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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 left"></span></div><aside class="sidebar"><!--[--><!----><!--]--><ul class="sidebar-links"><li><!--[--><a href="/home.html" class="nav-link sidebar-link sidebar-page" aria-label="一、前言"><!---->一、前言<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">二、Java核心</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.1 Java概述</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.2 Java语法基础</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable 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="Java数组"><!---->Java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 href="/string/immutable.html" class="nav-link sidebar-link sidebar-page" aria-label="为什么String是不可变的？"><!---->为什么String是不可变的？<!----></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="深入解析String.intern()方法"><!---->深入解析String.intern()方法<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/equals.html" class="nav-link sidebar-link sidebar-page" aria-label="如何判断两个字符串是否相等？"><!---->如何判断两个字符串是否相等？<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/string/join.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="Java字符串拼接"><!---->Java字符串拼接<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/split.html" class="nav-link sidebar-link sidebar-page" aria-label="Java字符串分割"><!---->Java字符串分割<!----></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 right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.5 集合框架（容器）</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 IO</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.7 异常处理</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.11 网络编程</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 NIO</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.13 并发编程</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.14 JVM</span><span class="arrow right"></span></button><!----></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">四、数据库</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">五、计算机基础</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">六、求职面试</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">七、学习建议</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">八、知识库搭建</span><span class="arrow right"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、联系作者</span><span class="arrow right"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->最优雅的Java字符串拼接是哪种方式？</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><!----><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span>2021年7月13日</span><meta property="datePublished" content="2021-07-13T07:01:13.000Z"></span><span class="category-info" aria-label="分类🌈" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 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><ul class="categories-wrapper"><li><span class="category category0 clickable" role="navigation">Java核心</span></li></ul><meta property="articleSection" content="Java核心"></span><span 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><ul class="tags-wrapper"><li><span class="tag tag5 clickable" role="navigation">数组&字符串</span></li></ul><meta property="keywords" content="数组&字符串"></span><span class="words-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 2677 字</span><meta property="wordCount" content="2677"></span><span class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 9 分钟</span><meta property="timeRequired" content="PT9M"></span></div><hr></div><!----><!----><div class="theme-hope-content"><p>“哥，你让我看的《<a href="https://tobebetterjavaer.com/nice-article/weixin/sulwgalcpdssbjavakfsc.html" target="_blank" rel="noopener noreferrer">Java 开发手册<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>》上有这么一段内容：循环体内，拼接字符串最好使用 StringBuilder 的 <code>append()</code> 方法，而不是 + 号操作符。这是为什么呀？”三妹疑惑地问。</p><p>“好的，三妹，哥来慢慢给你讲。”我回答。</p><p>三妹能在学习的过程中不断地发现问题，让我感到非常的开心。其实很多时候，我们不应该只是把知识点记在心里，还应该问一问自己，到底是为什么，只有迈出去这一步，才能真正的成长起来。</p><p>“+ 号操作符其实被 Java 在编译的时候重新解释了，换一种说法就是，+ 号操作符是一种语法糖，让字符串的拼接变得更简便了。”一边给三妹解释，我一边在 Intellij IDEA 中敲出了下面这段代码。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Demo</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> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"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>chenmo <span class="token operator">+</span> wanger<span class="token punctuation">)</span><span class="token punctuation">
1000
;;</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></div><p>在 Java 8 的环境下，使用 <code>javap -c Demo.class</code> 反编译字节码后，可以看到以下内容：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>Compiled from "Demo.java"
class Demo {
Demo();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc
7018
#2 // String 沉默
2: astore_1
3: ldc #3 // String 王二
5: astore_2
6: getstatic #4 // Field java/lang/System.out:Ljava/io/PrintStream;
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
}
</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></div><p>“你看，三妹，这里有一个 new 关键字，并且 class 类型为 <code>java/lang/StringBuilder</code>。”我指着标号为 9 的那行对三妹说，“这意味着新建了一个 StringBuilder 的对象。”</p><p>“然后看标号为 17 的这行，是一个 invokevirtual 指令，用于调用对象的方法，也就是 StringBuilder 对象的 <code>append()</code> 方法。”</p><p>“也就意味着把 chenmo 这个字符串添加到 StringBuilder 对象中了。”</p><p>“再往下看，标号为 21 的这行，又调用了一次 <code>append()</code> 方法，意味着把 wanger 这个字符串添加到 StringBuilder 对象中了。”</p><p>换成 Java 代码来表示的话，大概是这个样子：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Demo</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> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"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 punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>chenmo<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>wanger<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><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></div><p>“哦，原来编译的时候把“+”号操作符替换成了 StringBuilder 的 <code>append()</code> 方法啊。”三妹恍然大悟。</p><p>“是的，不过到了 Java 9，情况发生了一些改变，同样的代码，字节码指令完全不同了。”我说。</p><p>同样的代码，在 Java 11 的环境下，字节码指令是这样的：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>Compiled from "Demo.java"
public class com.itwanger.thirtyseven.Demo {
public com.itwanger.thirtyseven.Demo();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2 // String
2: astore_1
3: iconst_0
4: istore_2
5: iload_2
6: bipush 10
8: if_icmpge 41
11: new #3 // class java/lang/String
14: dup
15: ldc #4 // String 沉默
17: invokespecial #5 // Method java/lang/String."<init>":(Ljava/lang/String;)V
20: astore_3
21: ldc #6 // String 王二
23: astore 4
25: aload_1
26: aload_3
27: aload 4
29: invokedynamic #7, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
34: astore_1
35: iinc 2, 1
38: goto 5
41: return
}
</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></div><p>看标号为 29 的这行，字节码指令为 <code>invokedynamic</code>，该指令允许由应用级的代码来决定方法解析，所谓的应用级的代码其实是一个方法——被称为引导方法（Bootstrap Method），简称 BSM，BSM 会返回一个 CallSite（调用点） 对象，这个对象就和 <code>invokedynamic</code> 指令链接在一起。以后再执行这条 <code>invokedynamic</code> 指令时就不会创建新的 CallSite 对象。CallSite 其实就是一个 MethodHandle（方法句柄）的 holder，指向一个调用点真正执行的方法——此时就是 <code>StringConcatFactory.makeConcatWithConstants()</code> 方法。</p><p>“哥，你别再说了，再说我就听不懂了。”三妹打断了我的话。</p><p>“好吧，总之就是 Java 9 以后，JDK 用了另外一种方法来动态解释 + 号操作符，具体的实现方式在字节码指令层面已经看不到了，所以我就以 Java 8 来继续讲解吧。”</p><p>“再回到《Java 开发手册》上的那段内容：循环体内，拼接字符串最好使用 StringBuilder 的 <code>append()</code> 方法，而不是 + 号操作符。原因就在于循环体内如果用 + 号操作符的话，就会产生大量的 StringBuilder 对象，不仅占用了更多的内存空间，还会让 Java 虚拟机不停的进行垃圾回收，从而降低了程序的性能。”</p><p>更好的写法就是在循环的外部新建一个 StringBuilder 对象，然后使用 <code>append()</code> 方法将循环体内的字符串添加进来：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Demo</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">StringBuilder</span> sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</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">1</span><span class="token punctuation">;</span> i <span class="token operator"><</span> <span class="token number">10</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 class-name">String</span> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"王二"</span><span cl
9BA8
ass="token punctuation">;</span>
sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>chenmo<span class="token punctuation">)</span><span class="token punctuation">;</span>
sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>wanger<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>sb<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 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>来做个小测试。</p><p>第一个，for 循环中使用”+”号操作符。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> result <span class="token operator">=</span> <span class="token string">""</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> <span class="token number">100000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
result <span class="token operator">+=</span> <span class="token string">"六六六"</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>第二个，for 循环外部新建 StringBuilder，循环体内使用 <code>append()</code> 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">StringBuilder</span> sb <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</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> <span class="token number">100000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
sb<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"六六六"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>“这两个小测试分别会耗时多长时间呢？三妹你来运行下。”</p><p>“哇，第一个小测试的执行时间是 6212 毫秒，第二个只用了不到 1 毫秒，差距也太大了吧！”三妹说。</p><p>“是的，这下明白了原因吧？”我说。</p><p>“是的，哥，原来如此。”</p><p>“好了，三妹，来看一下 StringBuilder 类的 <code>append()</code> 方法的源码吧！”</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">StringBuilder</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</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></div><p>这 3 行代码其实没啥看的。我们来看父类 AbstractStringBuilder 的 <code>append()</code> 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">AbstractStringBuilder</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>str <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token function">appendNull</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> str<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>
<span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>count <span class="token operator">+</span> len<span class="token punctuation">)</span><span class="token punctuation">;</span>
str<span class="token punctuation">.</span><span class="token function">getChars</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> len<span class="token punctuation">,</span> value<span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
count <span class="token operator">+=</span> len<span class="token punctuation">;</span>
<span class="token keyword">return</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></div><p>1）判断拼接的字符串是不是 null，如果是，当做字符串“null”来处理。<code>appendNull()</code> 方法的源码如下：</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 class-name">AbstractStringBuilder</span> <span class="token function">appendNull</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> c <span class="token operator">=</span> count<span class="token punctuation">;</span>
<span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>c <span class="token operator">+</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">final</span> <span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> value <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>value<span class="token punctuation">;</span>
value<span class="token punctuation">[</span>c<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token char">'n'</span><span class="token punctuation">;</span>
value<span class="token punctuation">[</span>c<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token char">'u'</span><span class="token punctuation">;</span>
value<span class="token punctuation">[</span>c<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token char">'l'</span><span class="token punctuation">;</span>
value<span class="token punctuation">[</span>c<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token char">'l'</span><span class="token punctuation">;</span>
count <span class="token operator">=</span> c<span class="token punctuation">;</span>
<span class="token keyword">return</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></div><p>2）获取字符串的长度。</p><p>3）<code>ensureCapacityInternal()</code> 方法的源码如下：</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">void</span> <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span><span class="token keyword">int</span> minimumCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// overflow-conscious code</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>minimumCapacity <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>
value <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span>
<span class="token function">newCapacity</span><span class="token punctuation">(</span>minimumCapacity<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>
</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>由于字符串内部是用数组实现的，所以需要先判断拼接后的字符数组长度是否超过当前数组的长度，如果超过，先对数组进行扩容，然后把原有的值复制到新的数组中。</p><p>4）将拼接的字符串 str 复制到目标数组 value 中。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code>str<span class="token punctuation">.</span><span class="token function">getChars</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> len<span class="token punctuation">,</span> value<span class="token punctuation">,</span> count<span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>5）更新数组的长度 count。</p><p>“说到 StringBuilder 就必须得提一嘴 StringBuffer，两者就像是孪生双胞胎，该有的都有，只不过大哥 StringBuffer 因为多呼吸两口新鲜空气，所以是线程安全的。”我说，“它里面的方法基本上都加了 synchronized 关键字来做同步。”</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">synchronized</span> <span class="token class-name">StringBuffer</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
toStringCache <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</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></div><p>“除了可以使用 + 号操作符，StringBuilder 和 StringBuilder 的 <code>append()</code> 方法，还有其他的字符串拼接方法吗？”三妹问。</p><p>“有啊，比如说 String 类的 <code>concat()</code> 方法，有点像 StringBuilder 类的 <code>append()</code> 方法。”</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"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>chenmo<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span>wanger<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></div><p>可以来看一下 <code>concat()</code> 方法的源码。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">concat</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> otherLen <span class="token operator">=</span> str<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>
<span class="token keyword">if</span> <span class="token punctuation">(</span>otherLen <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> <span class="token keyword">this</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">char</span> buf<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> len <span class="token operator">+</span> otherLen<span class="token punctuation">)</span><span class="token punctuation">;</span>
str<span class="token punctuation">.</span><span class="token function">getChars</span><span class="token punctuation">(</span>buf<span class="token punctuation">,</span> len<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>buf<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>1）如果拼接的字符串的长度为 0，那么返回拼接前的字符串。</p><p>2）将原字符串的字符数组 value 复制到变量 buf 数组中。</p><p>3）把拼接的字符串 str 复制到字符数组 buf 中，并返回新的字符串对象。</p><p>我一行一行地给三妹解释着。</p><p>“和 <code>+</code> 号操作符相比，<code>concat()</code> 方法在遇到字符串为 null 的时候，会抛出 NullPointerException，而“+”号操作符会把 null 当做是“null”字符串来处理。”</p><p>如果拼接的字符串是一个空字符串（""），那么 concat 的效率要更高一点,毕竟不需要 <code>new StringBuilder</code> 对象。</p><p>如果拼接的字符串非常多，<code>concat()</code> 的效率就会下降，因为创建的字符串对象越来越多。</p><p>“还有吗？”三妹似乎对字符串拼接很感兴趣。</p><p>“有，当然有。”</p><p>String 类有一个静态方法 <code>join()</code>，可以这样来使用。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"王二"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> cmower <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">,</span> chenmo<span class="token punctuation">,</span> wanger<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>cmower<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>第一个参数为字符串连接符，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> message <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token string">"-"</span><span class="token punctuation">,</span> <span class="token string">"王二"</span><span class="token punctuation">,</span> <span class="token string">"太特么"</span><span class="token punctuation">,</span> <span class="token string">"有趣了"</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>输出结果为：<code>王二-太特么-有趣了</code>。</p><p>来看一下 join 方法的源码：</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 keyword">static</span> <span class="token class-name">String</span> <span class="token function">join</span><span class="token punctuation">(</span><span class="token class-name">CharSequence</span> delimiter<span class="token punctuation">,</span> <span class="token class-name">CharSequence</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> elements<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">requireNonNull</span><span class="token punctuation">(</span>delimiter<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">requireNonNull</span><span class="token punctuation">(</span>elements<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Number of elements not likely worth Arrays.stream overhead.</span>
<span class="token class-name">StringJoiner</span> joiner <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringJoiner</span><span class="token punctuation">(</span>delimiter<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">CharSequence</span> cs<span class="token operator">:</span> elements<span class="token punctuation">)</span> <span class="token punctuation">{</span>
joiner<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>cs<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> joiner<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<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>里面新建了一个叫 StringJoiner 的对象，然后通过 for-each 循环把可变参数添加了进来，最后调用 <code>toString()</code> 方法返回 String。</p><p>“实际的工作中，<code>org.apache.commons.lang3.StringUtils</code> 的 <code>join()</code> 方法也经常用来进行字符串拼接。”</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> chenmo <span class="token operator">=</span> <span class="token string">"沉默"</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> wanger <span class="token operator">=</span> <span class="token string">"王二"</span><span class="token punctuation">;</span>
<span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span>chenmo<span class="token punctuation">,</span> wanger<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>该方法不用担心 NullPointerException。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token keyword">null</span>
<span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token string">""</span>
<span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token keyword">null</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token string">""</span>
<span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">"a"</span><span class="token punctuation">,</span> <span class="token string">"b"</span><span class="token punctuation">,</span> <span class="token string">"c"</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token string">"abc"</span>
<span class="token class-name">StringUtils</span><span class="token punctuation">.</span><span class="token function">join</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">""</span><span class="token punctuation">,</span> <span class="token string">"a"</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token string">"a"</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>来看一下源码：</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 keyword">static</span> <span class="token class-name">String</span> <span class="token function">join</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array<span class="token punctuation">,</span> <span class="token class-name">String</span> separator<span class="token punctuation">,</span> <span class="token keyword">final</span> <span class="token keyword">int</span> startIndex<span class="token punctuation">,</span> <span class="token keyword">final</span> <span class="token keyword">int</span> endIndex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>array <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>separator <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
separator <span class="token operator">=</span> <span class="token constant">EMPTY</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">final</span> <span class="token class-name">StringBuilder</span> buf <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span>noOfItems <span class="token operator">*</span> <span class="token number">16</span><span class="token punctuation">)</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> startIndex<span class="token punctuation">;</span> i <span class="token operator"><</span> endIndex<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">if</span> <span class="token punctuation">(</span>i <span class="token operator">></span> startIndex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
buf<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>separator<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>array<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
buf<span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>array<span class="token punctuation">[</span>i<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">return</span> buf<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>内部使用的仍然是 StringBuilder。</p><p>“好了，三妹，关于字符串拼接的知识点我们就讲到这吧。注意 Java 9 以后，对 + 号操作符的解释和之前发生了变化，字节码指令已经不同了，等后面你学了字节码指令后我们再详细地讲一次。”我说。</p><p>“嗯，哥，你休息吧，我把这些例子再重新跑一遍。”三妹说。</p><hr><p>最近整理了一份牛逼的学习资料，包括但不限于Java基础部分（JVM、Java集合框架、多线程），还囊括了 <strong>数据库、计算机网络、算法与数据结构、设计模式、框架类Spring、Netty、微服务（Dubbo，消息队列） 网关</strong> 等等等等……详情戳：<a href="https://tobebetterjavaer.com/pdf/programmer-111.html" target="_blank" rel="noopener noreferrer">可以说是2022年全网最全的学习和找工作的PDF资源了<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>111</strong> 即可免费领取。</p><p><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/gongzhonghao.png" alt="" loading="lazy"></p></div><!----><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/itwanger/toBeBetterJavaer/edit/master/docs/string/join.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 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 190.883-192.61h267.754a60.46 60.46 0 0 1 0 120.92H252.891a69.962 69.962 0 0 0-69.098 69.099V771.98a69.962 69.962 0 0 0 69.098 69.098h475.911A69.962 69.962 0 0 0 797.9 771.98V503.363a60.46 60.46 0 1 1 120.922 0V771.98A190.883 190.883 0 0 1 728.802 962z"></path></svg><!--]-->编辑此页<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></div><div class="meta-item update-time"><span class="label">上次编辑于: </span><!----></div><div class="meta-item contributors"><span class="label">贡献者: </span><!--[--><!--[--><span class="contributor" title="email: www.qing_gee@163.com">itwanger</span>,<!--]--><!--[--><span class="contributor" title="email: www.qing_gee@163.com">沉默王二</span><!--]--><!--]--></div></footer><nav class="page-nav"><a href="/string/equals.html" class="nav-link prev" aria-label="如何判断两个字符串是否相等？"><div class="hint"><span class="arrow left"></span>上一页</div><div class="link"><!---->如何判断两个字符串是否相等？</div></a><a href="/string/split.html" class="nav-link next" aria-label="Java字符串分割"><div class="hint">下一页<span class="arrow right"></span></div><div class="link">Java字符串分割<!----></div></a></nav><div class="giscus-wrapper input-top" style="display:block;"><div style="text-align:center">Loading...</div></div><!----><!--]--></main><!--]--><footer class="footer-wrapper"><div class="footer"><a href="https://beian.miit.gov.cn/" target="_blank">豫ICP备2021038026号-1</a><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/beian.png" height="15px" width="15px" /><a target="_blank" href="http://www.beian.gov.cn/portal/registerSystemInfo?recordcode=41030502000411"><span>豫公网安备 41030502000411号</span></a></div><div class="copyright">Copyright © 2022 沉默王二</div></footer><!--]--></div><!--]--><!----><!----><!--]--></div>
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