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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>1.8. Stream</title><link rel="stylesheet" type="text/css" href="/docbook.css" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="home" href="index.html" title="Netkiller Java 手札" /><link rel="up" href="java.html" title="第 1 章 Java 14" /><link rel="prev" href="java.util.Queue.html" title="1.7. Queue" /><link rel="next" href="Optional.html" title="1.9. Optional" /></head><body><a xmlns="" href="//www.netkiller.cn/">Home</a> |
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<a xmlns="" href="//www.netkiller.cn/home/about.html">About</a><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">1.8. Stream</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="java.util.Queue.html">上一页</a> </td><th width="60%" align="center">第 1 章 Java 14</th><td width="20%" align="right"> <a accesskey="n" href="Optional.html">下一页</a></td></tr></table><hr /></div><table xmlns=""><tr><td><iframe src="//ghbtns.com/github-btn.html?user=netkiller&repo=netkiller.github.io&type=watch&count=true&size=large" height="30" width="170" frameborder="0" scrolling="0" style="width:170px; height: 30px;" allowTransparency="true"></iframe></td><td><iframe src="//ghbtns.com/github-btn.html?user=netkiller&repo=netkiller.github.io&type=fork&count=true&size=large" height="30" width="170" frameborder="0" scrolling="0" style="width:170px; height: 30px;" allowTransparency="true"></iframe></td><td><iframe src="//ghbtns.com/github-btn.html?user=netkiller&type=follow&count=true&size=large" height="30" width="240" frameborder="0" scrolling="0" style="width:240px; height: 30px;" allowTransparency="true"></iframe></td><td></td><td><a href="https://zhuanlan.zhihu.com/netkiller"><img src="/images/logo/zhihu-card-default.svg" height="25" /></a></td><td valign="middle"><a href="https://zhuanlan.zhihu.com/netkiller">知乎专栏</a> ｜ <a href="https://www.zhihu.com/club/1241768772601950208">多维度架构</a></td><td> | </td><td>微信号 netkiller-ebook </td><td> | </td><td>QQ群：128659835 请注明“读者”</td></tr></table><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="stream"></a>1.8. Stream</h2></div></div></div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp42"></a>1.8.1. Stream.of</h3></div></div></div>
<pre class="programlisting">
Stream<String> stream = Stream.of("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp43"></a>1.8.2. Stream.ofNullable</h3></div></div></div>
<p>增加单个参数构造方法，可为null</p>
<pre class="screen">
Stream.ofNullable(null).count(); // 0
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp44"></a>1.8.3. filter</h3></div></div></div>
<p>filter 方法用于通过设置的条件过滤出元素。以下代码片段使用 filter 方法过滤掉空字符串：</p>
<pre class="programlisting">
List<String> strings = Arrays.asList("Hollis", "", "HollisChuang", "H", "hollis");
strings.stream().filter(string -> !string.isEmpty()).forEach(System.out::println);
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp45"></a>1.8.4. map</h3></div></div></div>
<p>map 方法用于映射每个元素到对应的结果，以下代码片段使用 map 输出了元素对应的平方数：</p>
<pre class="programlisting">
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
numbers.stream().map( i -> i*i).forEach(System.out::println);
//9,4,4,9,49,9,25
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp46"></a>1.8.5. limit/skip</h3></div></div></div>
<p>limit 返回 Stream 的前面 n 个元素；skip 则是扔掉前 n 个元素。以下代码片段使用 limit 方法保理4个元素：</p>
<pre class="programlisting">
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
numbers.stream().limit(4).forEach(System.out::println);
//3,2,2,3
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp47"></a>1.8.6. sorted</h3></div></div></div>
<p>sorted 方法用于对流进行排序。以下代码片段使用 sorted 方法进行排序：</p>
<pre class="programlisting">
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
numbers.stream().sorted().forEach(System.out::println);
//2,2,3,3,3,5,7
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp48"></a>1.8.7. distinct</h3></div></div></div>
<p>distinct主要用来去重，以下代码片段使用 distinct 对元素进行去重：</p>
<pre class="programlisting">
List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
numbers.stream().distinct().forEach(System.out::println);
//3,2,7,5
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp49"></a>1.8.8. forEach</h3></div></div></div>
<p>Stream 提供了方法 'forEach' 来迭代流中的每个数据。以下代码片段使用 forEach 输出了10个随机数：</p>
<pre class="programlisting">
Random random = new Random();
random.ints().limit(10).forEach(System.out::println);
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp50"></a>1.8.9. count</h3></div></div></div>
<p>count用来统计流中的元素个数。</p>
<pre class="programlisting">
List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis","Hollis666", "Hello", "HelloWorld", "Hollis");
System.out.println(strings.stream().count());
//7
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp51"></a>1.8.10. collect</h3></div></div></div>
<p>collect就是一个归约操作，可以接受各种做法作为参数，将流中的元素累积成一个汇总结果：</p>
<pre class="programlisting">
List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis","Hollis666", "Hello", "HelloWorld", "Hollis");
strings = strings.stream().filter(string -> string.startsWith("Hollis")).collect(Collectors.toList());
System.out.println(strings);
//Hollis, HollisChuang, Hollis666, Hollis
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp52"></a>1.8.11. takeWhile 和 dropWhile</h3></div></div></div>
<p>增加 takeWhile 和 dropWhile 方法</p>
<pre class="screen">
Stream.of(1, 2, 3, 2, 1)
.takeWhile(n -> n < 3)
.collect(Collectors.toList()); // [1, 2]
</pre>
<p>从开始计算，当 n < 3 时就截止</p>
<pre class="screen">
Stream.of(1, 2, 3, 2, 1)
.dropWhile(n -> n < 3)
.collect(Collectors.toList()); // [3, 2, 1]
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp53"></a>1.8.12. List to Stream</h3></div></div></div>
<pre class="programlisting">
List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");
Stream<String> stream = strings.stream();
</pre>
</div>
<div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="idp54"></a>1.8.13. 混合使用的例子</h3></div></div></div>
<pre class="programlisting">
List<String> strings = Arrays.asList("Hollis", "HollisChuang", "hollis", "Hello", "HelloWorld", "Hollis");
Stream s = strings.stream().filter(string -> string.length()<= 6).map(String::length).sorted().limit(3)
.distinct();
</pre>
</div>
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