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href="/java8/optional.html" class="nav-link sidebar-link sidebar-page" aria-label="Optional最佳指南"><!---->Optional最佳指南<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/java8/Lambda.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="深入浅出Lambda表达式"><!---->深入浅出Lambda表达式<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/java8/Lambda.html#_01、初识-lambda" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="01、初识 Lambda"><!---->01、初识 Lambda<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/Lambda.html#_02、lambda-语法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="02、Lambda 语法"><!---->02、Lambda 语法<!----></a><ul class="sidebar-sub-headers"></ul></li><li 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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 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end"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->深入浅出Java 8 Lambda表达式：探索函数式编程的魅力</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" 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content="PT8M"></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="/java8/Lambda.html#_01、初识-lambda" class="router-link-active router-link-exact-active toc-link level3">01、初识 Lambda</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/Lambda.html#_02、lambda-语法" class="router-link-active router-link-exact-active toc-link level3">02、Lambda 语法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/Lambda.html#_03、lambda-和-this-关键字" class="router-link-active router-link-exact-active toc-link level3">03、Lambda 和 this 关键字</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/Lambda.html#_04、最后" class="router-link-active router-link-exact-active toc-link level3">04、最后</a></li><!----><!--]--></ul></div><!--[--><!----><!--]--></aside></div><!----><div class="theme-hope-content"><h1 id="_10-3-深入浅出lambda表达式" tabindex="-1"><a class="header-anchor" href="#_10-3-深入浅出lambda表达式" aria-hidden="true">#</a> 10.3 深入浅出Lambda表达式</h1><p>今天分享的主题是《Lambda 表达式入门》，这也是之前一些读者留言强烈要求我写一写的，不好意思，让你们久等了，现在来满足你们，为时不晚吧？</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/Lambda-1.jpg" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_01、初识-lambda" tabindex="-1"><a class="header-anchor" href="#_01、初识-lambda" aria-hidden="true">#</a> 01、初识 Lambda</h3><p>Lambda 表达式描述了一个代码块（或者叫匿名方法），可以将其作为参数传递给构造方法或者普通方法以便后续执行。考虑下面这段代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></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">"沉默王二"</span><span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>来从左到右解释一下，<code>()</code> 为 Lambda 表达式的参数列表（本例中没有参数），<code>-></code> 标识这串代码为 Lambda 表达式（也就是说，看到 <code>-></code> 就知道这是 Lambda），<code>System.out.println("沉默王二")</code> 为要执行的代码，即将“沉默王二”打印到标准输出流。</p><p>有点 Java 基础的同学应该不会对 Runnable 接口感到陌生，这是多线程的一个基础接口，它的定义如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token annotation punctuation">@FunctionalInterface</span>
<span class="token keyword">public</span> <span class="token keyword">interface</span> <span class="token class-name">Runnable</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">run</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></div><p>Runnable 接口非常简单，仅有一个抽象方法 <code>run()</code>；细心的同学会发现一个陌生的注解 <code>@FunctionalInterface</code>，这个注解是什么意思呢？</p><p>我看了它的源码，里面有这样一段注释：</p><blockquote><p>Note that instances of functional interfaces can be created with lambda expressions, method references, or constructor references.</p></blockquote><p>大致的意思就是说，通过 <code>@FunctionalInterface</code> 标记的接口可以通过 Lambda 表达式创建实例。具体怎么表现呢？</p><p>原来我们创建一个线程并启动它是这样的：</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">LamadaTest</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 keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"沉默王二"</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><span class="token function">start</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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>通过 Lambda 表达式呢？只需要下面这样：</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">LamadaTest</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 keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></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">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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></div><p>是不是很妙！比起匿名内部类，Lambda 表达式不仅易于理解，更大大简化了必须编写的代码数量。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/Lambda-2.jpg" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_02、lambda-语法" tabindex="-1"><a class="header-anchor" href="#_02、lambda-语法" aria-hidden="true">#</a> 02、Lambda 语法</h3><p>每个 Lambda 表达式都遵循以下法则：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>( parameter-list ) -> { expression-or-statements }
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p><code>()</code> 中的 <code>parameter-list</code> 是以逗号分隔的参数。你可以指定参数的类型，也可以不指定（编译器会根据上下文进行推断）。Java 11 后，还可以使用 <code>var</code> 关键字作为参数类型，有点 JavaScript 的味道。</p><p><code>-></code> 相当于 Lambda 的标识符，就好像见到圣旨就见到了皇上。</p><p><code>{}</code> 中的 <code>expression-or-statements</code> 为 Lambda 的主体，可以是一行语句，也可以多行。</p><p>可以通过 Lambda 表达式干很多事情，比如说</p><p>1）为变量赋值，示例如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></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">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
r<span class="token punctuation">.</span><span class="token function">run</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>2）作为 return 结果，示例如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token class-name">FileFilter</span> <span class="token function">getFilter</span><span class="token punctuation">(</span><span class="token class-name">String</span> ext<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span>pathname<span class="token punctuation">)</span> <span class="token operator">-></span> pathname<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 function">endsWith</span><span class="token punctuation">(</span>ext<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>3）作为数组元素，示例如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">final</span> <span class="token class-name">PathMatcher</span> matchers<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span>
<span class="token punctuation">{</span>
<span class="token punctuation">(</span>path<span class="token punctuation">)</span> <span class="token operator">-></span> path<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 function">endsWith</span><span class="token punctuation">(</span><span class="token string">"txt"</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token punctuation">(</span>path<span class="token punctuation">)</span> <span class="token operator">-></span> path<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 function">endsWith</span><span class="token punctuation">(</span><span class="token string">"java"</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></div><p>4）作为普通方法或者构造方法的参数，示例如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></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">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>需要注意 Lambda 表达式的作用域范围。</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 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 keyword">int</span> limit <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
<span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> limit <span class="token operator">=</span> <span class="token number">5</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> limit<span class="token punctuation">;</span> i<span class="token operator">++</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>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>
</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>上面这段代码在编译的时候会提示错误：变量 limit 已经定义过了。</p><p>和匿名内部类一样，不要在 Lambda 表达式主体内对方法内的局部变量进行修改，否则编译也不会通过：Lambda 表达式中使用的变量必须是 final 的。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/Lambda-3.jpg" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>这个问题发生的原因是因为 Java 规范中是这样规定的：</p><blockquote><p>Any local variable, formal parameter, or exception parameter used but not declared in a lambda expression must either be declared final or be effectively final <a href="http://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.12.4" target="_blank" rel="noopener noreferrer">(§4.12.4)<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>, or a compile-time error occurs where the use is attempted.</p></blockquote><p>大致的意思就是说，Lambda 表达式中要用到的，但又未在 Lambda 表达式中声明的变量，必须声明为 final 或者是 effectively final，否则就会出现编译错误。</p><p>关于 final 和 effectively final 的区别，可能有些小伙伴不太清楚，这里多说两句。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">final</span> <span class="token keyword">int</span> a<span class="token punctuation">;</span>
a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment">// a = 2;</span>
<span class="token comment">// 由于 a 是 final 的，所以不能被重新赋值</span>
<span class="token keyword">int</span> b<span class="token punctuation">;</span>
b <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment">// b 此后再未更改</span>
<span class="token comment">// b 就是 effectively final</span>
<span class="token keyword">int</span> c<span class="token punctuation">;</span>
c <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token comment">// c 先被赋值为 1，随后又被重新赋值为 2</span>
c <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token comment">// c 就不是 effectively final</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></div><p>明白了 final 和 effectively final 的区别后，我们了解到，如果把 limit 定义为 final，那就无法在 Lambda 表达式中修改变量的值。那有什么好的解决办法呢？既能让编译器不发出警告，又能修改变量的值。</p><p>思前想后，试来试去，我终于找到了 3 个可行的解决方案：</p><p>1）把 limit 变量声明为 static。</p><p>2）把 limit 变量声明为 AtomicInteger。</p><p>3）使用数组。</p><p>下面我们来详细地一一介绍下。</p><h4 id="_01-把-limit-变量声明为-static" tabindex="-1"><a class="header-anchor" href="#_01-把-limit-变量声明为-static" aria-hidden="true">#</a> 01）把 limit 变量声明为 static</h4><p>要想把 limit 变量声明为 static，就必须将 limit 变量放在 <code>main()</code> 方法外部，因为 <code>main()</code> 方法本身是 static 的。完整的代码示例如下所示。</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">ModifyVariable2StaticInsideLambda</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> limit <span class="token operator">=</span> <span class="token number">10</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">Runnable</span> r <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
limit <span class="token operator">=</span> <span class="token number">5</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> limit<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">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</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 punctuation">}</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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 class="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>来看一下程序输出的结果：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>0
1
2
3
4
</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>OK，该方案是可行的。</p><h4 id="_02-把-limit-变量声明为-atomicinteger" tabindex="-1"><a class="header-anchor" href="#_02-把-limit-变量声明为-atomicinteger" aria-hidden="true">#</a> 02）把 limit 变量声明为 AtomicInteger</h4><p>AtomicInteger 可以确保 int 值的修改是原子性的，可以使用 <code>set()</code> 方法设置一个新的 int 值，<code>get()</code> 方法获取当前的 int 值。</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">ModifyVariable2AtomicInsideLambda</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 keyword">final</span> <span class="token class-name">AtomicInteger</span> limit <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">AtomicInteger</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
limit<span class="token punctuation">.</span><span class="token function">set</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 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> limit<span class="token punctuation">.</span><span class="token function">get</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>
<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>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">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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 class="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>来看一下程序输出的结果：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>0
1
2
3
4
</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>OK，该方案也是可行的。</p><h4 id="_03-使用数组" tabindex="-1"><a class="header-anchor" href="#_03-使用数组" aria-hidden="true">#</a> 03）使用数组</h4><p>使用数组的方式略带一些欺骗的性质，在声明数组的时候设置为 final，但更改 int 的值时却修改的是数组的一个元素。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ModifyVariable2ArrayInsideLambda</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 keyword">final</span> <span class="token keyword">int</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> limits <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">10</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token punctuation">{</span>
limits<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token number">5</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> limits<span class="token punctuation">[</span><span class="token number">0</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>
<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>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">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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 class="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>来看一下程序输出的结果：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>0
1
2
3
4
</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>OK，该方案也是可行的。</p><h3 id="_03、lambda-和-this-关键字" tabindex="-1"><a class="header-anchor" href="#_03、lambda-和-this-关键字" aria-hidden="true">#</a> 03、Lambda 和 this 关键字</h3><p>Lambda 表达式并不会引入新的作用域，这一点和匿名内部类是不同的。也就是说，Lambda 表达式主体内使用的 this 关键字和其所在的类实例相同。</p><p>来看下面这个示例。</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">LamadaTest</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 keyword">new</span> <span class="token class-name">LamadaTest</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">work</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">void</span> <span class="token function">work</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"this = %s%n"</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"this = %s%n"</span><span class="token punctuation">,</span> <span class="token keyword">this</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">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span>r<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-></span> <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"this = %s%n"</span><span class="token punctuation">,</span> <span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">start</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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="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>Tips：<code>%s</code> 代表当前位置输出字符串，<code>%n</code> 代表换行符，也可以使用 <code>\n</code> 代替，但 <code>%n</code> 是跨平台的。</p><p><code>work()</code> 方法中的代码可以分为 3 个部分：</p><p>1）单独的 this 关键字</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"this = %s%n"</span><span class="token punctuation">,</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></div><p>其中 this 为 <code>main()</code> 方法中通过 new 关键字创建的 LamadaTest 对象——<code>new LamadaTest()</code>。</p><p>2）匿名内部类中的 this 关键字</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Runnable</span> r <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"this = %s%n"</span><span class="token punctuation">,</span> <span class="token keyword">this</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 class="line-number"></div></div></div><p>其中 this 为 <code>work()</code> 方法中通过 new 关键字创建的 Runnable 对象——<code>new Runnable(){...}</code>。</p><p>3）Lambda 表达式中的 this 关键字</p><p>其中 this 关键字和 1）中的相同。</p><p>我们来看一下程序的输出结果：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">this</span> <span class="token operator">=</span> <span class="token class-name"><span class="token namespace">com<span class="token punctuation">.</span>cmower<span class="token punctuation">.</span>java_demo<span class="token punctuation">.</span>journal<span class="token punctuation">.</span></span>LamadaTest</span><span class="token annotation punctuation">@3feba861</span>
<span class="token keyword">this</span> <span class="token operator">=</span> <span class="token class-name"><span class="token namespace">com<span class="token punctuation">.</span>cmower<span class="token punctuation">.</span>java_demo<span class="token punctuation">.</span>journal<span class="token punctuation">.</span></span>LamadaTest</span>$<span class="token number">1</span><span class="token annotation punctuation">@64f033cb</span>
<span class="token keyword">this</span> <span class="token operator">=</span> <span class="token class-name"><span class="token namespace">com<span class="token punctuation">.</span>cmower<span class="token punctuation">.</span>java_demo<span class="token punctuation">.</span>journal<span class="token punctuation">.</span></span>LamadaTest</span><span class="token annotation punctuation">@3feba861</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>符合我们分析的预期。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/Lambda-4.jpg" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_04、最后" tabindex="-1"><a class="header-anchor" href="#_04、最后" aria-hidden="true">#</a> 04、最后</h3><p>尽管 Lambda 表达式在简化 Java 编程方面做了很多令人惊讶的努力，但在某些情况下，不当的使用仍然会导致不必要的混乱，大家伙慎用。</p><p>好了，我亲爱的读者朋友们，以上就是本文的全部内容了。能在疫情期间坚持看技术文，二哥必须要伸出大拇指为你点个赞👍。原创不易，如果觉得有点用的话，请不要吝啬你手中<strong>点赞</strong>的权力——因为这将是我写作的最强动力。</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" 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