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class="nav-link sidebar-link sidebar-page" 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 href="/basic-extra-meal/enum.html" class="nav-link sidebar-link sidebar-page" 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">2.4 字符串&amp;数组</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.5 集合框架（容器）</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 IO</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.7 异常处理</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.11 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、知识库搭建</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、联系作者</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->Java抽象类</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="https://tobebetterjavaer.com" 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 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span>2021年10月22日</span><meta property="datePublished" content="2021-10-22T07:22:10.000Z"></span><span class="category-info" aria-label="分类🌈" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H593.927c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.31-40.31zM730.22 920.502H623.926c-40.925 0-74.22-33.388-74.22-74.425V623.992c0-41.038 33.387-74.424 74.425-74.424h222.085c41.038 0 74.424 33.226 74.424 74.067v114.233c0 10.244-8.304 18.548-18.547 18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></svg><ul class="categories-wrapper"><li class="category category0" role>Java核心</li><meta property="articleSection" content="Java核心"></ul></span><span aria-label="标签🏷" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="tag icon"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><ul class="tags-wrapper"><li class="tag tag4" role>Java</li></ul><meta property="keywords" content="Java"></span><span 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 5 分钟</span><meta property="timeRequired" content="PT5M"></span></div><hr></div><!----><!----><div class="theme-hope-content"><h1 id="java抽象类" tabindex="-1"><a class="header-anchor" href="#java抽象类" aria-hidden="true">#</a> Java抽象类</h1><p>“二哥，你这明显加快了更新的频率呀！”三妹对于我最近的肝劲由衷的佩服了起来。</p><p>“哈哈，是呀，这次不能再断更了，我要再更 175 篇，总计 200 篇，给广大的学弟学妹们一个完整的 Java 学习体系。”我对未来充满了信心。</p><p>“那就开始吧。”三妹说。</p><hr><p>定义抽象类的时候需要用到关键字 <code>abstract</code>，放在 <code>class</code> 关键字前，就像下面这样。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
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<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>关于抽象类的命名，《阿里的 Java 开发手册》上有强调，“抽象类命名要使用 Abstract 或 Base 开头”，这条规约还是值得遵守的。</p><p>抽象类是不能实例化的，尝试通过 <code>new</code> 关键字实例化的话，编译器会报错，提示“类是抽象的，不能实例化”。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/object-class/abstract-01.png" alt=""></p><p>虽然抽象类不能实例化，但可以有子类。子类通过 <code>extends</code> 关键字来继承抽象类。就像下面这样。</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">BasketballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</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>如果一个类定义了一个或多个抽象方法，那么这个类必须是抽象类。</p><p>当我们尝试在一个普通类中定义抽象方法的时候，编译器会有两处错误提示。第一处在类级别上，提示“这个类必须通过 <code>abstract</code> 关键字定义”，见下图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/object-class/abstract-02.png" alt=""></p><p>第二处在尝试定义 abstract 的方法上，提示“抽象方法所在的类不是抽象的”，见下图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/object-class/abstract-03.png" alt=""></p><p>抽象类中既可以定义抽象方法，也可以定义普通方法，就像下面这样：</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">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">play</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">sleep</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">&quot;运动员也要休息而不是挑战极限&quot;</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>抽象类派生的子类必须实现父类中定义的抽象方法。比如说，抽象类 AbstractPlayer 中定义了 <code>play()</code> 方法，子类 BasketballPlayer 中就必须实现。</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">BasketballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">play</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">&quot;我是张伯伦，篮球场上得过 100 分&quot;</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></div><p>如果没有实现的话，编译器会提示“子类必须实现抽象方法”，见下图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/object-class/abstract-04.png" alt=""></p><p>“二哥，抽象方法我明白了，那什么时候使用抽象方法呢？能给我讲讲它的应用场景吗？”三妹及时的插话道。</p><p>“这问题问的恰到好处呀！”我扶了扶眼镜继续说。</p><p><strong>第一种场景</strong>。</p><p>当我们希望一些通用的功能被多个子类复用的时候，就可以使用抽象类。比如说，AbstractPlayer 抽象类中有一个普通的方法 <code>sleep()</code>，表明所有运动员都需要休息，那么这个方法就可以被子类复用。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">sleep</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">&quot;运动员也要休息而不是挑战极限&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>子类 BasketballPlayer 继承了 AbstractPlayer 类：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">BasketballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</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>也就拥有了 <code>sleep()</code> 方法。BasketballPlayer 的对象可以直接调用父类的 <code>sleep()</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">BasketballPlayer</span> basketballPlayer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BasketballPlayer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
basketballPlayer<span class="token punctuation">.</span><span class="token function">sleep</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>子类 FootballPlayer 继承了 AbstractPlayer 类：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">FootballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</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>也拥有了 <code>sleep()</code> 方法，FootballPlayer 的对象也可以直接调用父类的 <code>sleep()</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">FootballPlayer</span> footballPlayer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FootballPlayer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
footballPlayer<span class="token punctuation">.</span><span class="token function">sleep</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>这样是不是就实现了代码的复用呢？</p><p><strong>第二种场景</strong>。</p><p>当我们需要在抽象类中定义好 API，然后在子类中扩展实现的时候就可以使用抽象类。比如说，AbstractPlayer 抽象类中定义了一个抽象方法 <code>play()</code>，表明所有运动员都可以从事某项运动，但需要对应子类去扩展实现，表明篮球运动员打篮球，足球运动员踢足球。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">play</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></div><p>BasketballPlayer 继承了 AbstractPlayer 类，扩展实现了自己的 <code>play()</code> 方法。</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">BasketballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">play</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">&quot;我是张伯伦，我篮球场上得过 100 分，&quot;</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></div><p>FootballPlayer 继承了 AbstractPlayer 类，扩展实现了自己的 <code>play()</code> 方法。</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">FootballPlayer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractPlayer</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">play</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">&quot;我是C罗，我能接住任意高度的头球&quot;</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></div><p>为了进一步展示抽象类的特性，我们再来看一个具体的示例。假设现在有一个文件，里面的内容非常简单，只有一个“Hello World”，现在需要有一个读取器将内容从文件中读取出来，最好能按照大写的方式，或者小写的方式来读。</p><p>这时候，最好定义一个抽象类 BaseFileReader：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">BaseFileReader</span> <span class="token punctuation">{</span>
    <span class="token keyword">protected</span> <span class="token class-name">Path</span> filePath<span class="token punctuation">;</span>

    <span class="token keyword">protected</span> <span class="token class-name">BaseFileReader</span><span class="token punctuation">(</span><span class="token class-name">Path</span> filePath<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>filePath <span class="token operator">=</span> filePath<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">public</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> <span class="token function">readFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IOException</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token class-name">Files</span><span class="token punctuation">.</span><span class="token function">lines</span><span class="token punctuation">(</span>filePath<span class="token punctuation">)</span>
                <span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token operator">::</span><span class="token function">mapFileLine</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</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">protected</span> <span class="token keyword">abstract</span> <span class="token class-name">String</span> <span class="token function">mapFileLine</span><span class="token punctuation">(</span><span class="token class-name">String</span> line<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></div><ul><li><p>filePath 为文件路径，使用 protected 修饰，表明该成员变量可以在需要时被子类访问到。</p></li><li><p><code>readFile()</code> 方法用来读取文件，方法体里面调用了抽象方法 <code>mapFileLine()</code>——需要子类来扩展实现大小写的不同读取方式。</p></li></ul><p>在我看来，BaseFileReader 类设计的就非常合理，并且易于扩展，子类只需要专注于具体的大小写实现方式就可以了。</p><p>小写的方式：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">LowercaseFileReader</span> <span class="token keyword">extends</span> <span class="token class-name">BaseFileReader</span> <span class="token punctuation">{</span>
    <span class="token keyword">protected</span> <span class="token class-name">LowercaseFileReader</span><span class="token punctuation">(</span><span class="token class-name">Path</span> filePath<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">super</span><span class="token punctuation">(</span>filePath<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">protected</span> <span class="token class-name">String</span> <span class="token function">mapFileLine</span><span class="token punctuation">(</span><span class="token class-name">String</span> line<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> line<span class="token punctuation">.</span><span class="token function">toLowerCase</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>大写的方式：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">UppercaseFileReader</span> <span class="token keyword">extends</span> <span class="token class-name">BaseFileReader</span> <span class="token punctuation">{</span>
    <span class="token keyword">protected</span> <span class="token class-name">UppercaseFileReader</span><span class="token punctuation">(</span><span class="token class-name">Path</span> filePath<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">super</span><span class="token punctuation">(</span>filePath<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">protected</span> <span class="token class-name">String</span> <span class="token function">mapFileLine</span><span class="token punctuation">(</span><span class="token class-name">String</span> line<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> line<span class="token punctuation">.</span><span class="token function">toUpperCase</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>从文件里面一行一行读取内容的代码被子类复用了。与此同时，子类只需要专注于自己该做的工作，LowercaseFileReader 以小写的方式读取文件内容，UppercaseFileReader 以大写的方式读取文件内容。</p><p>来看一下测试类 FileReaderTest：</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">FileReaderTest</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 keyword">throws</span> <span class="token class-name">URISyntaxException</span><span class="token punctuation">,</span> <span class="token class-name">IOException</span> <span class="token punctuation">{</span>
        <span class="token class-name">URL</span> location <span class="token operator">=</span> <span class="token class-name">FileReaderTest</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getResource</span><span class="token punctuation">(</span><span class="token string">&quot;helloworld.txt&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Path</span> path <span class="token operator">=</span> <span class="token class-name">Paths</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>location<span class="token punctuation">.</span><span class="token function">toURI</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">BaseFileReader</span> lowercaseFileReader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">LowercaseFileReader</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">BaseFileReader</span> uppercaseFileReader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">UppercaseFileReader</span><span class="token punctuation">(</span>path<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>lowercaseFileReader<span class="token punctuation">.</span><span class="token function">readFile</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>uppercaseFileReader<span class="token punctuation">.</span><span class="token function">readFile</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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>在项目的 resource 目录下建一个文本文件，名字叫 helloworld.txt，里面的内容就是“Hello World”。文件的具体位置如下图所示，我用的集成开发环境是 Intellij IDEA。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/object-class/abstract-05.png" alt=""></p><p>在 resource 目录下的文件可以通过 <code>ClassLoader.getResource()</code> 的方式获取到 URI 路径，然后就可以取到文本内容了。</p><p>输出结果如下所示：</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>[hello world]
[HELLO WORLD]
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</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><hr><p>“完了吗？二哥”三妹似乎还沉浸在聆听教诲的快乐中。</p><p>“是滴，这次我们系统化的学习了抽象类，可以说面面俱到了。三妹你可以把代码敲一遍，加强了一些印象，电脑交给你了。”说完，我就跑到阳台去抽烟了。</p><p>“呼。。。。。”一个大大的眼圈飘散开来，又是愉快的一天~</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/oo/abstract.md" rel="noopener noreferrer" target="_blank" aria-label="编辑此页" class="nav-link label"><!--[--><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 190.883-192.61h267.754a60.46 60.46 0 0 1 0 120.92H252.891a69.962 69.962 0 0 0-69.098 69.099V771.98a69.962 69.962 0 0 0 69.098 69.098h475.911A69.962 69.962 0 0 0 797.9 771.98V503.363a60.46 60.46 0 1 1 120.922 0V771.98A190.883 190.883 0 0 1 728.802 962z"></path></svg><!--]-->编辑此页<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span><!----></a></div><div class="meta-item update-time"><span class="label">上次编辑于: </span><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: www.qing_gee@163.com">itwanger</span>,<!--]--><!--[--><span class="contributor" title="email: www.qing_gee@163.com">沉默王二</span><!--]--><!--]--></div></footer><nav class="page-nav"><a href="/oo/code-init.html" class="nav-link prev" aria-label="代码初始化块"><div class="hint"><span class="arrow left"></span>上一页</div><div class="link"><!---->代码初始化块</div></a><a href="/oo/interface.html" class="nav-link next" aria-label="接口"><div class="hint">下一页<span class="arrow right"></span></div><div class="link">接口<!----></div></a></nav><!----><!----><!--]--></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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