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<meta name="keywords" content="java,hashcode,equals"><meta property="og:url" content="https://javabetter.cn/basic-extra-meal/hashcode.html"><meta property="og:site_name" content="二哥的Java进阶之路"><meta property="og:title" content="Java hashCode方法解析：C++实现的高效本地方法"><meta property="og:description" content="hashCode是Java 所有类都有的方法，它会返回一个整数哈希码，表示对象在内存中的近似位置。相等的对象应具有相同哈希码，因此自定义类时需同时重写hashCode()和equals()方法。"><meta property="og:type" content="article"><meta property="og:locale" content="zh-CN"><meta property="og:updated_time" content="2023-09-04T05:16:42.000Z"><meta property="article:author" content="沉默王二"><meta property="article:tag" content="Java重要知识点"><meta property="article:modified_time" content="2023-09-04T05:16:42.000Z"><script type="application/ld+json">{"@context":"https://schema.org","@type":"Article","headline":"Java hashCode方法解析：C++实现的高效本地方法","image":[""],"dateModified":"2023-09-04T05:16:42.000Z","author":[{"@type":"Person","name":"沉默王二"}]}</script><meta name="robots" content="all"><meta name="author" content="沉默王二"><meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate"><meta http-equiv="Pragma" content="no-cache"><meta http-equiv="Expires" content="0"><meta name="apple-mobile-web-app-capable" content="yes"><script>
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<div id="app"><!--[--><!--[--><!--[--><span tabindex="-1"></span><a href="#main-content" class="vp-skip-link sr-only">跳至主要內容</a><!--]--><div class="theme-container has-toc"><!--[--><header id="navbar" class="vp-navbar"><div class="vp-navbar-start"><button type="button" class="vp-toggle-sidebar-button" title="Toggle Sidebar"><span class="icon"></span></button><!--[--><!----><!--]--><!--[--><a class="vp-link vp-brand" href="/"><img class="vp-nav-logo" src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/logo-02.png" alt="二哥的Java进阶之路"><!----><span class="vp-site-name hide-in-pad">二哥的Java进阶之路</span></a><!--]--><!--[--><!----><!--]--></div><div class="vp-navbar-center"><!--[--><!----><!--]--><!--[--><nav class="vp-nav-links"><div class="nav-item hide-in-mobile"><a class="vp-link nav-link" href="/blog.html"><span class="font-icon icon iconfont icon-gaishu" style=""></span>博客<!----></a></div><div class="nav-item hide-in-mobile"><a class="vp-link nav-link" href="/home.html"><span 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class="vp-sidebar-heading clickable active" type="button"><!----><span class="vp-sidebar-title">二、Java基础</span><span class="vp-arrow down"></span></button><ul class="vp-sidebar-links"><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.1 Java概述及环境配置</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.2 Java语法基础</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.3 数组&字符串</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.4 面向对象编程</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.5 集合框架（容器）</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.6 Java IO</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.7 异常处理</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.8 常用工具类</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.9 Java新特性</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.10 网络编程</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.11 NIO</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable active" type="button"><!----><span class="vp-sidebar-title">2.12 Java重要知识点</span><span class="vp-arrow down"></span></button><ul class="vp-sidebar-links"><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/java-naming.html"><!---->Java命名规范<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/java-unicode.html"><!---->中文乱码及字符编码全攻略<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/box.html"><!---->深入剖析Java中的拆箱和装箱<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/deep-copy.html"><!---->深入理解Java浅拷贝与深拷贝<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link active vp-sidebar-link vp-sidebar-page active" href="/basic-extra-meal/hashcode.html"><!---->Java hashCode方法解析<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/pass-by-value.html"><!---->Java是值传递还是引用传递？<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/true-generic.html"><!---->Java为什么无法实现真正的泛型<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/basic-extra-meal/fanshe.html"><!---->掌握 Java 反射<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li></ul></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.13 并发编程</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">2.14 JVM</span><span class="vp-arrow end"></span></button><!----></section></li></ul></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">三、Java进阶</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">四、MySQL</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">五、Redis</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">六、计算机基础</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">七、求职面试</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">八、学习建议</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">九、知识库搭建</span><span class="vp-arrow end"></span></button><!----></section></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">十、联系作者</span><span class="vp-arrow end"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main id="main-content" class="vp-page"><!--[--><!----><!----><nav class="vp-breadcrumb disable"></nav><div class="vp-page-title"><h1><!---->Java hashCode方法解析：C++实现的高效本地方法</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><span class="page-author-item">沉默王二</span></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 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hashCode方法解析</h1><p>今天我们来谈谈 Java 中的 <code>hashCode()</code> 方法。众所周知，Java 是一门面向对象的编程语言，所有的类都会默认继承自 Object 类，而 Object 的中文意思就是“对象”。</p><p>Object 类中就包含了 <code>hashCode()</code> 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">native</span> <span class="token keyword">int</span> <span class="token function">hashCode</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>意味着所有的类都会有一个 <code>hashCode()</code> 方法，该方法会返回一个 int 类型的值。由于 <code>hashCode()</code> 方法是一个<a href="https://javabetter.cn/oo/native-method.html" target="_blank" rel="noopener noreferrer">本地方法<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>（<code>native</code> 关键字修饰的方法，用 <code>C/C++</code> 语言实现，由 Java 调用），意味着 Object 类中并没有给出具体的实现。</p><p>具体的实现可以参考 <code>jdk/src/hotspot/share/runtime/synchronizer.cpp</code>（源码可以到 <a href="https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/synchronizer.cpp" target="_blank" rel="noopener noreferrer">GitHub 上 OpenJDK 的仓库中下载<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>）。<code>get_next_hash()</code> 方法会根据 hashCode 的取值来决定采用哪一种哈希值的生成策略。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/basic-extra-meal/hashcode-1.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>Java 9 之后，<code>hashCode()</code> 方法会被 <code>@HotSpotIntrinsicCandidate</code> 注解修饰，表明它在 HotSpot 虚拟机中有一套高效的实现，基于 CPU 指令。</p><p>那大家有没有想过这样一个问题：<strong>为什么 Object 类需要一个 <code>hashCode()</code> 方法呢</strong>？</p><p>在 Java 中，<code>hashCode()</code> 方法的主要作用就是为了配合<a href="https://javabetter.cn/collection/hashmap.html" target="_blank" rel="noopener noreferrer">哈希表<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>使用的。</p><p>哈希表（Hash Table），也叫散列表，是一种可以通过关键码值（key-value）直接访问的数据结构，它最大的特点就是可以快速实现查找、插入和删除。其中用到的算法叫做哈希，就是把任意长度的输入，变换成固定长度的输出，该输出就是哈希值。像 MD5、SHA1 都用的是哈希算法。</p><p>像 Java 中的 HashSet、Hashtable（注意是小写的 t）、HashMap 都是基于哈希表的具体实现。其中的 <a href="https://javabetter.cn/collection/hashmap.html" target="_blank" rel="noopener noreferrer">HashMap<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> 就是最典型的代表，不仅面试官经常问，工作中的使用频率也非常的高。</p><p>大家想一下，如果没有哈希表，但又需要这样一个数据结构，它里面存放的数据是不允许重复的，该怎么办呢？</p><p>要不使用 <code>equals()</code> 方法进行逐个比较？这种方案当然是可行的。但如果数据量特别特别大，采用 <code>equals()</code> 方法进行逐个对比的效率肯定很低很低，最好的解决方案就是哈希表。</p><p>拿 HashMap 来说吧。当我们要在它里面添加对象时，先调用这个对象的 <code>hashCode()</code> 方法，得到对应的哈希值，然后将哈希值和对象一起放到 HashMap 中。当我们要再添加一个新的对象时：</p><ul><li>获取对象的哈希值；</li><li>和之前已经存在的哈希值进行比较，如果不相等，直接存进去；</li><li>如果有相等的，再调用 <code>equals()</code> 方法进行对象之间的比较，如果相等，不存了；</li><li>如果不等，说明哈希冲突了，增加一个链表，存放新的对象；</li><li>如果链表的长度大于 8，转为红黑树来处理。</li></ul><p>就这么一套下来，调用 <code>equals()</code> 方法的频率就大大降低了。也就是说，只要哈希算法足够的高效，把发生哈希冲突的频率降到最低，哈希表的效率就特别的高。</p><p>来看一下 HashMap 的哈希算法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">hash</span><span class="token punctuation">(</span><span class="token class-name">Object</span> key<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> h<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span> <span class="token punctuation">(</span>h <span class="token operator">=</span> key<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">16</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>先调用对象的 <code>hashCode()</code> 方法，然后对该值进行右移运算，然后再进行异或运算。</p><p>通常来说，String 会用来作为 HashMap 的键进行哈希运算，因此我们再来看一下 String 的 <code>hashCode()</code> 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> h <span class="token operator">=</span> hash<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>h <span class="token operator">==</span> <span class="token number">0</span> <span class="token operator">&&</span> value<span class="token punctuation">.</span>length <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">char</span> val<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token operator">=</span> value<span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator"><</span> value<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
h <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> h <span class="token operator">+</span> val<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
hash <span class="token operator">=</span> h<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> h<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>可想而知，经过这么一系列复杂的运算，再加上 JDK 作者这种大师级别的设计，哈希冲突的概率我相信已经降到了最低（我们在 HashMap 中深入探讨过）。</p><p>当然了，从理论上来说，对于两个不同对象，它们通过 <code>hashCode()</code> 方法计算后的值可能相同。因此，不能使用 <code>hashCode()</code> 方法来判断两个对象是否相等，必须得通过 <code>equals()</code> 方法。</p><p>也就是说：</p><ul><li>如果两个对象调用 <code>equals()</code> 方法得到的结果为 true，调用 <code>hashCode()</code> 方法得到的结果必定相等；</li><li>如果两个对象调用 <code>hashCode()</code> 方法得到的结果不相等，调用 <code>equals()</code> 方法得到的结果必定为 false；</li></ul><p>反之：</p><ul><li>如果两个对象调用 <code>equals()</code> 方法得到的结果为 false，调用 <code>hashCode()</code> 方法得到的结果不一定不相等；</li><li>如果两个对象调用 <code>hashCode()</code> 方法得到的结果相等，调用 <code>equals()</code> 方法得到的结果不一定为 true；</li></ul><p>来看下面这段代码。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</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">Student</span> s1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span> <span class="token string">"张三"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Map</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Student</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span> scores <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
scores<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>s1<span class="token punctuation">,</span> <span class="token number">98</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>scores<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span> <span class="token string">"张三"</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">class</span> <span class="token class-name">Student</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> age<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">Student</span><span class="token punctuation">(</span><span class="token keyword">int</span> age<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>age <span class="token operator">=</span> age<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 annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">equals</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Student</span> student <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">Student</span><span class="token punctuation">)</span> o<span class="token punctuation">;</span>
<span class="token keyword">return</span> age <span class="token operator">==</span> student<span class="token punctuation">.</span>age <span class="token operator">&&</span>
<span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>name<span class="token punctuation">,</span> student<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>我们重写了 Student 类的 <code>equals()</code> 方法，如果两个学生的年纪和姓名相同，我们就认为是同一个学生，虽然很离谱，但我们就是这么草率。</p><p>在 <code>main()</code> 方法中，18 岁的张三考试得了 98 分，很不错的成绩，我们把张三和成绩放到了 HashMap 中，然后准备输出张三的成绩：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>null
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>很不巧，结果为 null，而不是预期当中的 98。这是为什么呢？</p><p>原因就在于重写 <code>equals()</code> 方法的时候没有重写 <code>hashCode()</code> 方法。默认情况下，<code>hashCode()</code> 方法是一个本地方法，会返回对象的存储地址，显然 <code>put()</code> 中的 s1 和 <code>get()</code> 中的 <code>new Student(18, "张三")</code> 是两个对象，它们的存储地址肯定是不同的。</p><p>HashMap 的 <code>get()</code> 方法会调用 <code>hash(key.hashCode())</code> 计算对象的哈希值，虽然两个不同的 <code>hashCode()</code> 结果经过 <code>hash()</code> 方法计算后有可能得到相同的结果，但这种概率微乎其微，所以就导致 <code>scores.get(new Student(18, "张三"))</code> 无法得到预期的值 18。</p><p>怎么解决这个问题呢？很简单，重写 <code>hashCode()</code> 方法。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code> <span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">hashCode</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">Objects</span><span class="token punctuation">.</span><span class="token function">hash</span><span class="token punctuation">(</span>age<span class="token punctuation">,</span> name<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><a href="https://javabetter.cn/common-tool/Objects.html" target="_blank" rel="noopener noreferrer">Objects 类<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>的 <code>hash()</code> 方法可以针对不同数量的参数生成新的 <code>hashCode()</code> 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">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token class-name">Object</span> a<span class="token punctuation">[</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>a <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> result <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Object</span> element <span class="token operator">:</span> a<span class="token punctuation">)</span>
result <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> result <span class="token operator">+</span> <span class="token punctuation">(</span>element <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span> element<span class="token punctuation">.</span><span class="token function">hashCode</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> 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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>代码似乎很简单，归纳出的数学公式如下所示（n 为字符串长度）。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/basic-extra-meal/hashcode-2.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>注意：31 是个奇质数，不大不小，一般质数都非常适合哈希计算，偶数相当于移位运算，容易溢出，造成数据信息丢失。</p><p>这就意味着年纪和姓名相同的情况下，会得到相同的哈希值。<code>scores.get(new Student(18, "张三"))</code> 就会返回 98 的预期值了。</p><p>《Java 编程思想》这本圣经中有一段话，对 <code>hashCode()</code> 方法进行了一段描述。</p><blockquote><p>设计 <code>hashCode()</code> 时最重要的因素就是：无论何时，对同一个对象调用 <code>hashCode()</code> 都应该生成同样的值。如果在将一个对象用 <code>put()</code> 方法添加进 HashMap 时产生一个 <code>hashCode()</code> 值，而用 <code>get()</code> 方法取出时却产生了另外一个 <code>hashCode()</code> 值，那么就无法重新取得该对象了。所以，如果你的 <code>hashCode()</code> 方法依赖于对象中易变的数据，用户就要当心了，因为此数据发生变化时，<code>hashCode()</code> 就会生成一个不同的哈希值，相当于产生了一个不同的键。</p></blockquote><p>也就是说，如果在重写 <code>hashCode()</code> 和 <code>equals()</code> 方法时，对象中某个字段容易发生改变，那么最好舍弃这些字段，以免产生不可预期的结果。</p><p>好。有了上面这些内容作为基础后，我们回头再来看看本地方法 <code>hashCode()</code> 的 C++ 源码。</p><div class="language-cpp line-numbers-mode" data-ext="cpp"><pre class="language-cpp"><code><span class="token keyword">static</span> <span class="token keyword">inline</span> intptr_t <span class="token function">get_next_hash</span><span class="token punctuation">(</span>Thread<span class="token operator">*</span> current<span class="token punctuation">,</span> oop obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
intptr_t value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 这种形式使用全局的 Park-Miller 随机数生成器。</span>
<span class="token comment">// 在 MP 系统上，我们将对全局变量进行大量的读写访问，因此该机制会引发大量的一致性通信。</span>
value <span class="token operator">=</span> os<span class="token double-colon punctuation">::</span><span class="token function">random</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">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 这种变体在 STW（Stop The World）操作之间具有稳定（幂等）的特性。</span>
<span class="token comment">// 在一些 1-0 同步方案中，这可能很有用。</span>
intptr_t addr_bits <span class="token operator">=</span> <span class="token generic-function"><span class="token function">cast_from_oop</span><span class="token generic class-name"><span class="token operator"><</span>intptr_t<span class="token operator">></span></span></span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token operator">>></span> <span class="token number">3</span><span class="token punctuation">;</span>
value <span class="token operator">=</span> addr_bits <span class="token operator">^</span> <span class="token punctuation">(</span>addr_bits <span class="token operator">>></span> <span class="token number">5</span><span class="token punctuation">)</span> <span class="token operator">^</span> GVars<span class="token punctuation">.</span>stw_random<span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
value <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// 用于敏感性测试</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">3</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
value <span class="token operator">=</span> <span class="token operator">++</span>GVars<span class="token punctuation">.</span>hc_sequence<span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
value <span class="token operator">=</span> <span class="token generic-function"><span class="token function">cast_from_oop</span><span class="token generic class-name"><span class="token operator"><</span>intptr_t<span class="token operator">></span></span></span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
<span class="token comment">// Marsaglia 的异或移位方案，具有线程特定的状态</span>
<span class="token comment">// 这可能是最好的整体实现 -- 我们可能会在未来的版本中将其设为默认实现。</span>
<span class="token keyword">unsigned</span> t <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateX<span class="token punctuation">;</span>
t <span class="token operator">^=</span> <span class="token punctuation">(</span>t <span class="token operator"><<</span> <span class="token number">11</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
current<span class="token operator">-></span>_hashStateX <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateY<span class="token punctuation">;</span>
current<span class="token operator">-></span>_hashStateY <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateZ<span class="token punctuation">;</span>
current<span class="token operator">-></span>_hashStateZ <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateW<span class="token punctuation">;</span>
<span class="token keyword">unsigned</span> v <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateW<span class="token punctuation">;</span>
v <span class="token operator">=</span> <span class="token punctuation">(</span>v <span class="token operator">^</span> <span class="token punctuation">(</span>v <span class="token operator">>></span> <span class="token number">19</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>t <span class="token operator">^</span> <span class="token punctuation">(</span>t <span class="token operator">>></span> <span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
current<span class="token operator">-></span>_hashStateW <span class="token operator">=</span> v<span class="token punctuation">;</span>
value <span class="token operator">=</span> v<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
value <span class="token operator">&=</span> markWord<span class="token double-colon punctuation">::</span>hash_mask<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>value <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> value <span class="token operator">=</span> <span class="token number">0xBAD</span><span class="token punctuation">;</span>
<span class="token function">assert</span><span class="token punctuation">(</span>value <span class="token operator">!=</span> markWord<span class="token double-colon punctuation">::</span>no_hash<span class="token punctuation">,</span> <span class="token string">"invariant"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> value<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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="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 C++ 基础的话，不用细致去看每一行代码，我们只通过表面去了解一下 <code>get_next_hash()</code> 这个方法就行。其中的 <code>hashCode</code> 变量是 JVM 启动时的一个全局参数，可以通过它来切换哈希值的生成策略。</p><ul><li><code>hashCode==0</code>，调用操作系统 OS 的 <code>random()</code> 方法返回随机数。</li><li><code>hashCode == 1</code>，在 STW（stop-the-world）操作中，这种策略通常用于同步方案中。利用对象地址进行计算，使用不经常更新的随机数（<code>GVars.stw_random</code>）参与其中。</li><li><code>hashCode == 2</code>，使用返回 1，用于某些情况下的测试。</li><li><code>hashCode == 3</code>，从 0 开始计算哈希值，不是线程安全的，多个线程可能会得到相同的哈希值。</li><li><code>hashCode == 4</code>，与创建对象的内存位置有关，原样输出。</li><li><code>hashCode == 5</code>，默认值，支持多线程，使用了 Marsaglia 的 xor-shift 算法产生伪随机数。所谓的 xor-shift 算法，简单来说，看起来就是一个移位寄存器，每次移入的位由寄存器中若干位取异或生成。所谓的伪随机数，不是完全随机的，但是真随机生成比较困难，所以只要能通过一定的随机数统计检测，就可以当作真随机数来使用。</li></ul><p>这里简单总结下。</p><p>在 Java 中，<code>hashCode()</code>方法是定义在 <code>java.lang.Object</code> 类中的一个方法，该类是所有 Java 所有类的父类。因此，每个 Java 对象都可以调用 <code>hashCode()</code>方法。<code>hashCode()</code>方法主要用于支持哈希表（如 java.util.HashMap），这些数据结构使用哈希算法能实现快速查找、插入和删除操作。</p><p><code>hashCode()</code>方法的主要目的是返回一个整数，这个整数称为哈希码，它代表了对象在内存中的一种近似表示。哈希码用于将对象映射到哈希表中的一个特定的位置。两个相等的对象（根据 <a href="https://javabetter.cn/string/equals.html" target="_blank" rel="noopener noreferrer"><code>equals()</code>方法比较<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 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