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class="title">二、Java基础</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.1 Java概述及环境配置</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.2 Java语法基础</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.3 数组&字符串</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.4 面向对象编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.5 集合框架（容器）</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 Java IO</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.7 异常处理</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">2.9 Java新特性</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/java8/stream.html" class="nav-link sidebar-link sidebar-page" aria-label="掌握Stream流"><!---->掌握Stream流<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/java8/optional.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="Optional最佳指南"><!---->Optional最佳指南<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_01、没有-optional-会有什么问题" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="01、没有 Optional 会有什么问题"><!---->01、没有 Optional 会有什么问题<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_02、optional-是如何解决这个问题的" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="02、Optional 是如何解决这个问题的"><!---->02、Optional 是如何解决这个问题的<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_03、创建-optional-对象" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="03、创建 Optional 对象"><!---->03、创建 Optional 对象<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_04、判断值是否存在" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="04、判断值是否存在"><!---->04、判断值是否存在<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_05、非空表达式" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="05、非空表达式"><!---->05、非空表达式<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_06、设置-获取-默认值" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="06、设置（获取）默认值"><!---->06、设置（获取）默认值<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_07、获取值" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="07、获取值"><!---->07、获取值<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_08、过滤值" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="08、过滤值"><!---->08、过滤值<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java8/optional.html#_09、转换值" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="09、转换值"><!---->09、转换值<!----></a><ul class="sidebar-sub-headers"></ul></li></ul><!--]--></li><li><!--[--><a href="/java8/Lambda.html" class="nav-link sidebar-link sidebar-page" aria-label="深入浅出Lambda表达式"><!---->深入浅出Lambda表达式<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/java8/java14.html" class="nav-link sidebar-link sidebar-page" aria-label="Java 14 新特性"><!---->Java 14 新特性<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 网络编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.11 NIO</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 Java重要知识点</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.13 并发编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.14 JVM</span><span class="arrow end"></span></button><!----></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java进阶</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">四、数据库</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">五、计算机基础</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">六、求职面试</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">七、学习建议</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">八、知识库搭建</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、联系作者</span><span class="arrow end"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->Java 8 Optional最佳指南：解决空指针问题的优雅之选</h1><div class="page-info"><span class="page-author-info" aria-label="作者🖊" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="page-author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><!----><span class="page-date-info" aria-label="写作日期📅" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span><!----></span><meta property="datePublished" content="2022-03-15T14:42:30.000Z"></span><span class="page-category-info" aria-label="分类🌈" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 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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 10 分钟</span><meta property="timeRequired" content="PT10M"></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/optional.html#_01、没有-optional-会有什么问题" class="router-link-active router-link-exact-active toc-link level3">01、没有 Optional 会有什么问题</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_02、optional-是如何解决这个问题的" class="router-link-active router-link-exact-active toc-link level3">02、Optional 是如何解决这个问题的</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_03、创建-optional-对象" class="router-link-active router-link-exact-active toc-link level3">03、创建 Optional 对象</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_04、判断值是否存在" class="router-link-active router-link-exact-active toc-link level3">04、判断值是否存在</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_05、非空表达式" class="router-link-active router-link-exact-active toc-link level3">05、非空表达式</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_06、设置-获取-默认值" class="router-link-active router-link-exact-active toc-link level3">06、设置（获取）默认值</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_07、获取值" class="router-link-active router-link-exact-active toc-link level3">07、获取值</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_08、过滤值" class="router-link-active router-link-exact-active toc-link level3">08、过滤值</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java8/optional.html#_09、转换值" class="router-link-active router-link-exact-active toc-link level3">09、转换值</a></li><!----><!--]--></ul></div><!--[--><!----><!--]--></aside></div><!----><div class="theme-hope-content"><h1 id="_10-2-optional最佳指南" tabindex="-1"><a class="header-anchor" href="#_10-2-optional最佳指南" aria-hidden="true">#</a> 10.2 Optional最佳指南</h1><p>想学习，永远都不晚，尤其是针对 Java 8 里面的好东西，Optional 就是其中之一，该类提供了一种用于表示可选值而非空引用的类级别解决方案。作为一名 Java 程序员，我真的是烦透了 <a href="https://tobebetterjavaer.com/exception/npe.html" target="_blank" rel="noopener noreferrer">NullPointerException（NPE）<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>，尽管和它熟得就像一位老朋友，知道它也是迫不得已——程序正在使用一个对象却发现这个对象的值为 null，于是 Java 虚拟机就怒发冲冠地把它抛了出来当做替罪羊。</p><p>当然了，我们程序员是富有责任心的，不会坐视不管，于是就有了大量的 null 值检查。尽管有时候这种检查完全没有必要，但我们已经习惯了例行公事。终于，Java 8 看不下去了，就引入了 Optional，以便我们编写的代码不再那么刻薄呆板。</p><figure><img src="https://cdn.tobebetterjavaer.com/stutymore/guava-20230329172935.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_01、没有-optional-会有什么问题" tabindex="-1"><a class="header-anchor" href="#_01、没有-optional-会有什么问题" aria-hidden="true">#</a> 01、没有 Optional 会有什么问题</h3><p>我们来模拟一个实际的应用场景。小王第一天上班，领导老马就给他安排了一个任务，要他从数据库中根据会员 ID 拉取一个会员的姓名，然后将姓名打印到控制台。虽然是新来的，但这个任务难不倒小王，于是他花了 10 分钟写下了这段代码：</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">class</span> <span class="token class-name">WithoutOptionalDemo</span> <span class="token punctuation">{</span>
<span class="token keyword">class</span> <span class="token class-name">Member</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> name<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">setName</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">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">Member</span> mem <span class="token operator">=</span> <span class="token function">getMemberByIdFromDB</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>mem <span class="token operator">!=</span> <span class="token keyword">null</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>mem<span class="token punctuation">.</span><span class="token function">getName</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 keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Member</span> <span class="token function">getMemberByIdFromDB</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 当前 ID 的会员不存在</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 punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>由于当前 ID 的会员不存在，所以 <code>getMemberByIdFromDB()</code> 方法返回了 null 来作为没有获取到该会员的结果，那就意味着在打印会员姓名的时候要先对 mem 判空，否则就会抛出 NPE 异常，不信？让小王把 <code>if (mem != null)</code> 去掉试试，控制台立马打印错误堆栈给你颜色看看。</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>Exception in thread "main" java.lang.NullPointerException
at com.cmower.dzone.optional.WithoutOptionalDemo.main(WithoutOptionalDemo.java:24)
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="_02、optional-是如何解决这个问题的" tabindex="-1"><a class="header-anchor" href="#_02、optional-是如何解决这个问题的" aria-hidden="true">#</a> 02、Optional 是如何解决这个问题的</h3><p>小王把代码提交后，就兴高采烈地去找老马要新的任务了。本着虚心学习的态度，小王请求老马看一下自己的代码，于是老王就告诉他应该尝试一下 Optional，可以避免没有必要的 null 值检查。现在，让我们来看看小王是如何通过 Optional 来解决上述问题的。</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">OptionalDemo</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">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Member</span><span class="token punctuation">></span></span> optional <span class="token operator">=</span> <span class="token function">getMemberByIdFromDB</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
optional<span class="token punctuation">.</span><span class="token function">ifPresent</span><span class="token punctuation">(</span>mem <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">"会员姓名是："</span> <span class="token operator">+</span> mem<span class="token punctuation">.</span><span class="token function">getName</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 punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Member</span><span class="token punctuation">></span></span> <span class="token function">getMemberByIdFromDB</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">boolean</span> hasName <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>hasName<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Member</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 keyword">return</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">empty</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">class</span> <span class="token class-name">Member</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// getter / setter</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>getMemberByIdFromDB()</code> 方法返回了 <code>Optional<Member></code> 作为结果，这样就表明 Member 可能存在，也可能不存在，这时候就可以在 Optional 的 <code>ifPresent()</code> 方法中使用 Lambda 表达式来直接打印结果。</p><p>Optional 之所以可以解决 NPE 的问题，是因为它明确的告诉我们，不需要对它进行判空。它就好像十字路口的路标，明确地告诉你该往哪走。</p><h3 id="_03、创建-optional-对象" tabindex="-1"><a class="header-anchor" href="#_03、创建-optional-对象" aria-hidden="true">#</a> 03、创建 Optional 对象</h3><p>1）可以使用静态方法 <code>empty()</code> 创建一个空的 Optional 对象</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> empty <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">empty</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>empty<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：Optional.empty</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>2）可以使用静态方法 <code>of()</code> 创建一个非空的 Optional 对象</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</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 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>opt<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：Optional[沉默王二]</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>当然了，传递给 <code>of()</code> 方法的参数必须是非空的，也就是说不能为 null，否则仍然会抛出 NullPointerException。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optnull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span>name<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>3）可以使用静态方法 <code>ofNullable()</code> 创建一个即可空又可非空的 Optional 对象</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optOrNull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<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>optOrNull<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：Optional.empty</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><code>ofNullable()</code> 方法内部有一个三元表达式，如果为参数为 null，则返回私有常量 EMPTY；否则使用 new 关键字创建了一个新的 Optional 对象——不会再抛出 NPE 异常了。</p><h3 id="_04、判断值是否存在" tabindex="-1"><a class="header-anchor" href="#_04、判断值是否存在" aria-hidden="true">#</a> 04、判断值是否存在</h3><p>可以通过方法 <code>isPresent()</code> 判断一个 Optional 对象是否存在，如果存在，该方法返回 true，否则返回 false——取代了 <code>obj != null</code> 的判断。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</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 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>opt<span class="token punctuation">.</span><span class="token function">isPresent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：true</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optOrNull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</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 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>optOrNull<span class="token punctuation">.</span><span class="token function">isPresent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：false</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>Java 11 后还可以通过方法 <code>isEmpty()</code> 判断与 <code>isPresent()</code> 相反的结果。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</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 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>opt<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：false</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optOrNull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</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 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>optOrNull<span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：true</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="_05、非空表达式" tabindex="-1"><a class="header-anchor" href="#_05、非空表达式" aria-hidden="true">#</a> 05、非空表达式</h3><p>Optional 类有一个非常现代化的方法——<code>ifPresent()</code>，允许我们使用函数式编程的方式执行一些代码，因此，我把它称为非空表达式。如果没有该方法的话，我们通常需要先通过 <code>isPresent()</code> 方法对 Optional 对象进行判空后再执行相应的代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optOrNull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</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 keyword">if</span> <span class="token punctuation">(</span>optOrNull<span class="token punctuation">.</span><span class="token function">isPresent</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>optOrNull<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><span class="token function">length</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></div><p>有了 <code>ifPresent()</code> 之后，情况就完全不同了，可以直接将 Lambda 表达式传递给该方法，代码更加简洁，更加直观。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</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>
opt<span class="token punctuation">.</span><span class="token function">ifPresent</span><span class="token punctuation">(</span>str <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>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 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>Java 9 后还可以通过方法 <code>ifPresentOrElse(action, emptyAction)</code> 执行两种结果，非空时执行 action，空时执行 emptyAction。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</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>
opt<span class="token punctuation">.</span><span class="token function">ifPresentOrElse</span><span class="token punctuation">(</span>str <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>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 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">"为空"</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><h3 id="_06、设置-获取-默认值" tabindex="-1"><a class="header-anchor" href="#_06、设置-获取-默认值" aria-hidden="true">#</a> 06、设置（获取）默认值</h3><p>有时候，我们在创建（获取） Optional 对象的时候，需要一个默认值，<code>orElse()</code> 和 <code>orElseGet()</code> 方法就派上用场了。</p><p><code>orElse()</code> 方法用于返回包裹在 Optional 对象中的值，如果该值不为 null，则返回；否则返回默认值。该方法的参数类型和值的类型一致。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> nullName <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>nullName<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElse</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 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>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：沉默王二</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><code>orElseGet()</code> 方法与 <code>orElse()</code> 方法类似，但参数类型不同。如果 Optional 对象中的值为 null，则执行参数中的函数。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">String</span> nullName <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>nullName<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElseGet</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">"沉默王二"</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>name<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 输出：沉默王二</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>从输出结果以及代码的形式上来看，这两个方法极其相似，这不免引起我们的怀疑，Java 类库的设计者有必要这样做吗？</p><p>假设现在有这样一个获取默认值的方法，很传统的方式。</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">getDefaultValue</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">"getDefaultValue"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token string">"沉默王二"</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>然后，通过 <code>orElse()</code> 方法和 <code>orElseGet()</code> 方法分别调用 <code>getDefaultValue()</code> 方法返回默认值。</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">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> name <span class="token operator">=</span> <span class="token keyword">null</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">"orElse"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name2 <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElse</span><span class="token punctuation">(</span><span class="token function">getDefaultValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">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">"orElseGet"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name3 <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElseGet</span><span class="token punctuation">(</span><span class="token class-name">OrElseOptionalDemo</span><span class="token operator">::</span><span class="token function">getDefaultValue</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>注：<code>类名 :: 方法名</code>是 Java 8 引入的语法，方法名后面是没有 <code>()</code> 的，表明该方法并不一定会被调用。</p><p>输出结果如下所示：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code>orElse
getDefaultValue
orElseGet
getDefaultValue
</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>输出结果是相似的，没什么太大的不同，这是在 Optional 对象的值为 null 的情况下。假如 Optional 对象的值不为 null 呢？</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 class-name">String</span> name <span class="token operator">=</span> <span class="token string">"沉默王三"</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">"orElse"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name2 <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElse</span><span class="token punctuation">(</span><span class="token function">getDefaultValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">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">"orElseGet"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> name3 <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">orElseGet</span><span class="token punctuation">(</span><span class="token class-name">OrElseOptionalDemo</span><span class="token operator">::</span><span class="token function">getDefaultValue</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>输出结果如下所示：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code>orElse
getDefaultValue
orElseGet
</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>咦，<code>orElseGet()</code> 没有去调用 <code>getDefaultValue()</code>。哪个方法的性能更佳，你明白了吧？</p><h3 id="_07、获取值" tabindex="-1"><a class="header-anchor" href="#_07、获取值" aria-hidden="true">#</a> 07、获取值</h3><p>直观从语义上来看，<code>get()</code> 方法才是最正宗的获取 Optional 对象值的方法，但很遗憾，该方法是有缺陷的，因为假如 Optional 对象的值为 null，该方法会抛出 NoSuchElementException 异常。这完全与我们使用 Optional 类的初衷相悖。</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">GetOptionalDemo</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> name <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> optOrNull <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>name<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>optOrNull<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><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>这段程序在运行时会抛出异常：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>Exception in thread "main" java.util.NoSuchElementException: No value present
at java.base/java.util.Optional.get(Optional.java:141)
at com.cmower.dzone.optional.GetOptionalDemo.main(GetOptionalDemo.java:9)
</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>尽管抛出的异常是 NoSuchElementException 而不是 NPE，但在我们看来，显然是在“五十步笑百步”。建议 <code>orElseGet()</code> 方法获取 Optional 对象的值。</p><h3 id="_08、过滤值" tabindex="-1"><a class="header-anchor" href="#_08、过滤值" aria-hidden="true">#</a> 08、过滤值</h3><p>小王通过 Optional 类对之前的代码进行了升级，完成后又兴高采烈地跑去找老马要任务了。老马觉得这小伙子不错，头脑灵活，又干活积极，很值得培养，就又交给了小王一个新的任务：用户注册时对密码的长度进行检查。</p><p>小王拿到任务后，乐开了花，因为他刚要学习 Optional 类的 <code>filter()</code> 方法，这就派上了用场。</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">FilterOptionalDemo</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> password <span class="token operator">=</span> <span class="token string">"12345"</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>password<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>opt<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">isPresent</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><code>filter()</code> 方法的参数类型为 Predicate（Java 8 新增的一个函数式接口），也就是说可以将一个 Lambda 表达式传递给该方法作为条件，如果表达式的结果为 false，则返回一个 EMPTY 的 Optional 对象，否则返回过滤后的 Optional 对象。</p><p>在上例中，由于 password 的长度为 5 ，所以程序输出的结果为 false。假设密码的长度要求在 6 到 10 位之间，那么还可以再追加一个条件。来看小王增加难度后的代码。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token class-name">Predicate</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> len6 <span class="token operator">=</span> pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">6</span><span class="token punctuation">;</span>
<span class="token class-name">Predicate</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> len10 <span class="token operator">=</span> pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token number">10</span><span class="token punctuation">;</span>
password <span class="token operator">=</span> <span class="token string">"1234567"</span><span class="token punctuation">;</span>
opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>password<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">boolean</span> result <span class="token operator">=</span> opt<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>len6<span class="token punctuation">.</span><span class="token function">and</span><span class="token punctuation">(</span>len10<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">isPresent</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>result<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>这次程序输出的结果为 true，因为密码变成了 7 位，在 6 到 10 位之间。想象一下，假如小王使用 if-else 来完成这个任务，代码该有多冗长。</p><h3 id="_09、转换值" tabindex="-1"><a class="header-anchor" href="#_09、转换值" aria-hidden="true">#</a> 09、转换值</h3><p>小王检查完了密码的长度，仍然觉得不够尽兴，觉得要对密码的强度也进行检查，比如说密码不能是“password”，这样的密码太弱了。于是他又开始研究起了 <code>map()</code> 方法，该方法可以按照一定的规则将原有 Optional 对象转换为一个新的 Optional 对象，原有的 Optional 对象不会更改。</p><p>先来看小王写的一个简单的例子：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">OptionalMapDemo</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> name <span class="token operator">=</span> <span class="token string">"沉默王二"</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> nameOptional <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> intOpt <span class="token operator">=</span> nameOptional
<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token operator">::</span><span class="token function">length</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token 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> intOpt<span class="token punctuation">.</span><span class="token function">orElse</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token 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>在上面这个例子中，<code>map()</code> 方法的参数 <code>String::length</code>，意味着要 将原有的字符串类型的 Optional 按照字符串长度重新生成一个新的 Optional 对象，类型为 Integer。</p><p>搞清楚了 <code>map()</code> 方法的基本用法后，小王决定把 <code>map()</code> 方法与 <code>filter()</code> 方法结合起来用，前者用于将密码转化为小写，后者用于判断长度以及是否是“password”。</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">OptionalMapFilterDemo</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> password <span class="token operator">=</span> <span class="token string">"password"</span><span class="token punctuation">;</span>
<span class="token class-name">Optional</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> opt <span class="token operator">=</span> <span class="token class-name">Optional</span><span class="token punctuation">.</span><span class="token function">ofNullable</span><span class="token punctuation">(</span>password<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Predicate</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> len6 <span class="token operator">=</span> pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">6</span><span class="token punctuation">;</span>
<span class="token class-name">Predicate</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> len10 <span class="token operator">=</span> pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token number">10</span><span class="token punctuation">;</span>
<span class="token class-name">Predicate</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> eq <span class="token operator">=</span> pwd <span class="token operator">-></span> pwd<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">"password"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">boolean</span> result <span class="token operator">=</span> opt<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token operator">::</span><span class="token function">toLowerCase</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>len6<span class="token punctuation">.</span><span class="token function">and</span><span class="token punctuation">(</span>len10 <span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">and</span><span class="token punctuation">(</span>eq<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">isPresent</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>result<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></div><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/java8/optional-2.jpg" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>好了，我亲爱的读者朋友，以上就是本文的全部内容了——可以说是史上最佳 Optional 指南了，能看到这里的都是最优秀的程序员，二哥必须要伸出大拇指为你点个赞。</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" 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