型一致。</p><p>来看下面这个例子。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ReduceStreamDemo</span> <span class="token punctuation">{</span>

<span class="token class-name">Integer</span><span class="token punctuation">[</span><span class="token punctuation">]</span> ints <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> optional <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span> b<span class="token punctuation">)</span> <span class="token operator">-></span> a <span class="token operator">+</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> optional1 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token class-name">Integer</span><span class="token operator">::</span><span class="token function">sum</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>optional<span class="token punctuation">.</span><span class="token function">orElse</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>optional1<span class="token punctuation">.</span><span class="token function">orElse</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> reduce <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">,</span> <span class="token punctuation">(</span>a<span class="token punctuation">,</span> b<span class="token punctuation">)</span> <span class="token operator">-></span> a <span class="token operator">+</span> b<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> reduce1 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token number">6</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token operator">::</span><span class="token function">sum</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>运算规则可以是 <a href="https://mp.weixin.qq.com/s/ozr0jYHIc12WSTmmd_vEjw" target="_blank" rel="noopener noreferrer">Lambda 表达式<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>（比如 <code>(a, b) -> a + b</code>），也可以是类名::方法名（比如 <code>Integer::sum</code>）。</p><p>程序运行的结果如下所示：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token number">6</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>0、1、2、3 在没有起始值相加的时候结果为 6；有起始值 6 的时候结果为 12。</p><h2 id="_03、转换流" tabindex="-1"><a class="header-anchor" href="#_03、转换流" aria-hidden="true">#</a> 03、转换流</h2><p>既然可以把集合或者数组转成流，那么也应该有对应的方法，将流转换回去——<code>collect()</code> 方法就满足了这种需求。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">CollectStreamDemo</span> <span class="token punctuation">{</span>

<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> strArray <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toArray</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token operator">::</span><span class="token keyword">new</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span>strArray<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token operator">::</span><span class="token function">length</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list2 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toCollection</span><span class="token punctuation">(</span><span class="token class-name">ArrayList</span><span class="token operator">::</span><span class="token keyword">new</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">String</span> str <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">joining</span><span class="token punctuation">(</span><span class="token string">", "</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>toArray()</code> 方法可以将流转换成数组，你可能比较好奇的是 <code>String[]::new</code>，它是什么东东呢？来看一下 <code>toArray()</code> 方法的源码。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">A</span><span class="token punctuation">></span></span> <span class="token class-name">A</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">toArray</span><span class="token punctuation">(</span><span class="token class-name">IntFunction</span><span class="token operator"><</span><span class="token class-name">A</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token operator">></span> generator<span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>也就是说 <code>String[]::new</code> 是一个 IntFunction，一个可以产生所需的新数组的函数，可以通过反编译字节码看看它到底是什么：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> strArray <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toArray</span><span class="token punctuation">(</span><span class="token punctuation">(</span>x$<span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>

<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span>x$<span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span>strArray<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>也就是相当于返回了一个指定长度的字符串数组。</p><p>当我们需要把一个集合按照某种规则转成另外一个集合的时候，就可以配套使用 <code>map()</code> 方法和 <code>collect()</code> 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token operator">::</span><span class="token function">length</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>通过 <code>stream()</code> 方法创建集合的流后，再通过 <code>map(String:length)</code> 将其映射为字符串长度的一个新流，最后通过 <code>collect()</code> 方法将其转换成新的集合。</p><p>Collectors 是一个收集器的工具类，内置了一系列收集器实现，比如说 <code>toList()</code> 方法将元素收集到一个新的 <code>java.util.List</code> 中；比如说 <code>toCollection()</code> 方法将元素收集到一个新的 <code> java.util.ArrayList</code> 中；比如说 <code>joining()</code> 方法将元素收集到一个可以用分隔符指定的字符串中。</p><p>来看一下程序的输出结果：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token punctuation">[</span>周杰伦<span class="token punctuation">,</span> 王力宏<span class="token punctuation">,</span> 陶喆<span class="token punctuation">,</span> 林俊杰<span class="token punctuation">]</span>

周杰伦<span class="token punctuation">,</span> 王力宏<span class="token punctuation">,</span> 陶喆<span class="token punctuation">,</span> 林俊杰

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