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<meta name="keywords" content="Java, String类, 源码, 源码解析, 字符串操作,String"><meta property="og:url" content="https://tobebetterjavaer.com/string/string-source.html"><meta property="og:site_name" content="Java程序员进阶之路"><meta property="og:title" content="深入解读String类源码及其应用技巧"><meta property="og:description" content="本文将对Java String类的源码进行深入分析，帮助你理解字符串在Java中的实现原理、操作方式和性能优化策略。通过详细解读String源码，我们将揭示字符串类的内部机制，掌握如何高效地进行字符串操作，以及在实际编程中如何应用这些知识来提高代码质量和性能。"><meta property="og:type" content="article"><meta property="og:locale" content="zh-CN"><meta property="og:updated_time" content="2023-04-24T02:39:04.000Z"><meta property="article:tag" content="数组&字符串"><meta property="article:modified_time" content="2023-04-24T02:39:04.000Z"><script type="application/ld+json">{"@context":"https://schema.org","@type":"Article","headline":"深入解读String类源码及其应用技巧","image":[""],"dateModified":"2023-04-24T02:39:04.000Z","author":[]}</script><meta name="robots" content="all"><meta name="author" content="沉默王二"><meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate"><meta http-equiv="Pragma" content="no-cache"><meta http-equiv="Expires" content="0"><meta name="apple-mobile-web-app-capable" content="yes"><script>
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99.2-25.6 0 0 6.4-12.8 9.6-19.2 54.4-105.6 275.2-524.8 288-553.6 6.4-19.2-3.2-32-19.2-32C777.6 76.8 771.2 80 764.8 86.4z"></path></svg><div class="outlook-dropdown"><!----></div></button></div><div id="docsearch-container"></div><!--[--><!----><!--]--><button class="toggle-navbar-button" aria-label="Toggle Navbar" aria-expanded="false" aria-controls="nav-screen"><span class="button-container"><span class="button-top"></span><span class="button-middle"></span><span class="button-bottom"></span></span></button></div></header><!----><!--]--><!----><div class="toggle-sidebar-wrapper"><span class="arrow start"></span></div><aside class="sidebar" id="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基础</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概述及环境配置</span><span class="arrow end"></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 end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">2.3 数组&字符串</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/array/array.html" class="nav-link sidebar-link sidebar-page" aria-label="掌握Java数组"><!---->掌握Java数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/array/double-array.html" class="nav-link sidebar-link sidebar-page" aria-label="掌握Java二维数组"><!---->掌握Java二维数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/array/print.html" class="nav-link sidebar-link sidebar-page" aria-label="打印Java数组"><!---->打印Java数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/string/string-source.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="解读String类源码"><!---->解读String类源码<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的声明" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的声明"><!---->String 类的声明<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的底层实现" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的底层实现"><!---->String 类的底层实现<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的-hashcode-方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的 hashCode 方法"><!---->String 类的 hashCode 方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的-substring-方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的 substring 方法"><!---->String 类的 substring 方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的-indexof-方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的 indexOf 方法"><!---->String 类的 indexOf 方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/string/string-source.html#string-类的其他方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="String 类的其他方法"><!---->String 类的其他方法<!----></a><ul class="sidebar-sub-headers"></ul></li></ul><!--]--></li><li><!--[--><a href="/string/immutable.html" class="nav-link sidebar-link sidebar-page" aria-label="String为什么不可变"><!---->String为什么不可变<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/constant-pool.html" class="nav-link sidebar-link sidebar-page" aria-label="深入理解Java字符串常量池"><!---->深入理解Java字符串常量池<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/intern.html" class="nav-link sidebar-link sidebar-page" aria-label="详解 String.intern 方法"><!---->详解 String.intern 方法<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/builder-buffer.html" class="nav-link sidebar-link sidebar-page" aria-label="StringBuilder和StringBuffer"><!---->StringBuilder和StringBuffer<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/equals.html" class="nav-link sidebar-link sidebar-page" aria-label="String相等判断"><!---->String相等判断<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/join.html" class="nav-link sidebar-link sidebar-page" aria-label="String拼接"><!---->String拼接<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/split.html" class="nav-link sidebar-link sidebar-page" aria-label="String拆分"><!---->String拆分<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.4 面向对象编程</span><span class="arrow end"></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 end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 Java IO</span><span class="arrow end"></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 end"></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 end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 网络编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.11 NIO</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 Java重要知识点</span><span class="arrow end"></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 end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.14 JVM</span><span class="arrow end"></span></button><!----></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java进阶</span><span class="arrow end"></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 end"></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 end"></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 end"></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 end"></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 end"></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 end"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->深入解读String类源码及其应用技巧</h1><div class="page-info"><span class="page-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="page-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="page-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><!----></span><meta property="datePublished" content="2023-04-09T15:07:16.000Z"></span><span class="page-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 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H593.927c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.31-40.31zM730.22 920.502H623.926c-40.925 0-74.22-33.388-74.22-74.425V623.992c0-41.038 33.387-74.424 74.425-74.424h222.085c41.038 0 74.424 33.226 74.424 74.067v114.233c0 10.244-8.304 18.548-18.547 18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></svg><span class="page-category-item category0 clickable" role="navigation">Java核心</span><meta property="articleSection" content="Java核心"></span><span class="page-tag-info" aria-label="标签🏷" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="tag icon"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><span class="page-tag-item tag5 clickable" role="navigation">数组&字符串</span><meta property="keywords" content="数组&字符串"></span><span class="page-word-info" aria-label="字数🔠" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon word-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="word icon"><path d="M518.217 432.64V73.143A73.143 73.143 0 01603.43 1.097a512 512 0 01419.474 419.474 73.143 73.143 0 01-72.046 85.212H591.36a73.143 73.143 0 01-73.143-73.143z"></path><path d="M493.714 566.857h340.297a73.143 73.143 0 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 4646 字</span><meta property="wordCount" content="4646"></span><span class="page-reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down"><svg 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15 分钟</span><meta property="timeRequired" content="PT15M"></span></div><hr></div><div class="toc-place-holder"><aside id="toc"><!--[--><!----><!--]--><div class="toc-header">此页内容<button class="print-button" title="print"><svg xmlns="http://www.w3.org/2000/svg" class="icon print-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="print icon"><path d="M819.2 364.8h-44.8V128c0-17.067-14.933-32-32-32H281.6c-17.067 0-32 14.933-32 32v236.8h-44.8C145.067 364.8 96 413.867 96 473.6v192c0 59.733 49.067 108.8 108.8 108.8h44.8V896c0 17.067 14.933 32 32 32h460.8c17.067 0 32-14.933 32-32V774.4h44.8c59.733 0 108.8-49.067 108.8-108.8v-192c0-59.733-49.067-108.8-108.8-108.8zM313.6 160h396.8v204.8H313.6V160zm396.8 704H313.6V620.8h396.8V864zM864 665.6c0 25.6-19.2 44.8-44.8 44.8h-44.8V588.8c0-17.067-14.933-32-32-32H281.6c-17.067 0-32 14.933-32 32v121.6h-44.8c-25.6 0-44.8-19.2-44.8-44.8v-192c0-25.6 19.2-44.8 44.8-44.8h614.4c25.6 0 44.8 19.2 44.8 44.8v192z"></path></svg></button></div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的声明" class="router-link-active router-link-exact-active toc-link level3">String 类的声明</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的底层实现" class="router-link-active router-link-exact-active toc-link level3">String 类的底层实现</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的-hashcode-方法" class="router-link-active router-link-exact-active toc-link level3">String 类的 hashCode 方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的-substring-方法" class="router-link-active router-link-exact-active toc-link level3">String 类的 substring 方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的-indexof-方法" class="router-link-active router-link-exact-active toc-link level3">String 类的 indexOf 方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/string/string-source.html#string-类的其他方法" class="router-link-active router-link-exact-active toc-link level3">String 类的其他方法</a></li><!----><!--]--></ul></div><!--[--><!----><!--]--></aside></div><!----><div class="theme-hope-content"><h1 id="_4-4-解读string类源码" tabindex="-1"><a class="header-anchor" href="#_4-4-解读string类源码" aria-hidden="true">#</a> 4.4 解读String类源码</h1><p>我正坐在沙发上津津有味地读刘欣大佬的《码农翻身》——Java 帝国这一章，门铃响了。起身打开门一看，是三妹，她从学校回来了。</p><p>“三妹，你回来的真及时，今天我们打算讲 Java 中的字符串呢。”等三妹换鞋的时候我说。</p><p>“哦，可以呀，哥。听说字符串的细节特别多，什么<a href="https://tobebetterjavaer.com/string/constant-pool.html" target="_blank" rel="noopener noreferrer">字符串常量池<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>了、字符串不可变性了、<a href="https://tobebetterjavaer.com/string/join.html" target="_blank" rel="noopener noreferrer">字符串拼接<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>了、字符串长度限制了等等，你最好慢慢讲，否则我可能一时半会消化不了。”三妹的态度显得很诚恳。</p><p>“嗯，我已经想好了，今天就只带你大概认识一下字符串，主要读一读它的源码，其他的细节咱们后面再慢慢讲，保证你能及时消化。”</p><p>“好，那就开始吧。”三妹已经准备好坐在了电脑桌的边上。</p><p>我应了一声后走到电脑桌前坐下来，顺手打开 <a href="https://tobebetterjavaer.com/overview/IDEA-install-config.html" target="_blank" rel="noopener noreferrer">Intellij IDEA<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>，并找到了 String 的源码（Java 8）。</p><h3 id="string-类的声明" tabindex="-1"><a class="header-anchor" href="#string-类的声明" aria-hidden="true">#</a> String 类的声明</h3><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">final</span> <span class="token keyword">class</span> <span class="token class-name">String</span>
<span class="token keyword">implements</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span><span class="token punctuation">,</span> <span class="token class-name">Comparable</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">,</span> <span class="token class-name">CharSequence</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></div><p>“第一，String 类是 <a href="https://tobebetterjavaer.com/oo/final.html" target="_blank" rel="noopener noreferrer">final<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> 的，意味着它不能被子类继承。”</p><p>“第二，String 类实现了 <a href="https://tobebetterjavaer.com/io/Serializbale.html" target="_blank" rel="noopener noreferrer">Serializable 接口<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>，意味着它可以<a href="https://tobebetterjavaer.com/io/serialize.html" target="_blank" rel="noopener noreferrer">序列化<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><p>“第三，String 类实现了 <a href="https://tobebetterjavaer.com/basic-extra-meal/comparable-omparator.html" target="_blank" rel="noopener noreferrer">Comparable 接口<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>，意味着最好不要用‘==’来<a href="https://tobebetterjavaer.com/string/equals.html" target="_blank" rel="noopener noreferrer">比较两个字符串是否相等<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>，而应该用 <code>compareTo()</code> 方法去比较。”</p><p>因为 == 是用来比较两个对象的地址，这个在讲字符串比较的时候会详细讲。如果只是说比较字符串内容的话，可以使用 String 类的 equals 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">boolean</span> <span class="token function">equals</span><span class="token punctuation">(</span><span class="token class-name">Object</span> anObject<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">==</span> anObject<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>anObject <span class="token keyword">instanceof</span> <span class="token class-name">String</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> anotherString <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span> anObject<span class="token punctuation">;</span>
<span class="token keyword">int</span> n <span class="token operator">=</span> value<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>n <span class="token operator">==</span> anotherString<span class="token punctuation">.</span>value<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">char</span> v1<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
<span class="token keyword">char</span> v2<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> anotherString<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
<span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span>n<span class="token operator">--</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>v1<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">!=</span> v2<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
i<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token boolean">false</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 class="line-number"></div></div></div><p>“第四，<a href="https://tobebetterjavaer.com/string/builder-buffer.html" target="_blank" rel="noopener noreferrer">StringBuffer、StringBuilder 和 String<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> 一样，都实现了 CharSequence 接口，所以它们仨属于近亲。由于 String 是不可变的，所以遇到字符串拼接的时候就可以考虑一下 String 的另外两个好兄弟，StringBuffer 和 StringBuilder，它俩是可变的。”</p><h3 id="string-类的底层实现" tabindex="-1"><a class="header-anchor" href="#string-类的底层实现" aria-hidden="true">#</a> String 类的底层实现</h3><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">char</span> value<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>“第五，Java 9 以前，String 是用 char 型<a href="https://tobebetterjavaer.com/array/array.html" target="_blank" rel="noopener noreferrer">数组<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>实现的，之后改成了 byte 型数组实现，并增加了 coder 来表示编码。这样做的好处是在 Latin1 字符为主的程序里，可以把 String 占用的内存减少一半。当然，天下没有免费的午餐，这个改进在节省内存的同时引入了编码检测的开销。”</p><blockquote><p>Latin1（Latin-1）是一种单字节字符集（即每个字符只使用一个字节的编码方式），也称为ISO-8859-1（国际标准化组织8859-1），它包含了西欧语言中使用的所有字符，包括英语、法语、德语、西班牙语、葡萄牙语、意大利语等等。在Latin1编码中，每个字符使用一个8位（即一个字节）的编码，可以表示256种不同的字符，其中包括ASCII字符集中的所有字符，即0x00到0x7F，以及其他西欧语言中的特殊字符，例如é、ü、ñ等等。由于Latin1只使用一个字节表示一个字符，因此在存储和传输文本时具有较小的存储空间和较快的速度</p></blockquote><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">final</span> <span class="token keyword">class</span> <span class="token class-name">String</span>
<span class="token keyword">implements</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span><span class="token punctuation">,</span> <span class="token class-name">Comparable</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">,</span> <span class="token class-name">CharSequence</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Stable</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> value<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">byte</span> coder<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> hash<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></div><p>接下来，我们来详细地说一下。</p><p>从 <code>char[]</code> 到 <code>byte[]</code>，最主要的目的是<strong>节省字符串占用的内存空间</strong>。内存占用减少带来的另外一个好处，就是 GC 次数也会减少。</p><p>我们使用 <code>jmap -histo:live pid | head -n 10</code> 命令就可以查看到堆内对象示例的统计信息、查看 ClassLoader 的信息以及 finalizer 队列。</p><p>以我正在运行着的<a href="https://github.com/itwanger/coding-more" target="_blank" rel="noopener noreferrer">编程喵<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>项目实例（基于 Java 8）来说，结果是这样的。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/basic-extra-meal/jdk9-char-byte-string-d826ce88-bbbe-47a3-a1a9-4dd86dd3632f.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>其中 String 对象有 17638 个，占用了 423312 个字节的内存，排在第三位。</p><p>由于 Java 8 的 String 内部实现仍然是 <code>char[]</code>，所以我们可以看到内存占用排在第 1 位的就是 char 数组。</p><p><code>char[]</code> 对象有 17673 个，占用了 1621352 个字节的内存，排在第一位。</p><p>那也就是说优化 String 节省内存空间是非常有必要的，如果是去优化一个使用频率没有 String 这么高的类，就没什么必要，对吧？</p><p>众所周知，char 类型的数据在 JVM 中是占用两个字节的，并且使用的是 UTF-8 <a href="https://tobebetterjavaer.com/basic-extra-meal/java-unicode.html" target="_blank" rel="noopener noreferrer">编码<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>，其值范围在 '\u0000'（0）和 '\uffff'（65,535）（包含）之间。</p><p>也就是说，使用 <code>char[]</code> 来表示 String 就会导致，即使 String 中的字符只用一个字节就能表示，也得占用两个字节。</p><blockquote><p>PS：在计算机中，单字节字符通常指的是一个字节（8位）可以表示的字符，而双字节字符则指需要两个字节（16位）才能表示的字符。单字节字符和双字节字符的定义是相对的，不同的编码方式对应的单字节和双字节字符集也不同。常见的单字节字符集有ASCII（美国信息交换标准代码）、ISO-8859（国际标准化组织标准编号8859）、GBK（汉字内码扩展规范）、GB2312（中国国家标准，现在已经被GBK取代），像拉丁字母、数字、标点符号、控制字符都是单字节字符。双字节字符集包括 Unicode、UTF-8、GB18030（中国国家标准），中文、日文、韩文、拉丁文扩展字符属于双字节字符。</p></blockquote><p>当然了，仅仅将 <code>char[]</code> 优化为 <code>byte[]</code> 是不够的，还要配合 Latin-1 的编码方式，该编码方式是用单个字节来表示字符的，这样就比 UTF-8 编码节省了更多的空间。</p><p>换句话说，对于：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token string">"jack"</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>这样的，使用 Latin-1 编码，占用 4 个字节就够了。</p><p>但对于：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token string">"小二"</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>这种，木的办法，只能使用 UTF16 来编码。</p><p>针对 JDK 9 的 String 源码里，为了区别编码方式，追加了一个 coder 字段来区分。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token doc-comment comment">/**
* The identifier of the encoding used to encode the bytes in
* <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java">value</span></span><span class="token punctuation">}</span>. The supported values in this implementation are
*
* LATIN1
* UTF16
*
* <span class="token keyword">@implNote</span> This field is trusted by the VM, and is a subject to
* constant folding if String instance is constant. Overwriting this
* field after construction will cause problems.
*/</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">byte</span> coder<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></div><p>Java 会根据字符串的内容自动设置为相应的编码，要么 Latin-1 要么 UTF16。</p><p>也就是说，从 <code>char[]</code> 到 <code>byte[]</code>，<strong>中文是两个字节，纯英文是一个字节，在此之前呢，中文是两个字节，英文也是两个字节</strong>。</p><p>在 UTF-8 中，0-127 号的字符用 1 个字节来表示，使用和 ASCII 相同的编码。只有 128 号及以上的字符才用 2 个、3 个或者 4 个字节来表示。</p><ul><li>如果只有一个字节，那么最高的比特位为 0；</li><li>如果有多个字节，那么第一个字节从最高位开始，连续有几个比特位的值为 1，就使用几个字节编码，剩下的字节均以 10 开头。</li></ul><p>具体的表现形式为：</p><ul><li>0xxxxxxx：一个字节；</li><li>110xxxxx 10xxxxxx：两个字节编码形式（开始两个 1）；</li><li>1110xxxx 10xxxxxx 10xxxxxx：三字节编码形式（开始三个 1）；</li><li>11110xxx 10xxxxxx 10xxxxxx 10xxxxxx：四字节编码形式（开始四个 1）。</li></ul><p>也就是说，UTF-8 是变长的，那对于 String 这种有随机访问方法的类来说，就很不方便。所谓的随机访问，就是charAt、subString这种方法，随便指定一个数字，String要能给出结果。如果字符串中的每个字符占用的内存是不定长的，那么进行随机访问的时候，就需要从头开始数每个字符的长度，才能找到你想要的字符。</p><p>那你可能会问，UTF-16也是变长的呢？一个字符还可能占用 4 个字节呢？</p><p>的确，UTF-16 使用 2 个或者 4 个字节来存储字符。</p><ul><li>对于 Unicode 编号范围在 0 ~ FFFF 之间的字符，UTF-16 使用两个字节存储。</li><li>对于 Unicode 编号范围在 10000 ~ 10FFFF 之间的字符，UTF-16 使用四个字节存储，具体来说就是：将字符编号的所有比特位分成两部分，较高的一些比特位用一个值介于 D800~DBFF 之间的双字节存储，较低的一些比特位（剩下的比特位）用一个值介于 DC00~DFFF 之间的双字节存储。</li></ul><p>但是在 Java 中，一个字符（char）就是 2 个字节，占 4 个字节的字符，在 Java 里也是用两个 char 来存储的，而String的各种操作，都是以Java的字符（char）为单位的，charAt是取得第几个char，subString取的也是第几个到第几个char组成的子串，甚至length返回的都是char的个数。</p><p>所以UTF-16在Java的世界里，就可以视为一个定长的编码。</p><blockquote><p>参考链接：<a href="https://www.zhihu.com/question/447224628" target="_blank" rel="noopener noreferrer">https://www.zhihu.com/question/447224628<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></blockquote><h3 id="string-类的-hashcode-方法" tabindex="-1"><a class="header-anchor" href="#string-类的-hashcode-方法" aria-hidden="true">#</a> String 类的 hashCode 方法</h3><p>“第六，每一个字符串都会有一个 hash 值，这个哈希值在很大概率是不会重复的，因此 String 很适合来作为 <a href="https://tobebetterjavaer.com/collection/hashmap.html" target="_blank" rel="noopener noreferrer">HashMap<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> 的键值。”</p><p>来看 String 类的 hashCode 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">int</span> hash<span class="token punctuation">;</span> <span class="token comment">// Cache the hash code for the string</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> h <span class="token operator">=</span> hash<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>h <span class="token operator">==</span> <span class="token number">0</span> <span class="token operator">&&</span> value<span class="token punctuation">.</span>length <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">char</span> val<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> value<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
h <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> h <span class="token operator">+</span> val<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
hash <span class="token operator">=</span> h<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> h<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>hashCode 方法首先检查是否已经计算过哈希码，如果已经计算过，则直接返回缓存的哈希码。否则，方法将使用一个循环遍历字符串的所有字符，并使用一个乘法和加法的组合计算哈希码。这种计算方法被称为“31 倍哈希法”。计算完成后，将得到的哈希值存储在 hash 成员变量中，以便下次调用 hashCode 方法时直接返回该值，而不需要重新计算。这是一种缓存优化，称为“惰性计算”。</p><p>31倍哈希法（31-Hash）是一种简单有效的字符串哈希算法，常用于对字符串进行哈希处理。该算法的基本思想是将字符串中的每个字符乘以一个固定的质数31的幂次方，并将它们相加得到哈希值。具体地，假设字符串为s，长度为n，则31倍哈希值计算公式如下：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>H(s) = (s[0] * 31^(n-1)) + (s[1] * 31^(n-2)) + ... + (s[n-1] * 31^0)
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>其中，s[i]表示字符串s中第i个字符的ASCII码值，<code>^</code>表示幂运算。</p><p>31倍哈希法的优点在于简单易实现，计算速度快，同时也比较均匀地分布在哈希表中。</p><p><a href="https://tobebetterjavaer.com/basic-extra-meal/hashcode.html" target="_blank" rel="noopener noreferrer">hashCode 方法<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>，我们会在另外一个章节里详细讲，戳前面的链接了解。</p><p>我们可以通过以下方法模拟 String 的 hashCode 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">HashCodeExample</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> text <span class="token operator">=</span> <span class="token string">"沉默王二"</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> hashCode <span class="token operator">=</span> <span class="token function">computeHashCode</span><span class="token punctuation">(</span>text<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 string">"字符串 \""</span> <span class="token operator">+</span> text <span class="token operator">+</span> <span class="token string">"\" 的哈希码是: "</span> <span class="token operator">+</span> hashCode<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 string">"String 的 hashCode "</span> <span class="token operator">+</span> text<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">computeHashCode</span><span class="token punctuation">(</span><span class="token class-name">String</span> text<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> h <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> text<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
h <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> h <span class="token operator">+</span> text<span class="token punctuation">.</span><span class="token function">charAt</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> h<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></div><p>看一下结果：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>字符串 "沉默王二" 的哈希码是: 867758096
String 的 hashCode 867758096
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>结果是一样的，又学到了吧？</p><h3 id="string-类的-substring-方法" tabindex="-1"><a class="header-anchor" href="#string-类的-substring-方法" aria-hidden="true">#</a> String 类的 substring 方法</h3><p>String 类中还有一个方法比较常用 substring，用来截取字符串的，来看源码。</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 class-name">String</span> <span class="token function">substring</span><span class="token punctuation">(</span><span class="token keyword">int</span> beginIndex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>beginIndex <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">StringIndexOutOfBoundsException</span><span class="token punctuation">(</span>beginIndex<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">int</span> subLen <span class="token operator">=</span> value<span class="token punctuation">.</span>length <span class="token operator">-</span> beginIndex<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>subLen <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">StringIndexOutOfBoundsException</span><span class="token punctuation">(</span>subLen<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span>beginIndex <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token keyword">this</span> <span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> beginIndex<span class="token punctuation">,</span> subLen<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></div><p>substring 方法首先检查参数的有效性，如果参数无效，则抛出 StringIndexOutOfBoundsException <a href="https://tobebetterjavaer.com/exception/gailan.html" target="_blank" rel="noopener noreferrer">异常<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>。接下来，方法根据参数计算子字符串的长度。如果子字符串长度小于零，抛出StringIndexOutOfBoundsException异常。</p><p>如果 beginIndex 为 0，且 endIndex 等于字符串的长度，说明子串与原字符串相同，因此直接返回原字符串。否则，使用 value 数组（原字符串的字符数组）的一部分创建一个新的 String 对象并返回。</p><p>下面是几个使用 substring 方法的示例：</p><p>①、提取字符串中的一段子串：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> subStr <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">,</span> <span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 从第7个字符（包括）提取到第12个字符（不包括）</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>subStr<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 "world"</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>②、提取字符串中的前缀或后缀：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> prefix <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 提取前5个字符，即 "Hello,"</span>
<span class="token class-name">String</span> suffix <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 提取从第7个字符开始的所有字符，即 "world!"</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>③、处理字符串中的空格和分隔符：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">" Hello, world! "</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> trimmed <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 去除字符串开头和结尾的空格</span>
<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> words <span class="token operator">=</span> trimmed<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">"\\s+"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 将字符串按照空格分隔成单词数组</span>
<span class="token class-name">String</span> firstWord <span class="token operator">=</span> words<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">substring</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 punctuation">;</span> <span class="token comment">// 提取第一个单词的首字母</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>firstWord<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 "H"</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></div><p>④、处理字符串中的数字和符号：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"1234-5678-9012-3456"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> parts <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">"-"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 将字符串按照连字符分隔成四个部分</span>
<span class="token class-name">String</span> last4Digits <span class="token operator">=</span> parts<span class="token punctuation">[</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">substring</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 提取最后一个部分的后三位数字</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>last4Digits<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 "456"</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>总之，substring 方法可以根据需求灵活地提取字符串中的子串，为字符串处理提供了便利。</p><h3 id="string-类的-indexof-方法" tabindex="-1"><a class="header-anchor" href="#string-类的-indexof-方法" aria-hidden="true">#</a> String 类的 indexOf 方法</h3><p>indexOf 方法用于查找一个子字符串在原字符串中第一次出现的位置，并返回该位置的索引。来看该方法的源码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">/*
* 查找字符数组 target 在字符数组 source 中第一次出现的位置。
* sourceOffset 和 sourceCount 参数指定 source 数组中要搜索的范围，
* targetOffset 和 targetCount 参数指定 target 数组中要搜索的范围，
* fromIndex 参数指定开始搜索的位置。
* 如果找到了 target 数组，则返回它在 source 数组中的位置索引（从0开始），
* 否则返回-1。
*/</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> source<span class="token punctuation">,</span> <span class="token keyword">int</span> sourceOffset<span class="token punctuation">,</span> <span class="token keyword">int</span> sourceCount<span class="token punctuation">,</span>
<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> target<span class="token punctuation">,</span> <span class="token keyword">int</span> targetOffset<span class="token punctuation">,</span> <span class="token keyword">int</span> targetCount<span class="token punctuation">,</span>
<span class="token keyword">int</span> fromIndex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 如果开始搜索的位置已经超出 source 数组的范围，则直接返回-1（如果 target 数组为空，则返回 sourceCount）</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>fromIndex <span class="token operator">>=</span> sourceCount<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span>targetCount <span class="token operator">==</span> <span class="token number">0</span> <span class="token operator">?</span> sourceCount <span class="token operator">:</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 punctuation">}</span>
<span class="token comment">// 如果开始搜索的位置小于0，则从0开始搜索</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>fromIndex <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
fromIndex <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 如果 target 数组为空，则直接返回开始搜索的位置</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>targetCount <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> fromIndex<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 查找 target 数组的第一个字符在 source 数组中的位置</span>
<span class="token keyword">char</span> first <span class="token operator">=</span> target<span class="token punctuation">[</span>targetOffset<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> max <span class="token operator">=</span> sourceOffset <span class="token operator">+</span> <span class="token punctuation">(</span>sourceCount <span class="token operator">-</span> targetCount<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 循环查找 target 数组在 source 数组中的位置</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> sourceOffset <span class="token operator">+</span> fromIndex<span class="token punctuation">;</span> i <span class="token operator"><=</span> max<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">/* Look for first character. */</span>
<span class="token comment">// 如果 source 数组中当前位置的字符不是 target 数组的第一个字符，则在 source 数组中继续查找 target 数组的第一个字符</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>source<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">!=</span> first<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token operator">++</span>i <span class="token operator"><=</span> max <span class="token operator">&&</span> source<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">!=</span> first<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">/* Found first character, now look at the rest of v2 */</span>
<span class="token comment">// 如果在 source 数组中找到了 target 数组的第一个字符，则继续查找 target 数组的剩余部分是否匹配</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator"><=</span> max<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> j <span class="token operator">=</span> i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> end <span class="token operator">=</span> j <span class="token operator">+</span> targetCount <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> k <span class="token operator">=</span> targetOffset <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span> j <span class="token operator"><</span> end <span class="token operator">&&</span> source<span class="token punctuation">[</span>j<span class="token punctuation">]</span>
<span class="token operator">==</span> target<span class="token punctuation">[</span>k<span class="token punctuation">]</span><span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">,</span> k<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 如果 target 数组全部匹配，则返回在 source 数组中的位置索引</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>j <span class="token operator">==</span> end<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">/* Found whole string. */</span>
<span class="token keyword">return</span> i <span class="token operator">-</span> sourceOffset<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">// 没有找到 target 数组，则返回-1</span>
<span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</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 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 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>来看示例。</p><p>①、示例1：查找子字符串的位置</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> index <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">"world"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 查找 "world" 子字符串在 str 中第一次出现的位置</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>index<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 7</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>②、示例2：查找字符串中某个字符的位置</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> index <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">","</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 查找逗号在 str 中第一次出现的位置</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>index<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 5</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>③、示例3：查找子字符串的位置（从指定位置开始查找）</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> index <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">"l"</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 comment">// 从索引为3的位置开始查找 "l" 子字符串在 str 中第一次出现的位置</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>index<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 3</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>④、示例4：查找多个子字符串</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token string">"Hello, world!"</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> index1 <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">"o"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 查找 "o" 子字符串在 str 中第一次出现的位置</span>
<span class="token keyword">int</span> index2 <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">"o"</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 从索引为5的位置开始查找 "o" 子字符串在 str 中第一次出现的位置</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>index1<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 4</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>index2<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出 8</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></div><h3 id="string-类的其他方法" tabindex="-1"><a class="header-anchor" href="#string-类的其他方法" aria-hidden="true">#</a> String 类的其他方法</h3><p>比如说 <code>length()</code> 用于返回字符串长度。</p><p>比如说 <code>isEmpty()</code> 用于判断字符串是否为空。</p><p>比如说 <code>charAt()</code> 用于返回指定索引处的字符。</p><p>比如说 <code>getBytes()</code> 用于返回字符串的字节数组，可以指定编码方式，比如说：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> text <span class="token operator">=</span> <span class="token string">"沉默王二"</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>text<span class="token punctuation">.</span><span class="token function">getBytes</span><span class="token punctuation">(</span><span class="token class-name">StandardCharsets</span><span class="token punctuation">.</span><span class="token constant">UTF_8</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 class="line-number"></div></div></div><p>比如说 <code>trim()</code> 用于去除字符串两侧的空白字符，来看源码：</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 class-name">String</span> <span class="token function">trim</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> len <span class="token operator">=</span> value<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
<span class="token keyword">int</span> st <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> val <span class="token operator">=</span> value<span class="token punctuation">;</span> <span class="token comment">/* avoid getfield opcode */</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>st <span class="token operator"><</span> len<span class="token punctuation">)</span> <span class="token operator">&&</span> <span class="token punctuation">(</span>val<span class="token punctuation">[</span>st<span class="token punctuation">]</span> <span class="token operator"><=</span> <span class="token char">' '</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
st<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>st <span class="token operator"><</span> len<span class="token punctuation">)</span> <span class="token operator">&&</span> <span class="token punctuation">(</span>val<span class="token punctuation">[</span>len <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">]</span> <span class="token operator"><=</span> <span class="token char">' '</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
len<span class="token operator">--</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>st <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">||</span> <span class="token punctuation">(</span>len <span class="token operator"><</span> value<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token function">substring</span><span class="token punctuation">(</span>st<span class="token punctuation">,</span> len<span class="token punctuation">)</span> <span class="token operator">:</span> <span class="token keyword">this</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></div><p>举例：<code>" 沉默王二 ".trim()</code> 会返回"沉默王二"</p><p>除此之外，还有 <a href="https://tobebetterjavaer.com/string/split.html" target="_blank" rel="noopener noreferrer">split<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>、<a href="https://tobebetterjavaer.com/string/equals.html" target="_blank" rel="noopener noreferrer">equals<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>、<a href="https://tobebetterjavaer.com/string/join.html" target="_blank" rel="noopener noreferrer">join<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> 等这些方法，我们后面会一一来细讲。</p><hr><p>GitHub 上标星 7600+ 的开源知识库《<a href="https://github.com/itwanger/toBeBetterJavaer" target="_blank" rel="noopener noreferrer">二哥的 Java 进阶之路<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>》第一版 PDF 终于来了！包括Java基础语法、数组&字符串、OOP、集合框架、Java IO、异常处理、Java 新特性、网络编程、NIO、并发编程、JVM等等，共计 32 万余字，可以说是通俗易懂、风趣幽默……详情戳：<a href="https://tobebetterjavaer.com/overview/" target="_blank" rel="noopener noreferrer">太赞了，GitHub 上标星 7600+ 的 Java 教程<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><p>微信搜 <strong>沉默王二</strong> 或扫描下方二维码关注二哥的原创公众号沉默王二，回复 <strong>222</strong> 即可免费领取。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/gongzhonghao.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure></div><!----><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/itwanger/toBeBetterJavaer/edit/master/docs/string/string-source.md" rel="noopener noreferrer" target="_blank" aria-label="编辑此页" class="nav-link label"><!--[--><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 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