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aria-label="不可变对象">不可变对象</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/basic-extra-meal/varables.html" class="nav-link sidebar-link sidebar-page" aria-label="可变参数">可变参数</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/basic-extra-meal/generic.html" class="nav-link sidebar-link sidebar-page" aria-label="泛型">泛型</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/basic-extra-meal/annotation.html" class="nav-link sidebar-link sidebar-page" aria-label="注解">注解</a><ul class="sidebar-sub-headers"></ul></li><li><a aria-current="page" href="/basic-extra-meal/enum.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="枚举">枚举</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/basic-extra-meal/fanshe.html" class="nav-link sidebar-link sidebar-page" aria-label="反射">反射</a><ul class="sidebar-sub-headers"></ul></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.5 集合框架（容器）</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.6 IO</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.7 异常处理</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.9 Java新特性</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.11 并发编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.12 JVM</span><span class="arrow right"></span></button></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习资源</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">十、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>Java枚举（enum）</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><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 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class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 6 分钟</span><meta property="timeRequired" content="PT6M"></span></div><hr></div><div class="theme-hope-content"><h1 id="java枚举-enum" tabindex="-1"><a class="header-anchor" href="#java枚举-enum" aria-hidden="true">#</a> Java枚举（enum）</h1><p>“今天我们来学习枚举吧，三妹！”我说，“同学让你去她家玩了两天，感觉怎么样呀？”</p><p>“心情放松了不少。”三妹说，“可以开始学 Java 了，二哥。”</p><p>“OK。”</p><p>“枚举（enum），是 Java 1.5 时引入的关键字，它表示一种特殊类型的类，继承自 java.lang.Enum。”</p><p>“我们来新建一个枚举 PlayerType。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">enum</span> <span class="token class-name">PlayerType</span> <span class="token punctuation">{</span>

TENNIS<span class="token punctuation">,</span>

FOOTBALL<span class="token punctuation">,</span>

BASKETBALL

</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>“二哥，我没看到有继承关系呀！”</p><p>“别着急，看一下反编译后的字节码，你就明白了。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">PlayerType</span> <span class="token keyword">extends</span> <span class="token class-name">Enum</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">PlayerType</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">values</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">PlayerType</span> <span class="token function">valueOf</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span>

<span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">)</span><span class="token class-name">Enum</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>com<span class="token operator">/</span>cmower<span class="token operator">/</span>baeldung<span class="token operator">/</span>enum1<span class="token operator">/</span><span class="token class-name">PlayerType</span><span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">PlayerType</span><span class="token punctuation">(</span><span class="token class-name">String</span> s<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span>

<span class="token keyword">super</span><span class="token punctuation">(</span>s<span class="token punctuation">,</span> i<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">final</span> <span class="token class-name">PlayerType</span> TENNIS<span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">PlayerType</span> $VALUES<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token keyword">static</span>

TENNIS <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">PlayerType</span><span class="token punctuation">(</span><span class="token string">"TENNIS"</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

FOOTBALL <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">PlayerType</span><span class="token punctuation">(</span><span class="token string">"FOOTBALL"</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

BASKETBALL <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">PlayerType</span><span class="token punctuation">(</span><span class="token string">"BASKETBALL"</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

$VALUES <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PlayerType</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token punctuation">{</span>

TENNIS<span class="token punctuation">,</span> FOOTBALL<span class="token punctuation">,</span> BASKETBALL

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>“看到没？Java 编译器帮我们做了很多隐式的工作，不然手写一个枚举就没那么省心省事了。”</p><ul><li>要继承 Enum 类；</li><li>要写构造方法；</li><li>要声明静态变量和数组；</li><li>要用 static 块来初始化静态变量和数组；</li><li>要提供静态方法，比如说 <code>values()</code> 和 <code>valueOf(String name)</code>。</li></ul><p>“确实，作为开发者，我们的代码量减少了，枚举看起来简洁明了。”三妹说。</p><p>“既然枚举是一种特殊的类，那它其实是可以定义在一个类的内部的，这样它的作用域就可以限定于这个外部类中使用。”我说。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Player</span> <span class="token punctuation">{</span>

<span class="token keyword">private</span> <span class="token class-name">PlayerType</span> type<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">enum</span> <span class="token class-name">PlayerType</span> <span class="token punctuation">{</span>

TENNIS<span class="token punctuation">,</span>

FOOTBALL<span class="token punctuation">,</span>

BASKETBALL

<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">isBasketballPlayer</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 function">getType</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token class-name">PlayerType</span><span class="token punctuation">.</span>BASKETBALL<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token class-name">PlayerType</span> <span class="token function">getType</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">return</span> type<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setType</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span> type<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>PlayerType 就相当于 Player 的内部类。</p><p>由于枚举是 final 的，所以可以确保在 Java 虚拟机中仅有一个常量对象，基于这个原因，我们可以使用“==”运算符来比较两个枚举是否相等，参照 <code>isBasketballPlayer()</code> 方法。</p><p>“那为什么不使用 <code>equals()</code> 方法判断呢？”三妹问。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">if</span><span class="token punctuation">(</span>player<span class="token punctuation">.</span><span class="token function">getType</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token class-name">Player<span class="token punctuation">.</span>PlayerType</span><span class="token punctuation">.</span>BASKETBALL<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></div><p>“我来给你解释下。”</p><p>“==”运算符比较的时候，如果两个对象都为 null，并不会发生 <code>NullPointerException</code>，而 <code>equals()</code> 方法则会。</p><p>另外， “==”运算符会在编译时进行检查，如果两侧的类型不匹配，会提示错误，而 <code>equals()</code> 方法则不会。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/enum/enum-01.png" alt=""></p><p>“枚举还可用于 switch 语句，和基本数据类型的用法一致。”我说。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">switch</span> <span class="token punctuation">(</span>playerType<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">case</span> TENNIS<span class="token operator">:</span>

<span class="token keyword">return</span> <span class="token string">"网球运动员费德勒"</span><span class="token punctuation">;</span>

<span class="token keyword">case</span> FOOTBALL<span class="token operator">:</span>

<span class="token keyword">return</span> <span class="token string">"足球运动员C罗"</span><span class="token punctuation">;</span>

<span class="token keyword">case</span> BASKETBALL<span class="token operator">:</span>

<span class="token keyword">return</span> <span class="token string">"篮球运动员詹姆斯"</span><span class="token punctuation">;</span>

<span class="token keyword">case</span> UNKNOWN<span class="token operator">:</span>

<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">"未知"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">default</span><span class="token operator">:</span>

<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span>

<span class="token string">"运动员类型: "</span> <span class="token operator">+</span> playerType<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token function">TENNIS</span><span class="token punctuation">(</span><span class="token string">"网球"</span><span class="token punctuation">)</span><span class="token punctuation">,</span>

<span class="token function">FOOTBALL</span><span class="token punctuation">(</span><span class="token string">"足球"</span><span class="token punctuation">)</span><span class="token punctuation">,</span>

<span class="token function">BASKETBALL</span><span class="token punctuation">(</span><span class="token string">"篮球"</span><span class="token punctuation">)</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 class-name">PlayerType</span><span class="token punctuation">(</span><span class="token class-name">String</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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="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>“get 了吧，三妹？”</p><p>“嗯，比较好理解。”</p><p>“那接下来，我就来说点不一样的。”</p><p>“来吧，我准备好了。”</p><p>“EnumSet 是一个专门针对枚举类型的 Set 接口（后面会讲）的实现类，它是处理枚举类型数据的一把利器，非常高效。”我说，“从名字上就可以看得出，EnumSet 不仅和 Set 有关系，和枚举也有关系。”</p><p>“因为 EnumSet 是一个抽象类，所以创建 EnumSet 时不能使用 new 关键字。不过，EnumSet 提供了很多有用的静态工厂方法。”</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/enum/enum-02.png" alt=""></p><p>“来看下面这个例子，我们使用 <code>noneOf()</code> 静态工厂方法创建了一个空的 PlayerType 类型的 EnumSet；使用 <code>allOf()</code> 静态工厂方法创建了一个包含所有 PlayerType 类型的 EnumSet。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">EnumSetTest</span> <span class="token punctuation">{</span>

<span class="token keyword">public</span> <span class="token keyword">enum</span> <span class="token class-name">PlayerType</span> <span class="token punctuation">{</span>

TENNIS<span class="token punctuation">,</span>

FOOTBALL<span class="token punctuation">,</span>

BASKETBALL

<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">EnumSet</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">PlayerType</span><span class="token punctuation">></span></span> enumSetNone <span class="token operator">=</span> <span class="token class-name">EnumSet</span><span class="token punctuation">.</span><span class="token function">noneOf</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">EnumSet</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">PlayerType</span><span class="token punctuation">></span></span> enumSetAll <span class="token operator">=</span> <span class="token class-name">EnumSet</span><span class="token punctuation">.</span><span class="token function">allOf</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token punctuation">[</span>TENNIS<span class="token punctuation">,</span> FOOTBALL<span class="token punctuation">,</span> BASKETBALL<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>有了 EnumSet 后，就可以使用 Set 的一些方法了，见下图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/enum/enum-03.png" alt=""></p><p>“除了 EnumSet，还有 EnumMap，是一个专门针对枚举类型的 Map 接口的实现类，它可以将枚举常量作为键来使用。EnumMap 的效率比 HashMap 还要高，可以直接通过数组下标（枚举的 ordinal 值）访问到元素。”</p><p>“和 EnumSet 不同，EnumMap 不是一个抽象类，所以创建 EnumMap 时可以使用 new 关键字。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">EnumMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">PlayerType</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span> enumMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">EnumMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span><span class="token keyword">class</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>有了 EnumMap 对象后就可以使用 Map 的一些方法了，见下图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/enum/enum-04.png" alt=""></p><p>和 HashMap（后面会讲）的使用方法大致相同，来看下面的例子。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">EnumMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">PlayerType</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span> enumMap <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">EnumMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

enumMap<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>BASKETBALL<span class="token punctuation">,</span><span class="token string">"篮球运动员"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

enumMap<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>FOOTBALL<span class="token punctuation">,</span><span class="token string">"足球运动员"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

enumMap<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>TENNIS<span class="token punctuation">,</span><span class="token string">"网球运动员"</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>enumMap<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>BASKETBALL<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>enumMap<span class="token punctuation">.</span><span class="token function">containsKey</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>BASKETBALL<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>enumMap<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token class-name">PlayerType</span><span class="token punctuation">.</span>BASKETBALL<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

篮球运动员

true

篮球运动员

</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>“除了以上这些，《Effective Java》这本书里还提到了一点，如果要实现单例的话，最好使用枚举的方式。”我说。</p><p>“等等二哥，单例是什么？”三妹没等我往下说，就连忙问道。</p><p>“单例（Singleton）用来保证一个类仅有一个对象，并提供一个访问它的全局访问点，在一个进程中。因为这个类只有一个对象，所以就不能再使用 <code>new</code> 关键字来创建新的对象了。”</p><p>“Java 标准库有一些类就是单例，比如说 Runtime 这个类。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Runtime</span> runtime <span class="token operator">=</span> <span class="token class-name">Runtime</span><span class="token punctuation">.</span><span class="token function">getRuntime</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>“Runtime 类可以用来获取 Java 程序运行时的环境。”</p><p>“关于单例，懂了些吧？”我问三妹。</p><p>“噢噢噢噢。”三妹点了点头。</p><p>“通常情况下，实现单例并非易事，来看下面这种写法。”</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>

<span class="token keyword">private</span> <span class="token keyword">volatile</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> singleton<span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">Singleton</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">Singleton</span> <span class="token function">getSingleton</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>singleton <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token class-name">Singleton</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

singleton <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> singleton<span class="token punctuation">;</span>

INSTANCE<span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>“就这？”三妹睁大了眼睛。</p><p>“对啊，枚举默认实现了 Serializable 接口，因此 Java 虚拟机可以保证该类为单例，这与传统的实现方式不大相同。传统方式中，我们必须确保单例在反序列化期间不能创建任何新实例。”我说。</p><p>“好了，关于枚举就讲这么多吧，三妹，你把这些代码都手敲一遍吧！”</p><p>“好勒，这就安排。二哥，你去休息吧。”</p><p>“嗯嗯。”讲了这么多，必须跑去抽烟机那里安排一根华子了。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/xingbiaogongzhonghao.png" alt=""></p></div><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/itwanger/toBeBetterJavaer/edit/master/docs/basic-extra-meal/enum.md" rel="noopener noreferrer" target="_blank" aria-label="在 GitHub 上编辑此页" class="nav-link label"><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 190.883-192.61h267.754a60.46 60.46 0 0 1 0 120.92H252.891a69.962 69.962 0 0 0-69.098 69.099V771.98a69.962 69.962 0 0 0 69.098 69.098h475.911A69.962 69.962 0 0 0 797.9 771.98V503.363a60.46 60.46 0 1 1 120.922 0V771.98A190.883 190.883 0 0 1 728.802 962z"></path></svg>在 GitHub 上编辑此页<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a></div><div class="meta-item update-time"><span class="label">上次编辑于: </span><span class="info">2022/5/14 上午7:14:28</span></div><div class="meta-item contributors"><span class="label">贡献者: </span><span class="contributor" title="email: [email protected]">itwanger</span>,<span class="contributor" title="email: [email protected]">沉默王二</span></div></footer><nav class="page-nav"><a href="/basic-extra-meal/annotation.html" class="nav-link prev" aria-label="注解"><div class="hint"><span class="arrow left"></span>上一页</div><div class="link">注解</div></a><a href="/basic-extra-meal/fanshe.html" class="nav-link next" aria-label="反射"><div class="hint">下一页<span class="arrow right"></span></div><div class="link">反射</div></a></nav><div class="giscus-wrapper input-top" style="display:block;">Loading...</div></main><footer class="footer-wrapper"><div class="footer"><a href="https://beian.miit.gov.cn/" target="_blank">豫ICP备2021038026号-1</a><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/beian.png" height="15px" width="15px" /><a target="_blank" href="http://www.beian.gov.cn/portal/registerSystemInfo?recordcode=41030502000411"><span>豫公网安备 41030502000411号</span></a></div><div class="copyright">Copyright © 2022 沉默王二</div></footer></div></div>

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