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1.5M3.205 9.303c-.09.448-.277 1.21-1.241 1.203C1 10.5.5 9.513.5 8V7c0-1.57.5-2.5 1.464-2.494.964.006 1.134.598 1.24 1.342M12.553 10.5h1.953" stroke-width="1.2" stroke="currentColor" fill="none" stroke-linecap="square"></path></svg></kbd><kbd class="DocSearch-Button-Key">K</kbd></span></button></div><!--]--><!--]--><!--[--><!----><!--]--><button type="button" class="vp-toggle-navbar-button" aria-label="Toggle Navbar" aria-expanded="false" aria-controls="nav-screen"><span><span class="vp-top"></span><span class="vp-middle"></span><span class="vp-bottom"></span></span></button></div></header><!----><!--]--><!----><div class="toggle-sidebar-wrapper"><span class="arrow start"></span></div><aside id="sidebar" class="vp-sidebar"><!--[--><!----><!--]--><ul class="vp-sidebar-links"><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/home.html"><!---->一、前言<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><section class="vp-sidebar-group"><button class="vp-sidebar-heading clickable" type="button"><!----><span class="vp-sidebar-title">二、Java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">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 active" type="button"><!----><span class="vp-sidebar-title">七、求职面试</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 active" type="button"><!----><span class="vp-sidebar-title">面试题&八股文</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="/interview/java-34.html"><!---->34 道 Java 精选面试题👍<!----></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="/interview/java-hashmap-13.html"><!---->13道HashMap精选面试题👍<!----></a><ul class="vp-sidebar-sub-headers"><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_01、hashmap的底层数据结构是什么"><!---->01、HashMap的底层数据结构是什么？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_02、为什么链表改为红黑树的阈值是-8"><!---->02、为什么链表改为红黑树的阈值是 8?<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_03、解决hash冲突的办法有哪些-hashmap用的哪种"><!---->03、解决hash冲突的办法有哪些？HashMap用的哪种？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_04、为什么在解决-hash-冲突的时候-不直接用红黑树-而选择先用链表-再转红黑树"><!---->04、为什么在解决 hash 冲突的时候，不直接用红黑树？而选择先用链表，再转红黑树?<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_05、hashmap默认加载因子是多少-为什么是-0-75-不是-0-6-或者-0-8"><!---->05、HashMap默认加载因子是多少？为什么是 0.75，不是 0.6 或者 0.8 ？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_06、hashmap-中-key-的存储索引是怎么计算的"><!---->06、HashMap 中 key 的存储索引是怎么计算的？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_07、jdk-8-为什么要-hashcode-异或其右移十六位的值"><!---->07、JDK 8 为什么要 hashcode 异或其右移十六位的值？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_08、为什么-hash-值要与length-1相与"><!---->08、为什么 hash 值要与length-1相与？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_09、hashmap数组的长度为什么是-2-的幂次方"><!---->09、HashMap数组的长度为什么是 2 的幂次方？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_10、hashmap-的put方法流程"><!---->10、HashMap 的put方法流程？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_11、hashmap-的扩容方式"><!---->11、HashMap 的扩容方式？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_12、一般用什么作为hashmap的key"><!---->12、一般用什么作为HashMap的key?<!----></a><ul class="vp-sidebar-sub-headers"></ul></li><li class="vp-sidebar-sub-header"><a class="vp-link nav-link vp-sidebar-link vp-heading" href="/interview/java-hashmap-13.html#_13、hashmap为什么线程不安全"><!---->13、HashMap为什么线程不安全？<!----></a><ul class="vp-sidebar-sub-headers"></ul></li></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/interview/mysql-60.html"><!---->60 道 MySQL 精选面试题👍<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/interview/mysql-suoyin-15.html"><!---->MySQL索引15连问，抗住！<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/interview/redis-12.html"><!---->12 道 Redis 精选面试题👍<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link vp-sidebar-page" href="/interview/nginx-40.html"><!---->40 道 Nginx 精选面试题👍<!----></a><ul class="vp-sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a class="vp-link nav-link vp-sidebar-link 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class="toc-header">此页内容<button type="button" class="print-button" title="打印"><svg xmlns="http://www.w3.org/2000/svg" class="icon print-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="print icon"><path d="M819.2 364.8h-44.8V128c0-17.067-14.933-32-32-32H281.6c-17.067 0-32 14.933-32 32v236.8h-44.8C145.067 364.8 96 413.867 96 473.6v192c0 59.733 49.067 108.8 108.8 108.8h44.8V896c0 17.067 14.933 32 32 32h460.8c17.067 0 32-14.933 32-32V774.4h44.8c59.733 0 108.8-49.067 108.8-108.8v-192c0-59.733-49.067-108.8-108.8-108.8zM313.6 160h396.8v204.8H313.6V160zm396.8 704H313.6V620.8h396.8V864zM864 665.6c0 25.6-19.2 44.8-44.8 44.8h-44.8V588.8c0-17.067-14.933-32-32-32H281.6c-17.067 0-32 14.933-32 32v121.6h-44.8c-25.6 0-44.8-19.2-44.8-44.8v-192c0-25.6 19.2-44.8 44.8-44.8h614.4c25.6 0 44.8 19.2 44.8 44.8v192z"></path></svg></button></div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_01、hashmap的底层数据结构是什么">01、HashMap的底层数据结构是什么？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_02、为什么链表改为红黑树的阈值是-8">02、为什么链表改为红黑树的阈值是 8?</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_03、解决hash冲突的办法有哪些-hashmap用的哪种">03、解决hash冲突的办法有哪些？HashMap用的哪种？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_04、为什么在解决-hash-冲突的时候-不直接用红黑树-而选择先用链表-再转红黑树">04、为什么在解决 hash 冲突的时候，不直接用红黑树？而选择先用链表，再转红黑树?</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_05、hashmap默认加载因子是多少-为什么是-0-75-不是-0-6-或者-0-8">05、HashMap默认加载因子是多少？为什么是 0.75，不是 0.6 或者 0.8 ？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_06、hashmap-中-key-的存储索引是怎么计算的">06、HashMap 中 key 的存储索引是怎么计算的？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_07、jdk-8-为什么要-hashcode-异或其右移十六位的值">07、JDK 8 为什么要 hashcode 异或其右移十六位的值？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_08、为什么-hash-值要与length-1相与">08、为什么 hash 值要与length-1相与？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_09、hashmap数组的长度为什么是-2-的幂次方">09、HashMap数组的长度为什么是 2 的幂次方？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_10、hashmap-的put方法流程">10、HashMap 的put方法流程？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_11、hashmap-的扩容方式">11、HashMap 的扩容方式？</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_12、一般用什么作为hashmap的key">12、一般用什么作为HashMap的key?</a></li><!----><!--]--><!--[--><li class="toc-item"><a class="vp-link toc-link level3" href="/#_13、hashmap为什么线程不安全">13、HashMap为什么线程不安全？</a></li><!----><!--]--></ul><div class="toc-marker" style="top:-1.7rem;"></div></div><!--[--><!----><!--]--></aside></div><!----><div class="theme-hope-content"><h1 id="java-hashmap精选面试题" tabindex="-1"><a class="header-anchor" href="#java-hashmap精选面试题" aria-hidden="true">#</a> Java HashMap精选面试题</h1><p>对于 Java 求职者来说，HashMap 可谓是重中之重，是面试的必考点。然而 HashMap 的知识点非常多，复习起来花费精力很大。</p><h3 id="_01、hashmap的底层数据结构是什么" tabindex="-1"><a class="header-anchor" href="#_01、hashmap的底层数据结构是什么" aria-hidden="true">#</a> 01、HashMap的底层数据结构是什么？</h3><p>JDK 7 中，HashMap 由“数组+链表”组成，数组是 HashMap 的主体，链表则是主要为了解决哈希冲突而存在的。</p><p>在 JDK 8 中，HashMap 由“数组+链表+红黑树”组成。链表过长，会严重影响 HashMap 的性能，而红黑树搜索的时间复杂度是 O(logn)，而链表是糟糕的 O(n)。因此，JDK 8 对数据结构做了进一步的优化，引入了红黑树，链表和红黑树在达到一定条件会进行转换：</p><ul><li>当链表超过 8 且数据总量超过 64 时会转红黑树。</li><li>将链表转换成红黑树前会判断，如果当前数组的长度小于 64，那么会选择先进行数组扩容，而不是转换为红黑树，以减少搜索时间。</li></ul><p>链表长度超过 8 体现在 putVal 方法中的这段代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token comment">//链表长度大于8转换为红黑树进行处理</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">>=</span> <span class="token constant">TREEIFY_THRESHOLD</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -1 for 1st</span>
<span class="token function">treeifyBin</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> hash<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>table 长度为 64 体现在 treeifyBin 方法中的这段代码：：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">treeifyBin</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> <span class="token keyword">int</span> hash<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> n<span class="token punctuation">,</span> index<span class="token punctuation">;</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> e<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>tab <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator"><</span> <span class="token constant">MIN_TREEIFY_CAPACITY</span><span class="token punctuation">)</span>
<span class="token function">resize</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>MIN_TREEIFY_CAPACITY 的值正好为 64。</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 constant">MIN_TREEIFY_CAPACITY</span> <span class="token operator">=</span> <span class="token number">64</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>JDK 8 中 HashMap 的结构示意图：</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/collection/hashmap-interview-01.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_02、为什么链表改为红黑树的阈值是-8" tabindex="-1"><a class="header-anchor" href="#_02、为什么链表改为红黑树的阈值是-8" aria-hidden="true">#</a> 02、为什么链表改为红黑树的阈值是 8?</h3><p>因为泊松分布，我们来看作者在源码中的注释：</p><blockquote><p>Because TreeNodes are about twice the size of regular nodes, we<br> use them only when bins contain enough nodes to warrant use<br> (see TREEIFY_THRESHOLD). And when they become too small (due to<br> removal or resizing) they are converted back to plain bins. In<br> usages with well-distributed user hashCodes, tree bins are<br> rarely used. Ideally, under random hashCodes, the frequency of<br> nodes in bins follows a Poisson distribution<br> (<a href="http://en.wikipedia.org/wiki/Poisson_distribution" target="_blank" rel="noopener noreferrer">http://en.wikipedia.org/wiki/Poisson_distribution<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>) with a<br> parameter of about 0.5 on average for the default resizing<br> threshold of 0.75, although with a large variance because of<br> resizing granularity. Ignoring variance, the expected<br> occurrences of list size k are (exp(-0.5) pow(0.5, k) /<br> factorial(k)). The first values are:<br> 0: 0.60653066<br><br> 1: 0.30326533<br><br> 2: 0.07581633<br><br> 3: 0.01263606<br><br> 4: 0.00157952<br><br> 5: 0.00015795<br><br> 6: 0.00001316<br><br> 7: 0.00000094<br><br> 8: 0.00000006<br><br> more: less than 1 in ten million</p></blockquote><p>翻译过来大概的意思是：理想情况下使用随机的哈希码，容器中节点分布在 hash 桶中的频率遵循泊松分布，按照泊松分布的计算公式计算出了桶中元素个数和概率的对照表，可以看到链表中元素个数为 8 时的概率已经非常小，再多的就更少了，所以原作者在选择链表元素个数时选择了 8，是根据概率统计而选择的。</p><h3 id="_03、解决hash冲突的办法有哪些-hashmap用的哪种" tabindex="-1"><a class="header-anchor" href="#_03、解决hash冲突的办法有哪些-hashmap用的哪种" aria-hidden="true">#</a> 03、解决hash冲突的办法有哪些？HashMap用的哪种？</h3><p>解决Hash冲突方法有：</p><ul><li>开放定址法：也称为再散列法，基本思想就是，如果p=H(key)出现冲突时，则以p为基础，再次hash，p1=H(p),如果p1再次出现冲突，则以p1为基础，以此类推，直到找到一个不冲突的哈希地址pi。因此开放定址法所需要的hash表的长度要大于等于所需要存放的元素，而且因为存在再次hash，所以只能在删除的节点上做标记，而不能真正删除节点。</li><li>再哈希法：双重散列，多重散列，提供多个不同的hash函数，当R1=H1(key1)发生冲突时，再计算R2=H2(key1)，直到没有冲突为止。这样做虽然不易产生堆集，但增加了计算的时间。</li><li>链地址法：拉链法，将哈希值相同的元素构成一个同义词的单链表，并将单链表的头指针存放在哈希表的第i个单元中，查找、插入和删除主要在同义词链表中进行。链表法适用于经常进行插入和删除的情况。</li><li>建立公共溢出区：将哈希表分为公共表和溢出表，当溢出发生时，将所有溢出数据统一放到溢出区。</li></ul><p>HashMap中采用的是链地址法 。</p><h3 id="_04、为什么在解决-hash-冲突的时候-不直接用红黑树-而选择先用链表-再转红黑树" tabindex="-1"><a class="header-anchor" href="#_04、为什么在解决-hash-冲突的时候-不直接用红黑树-而选择先用链表-再转红黑树" aria-hidden="true">#</a> 04、为什么在解决 hash 冲突的时候，不直接用红黑树？而选择先用链表，再转红黑树?</h3><p>因为红黑树需要进行左旋，右旋，变色这些操作来保持平衡，而单链表不需要。</p><p>当元素小于 8 个的时候，此时做查询操作，链表结构已经能保证查询性能。当元素大于 8 个的时候， 红黑树搜索时间复杂度是 O(logn)，而链表是 O(n)，此时需要红黑树来加快查询速度，但是新增节点的效率变慢了。</p><p>因此，如果一开始就用红黑树结构，元素太少，新增效率又比较慢，无疑这是浪费性能的。</p><h3 id="_05、hashmap默认加载因子是多少-为什么是-0-75-不是-0-6-或者-0-8" tabindex="-1"><a class="header-anchor" href="#_05、hashmap默认加载因子是多少-为什么是-0-75-不是-0-6-或者-0-8" aria-hidden="true">#</a> 05、HashMap默认加载因子是多少？为什么是 0.75，不是 0.6 或者 0.8 ？</h3><p>作为一般规则，默认负载因子（0.75）在时间和空间成本上提供了很好的折衷。</p><p><a href="https://mp.weixin.qq.com/s/a3qfatEWizKK1CpYaxVBbA" target="_blank" rel="noopener noreferrer">详情参照这篇<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></p><h3 id="_06、hashmap-中-key-的存储索引是怎么计算的" tabindex="-1"><a class="header-anchor" href="#_06、hashmap-中-key-的存储索引是怎么计算的" aria-hidden="true">#</a> 06、HashMap 中 key 的存储索引是怎么计算的？</h3><p>首先根据key的值计算出hashcode的值，然后根据hashcode计算出hash值，最后通过hash&（length-1）计算得到存储的位置。</p><p><a href="https://mp.weixin.qq.com/s/aS2dg4Dj1Efwujmv-6YTBg" target="_blank" rel="noopener noreferrer">详情参照这篇<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></p><h3 id="_07、jdk-8-为什么要-hashcode-异或其右移十六位的值" tabindex="-1"><a class="header-anchor" href="#_07、jdk-8-为什么要-hashcode-异或其右移十六位的值" aria-hidden="true">#</a> 07、JDK 8 为什么要 hashcode 异或其右移十六位的值？</h3><p>因为在JDK 7 中扰动了 4 次，计算 hash 值的性能会稍差一点点。</p><p>从速度、功效、质量来考虑，JDK 8 优化了高位运算的算法，通过hashCode()的高16位异或低16位实现：<code>(h = k.hashCode()) ^ (h >>> 16)</code>。</p><p>这么做可以在数组 table 的 length 比较小的时候，也能保证考虑到高低Bit都参与到Hash的计算中，同时不会有太大的开销。</p><h3 id="_08、为什么-hash-值要与length-1相与" tabindex="-1"><a class="header-anchor" href="#_08、为什么-hash-值要与length-1相与" aria-hidden="true">#</a> 08、为什么 hash 值要与length-1相与？</h3><ul><li>把 hash 值对数组长度取模运算，模运算的消耗很大，没有位运算快。</li><li>当 length 总是 2 的n次方时，<code>h& (length-1) </code>运算等价于对length取模，也就是 h%length，但是 & 比 % 具有更高的效率。</li></ul><h3 id="_09、hashmap数组的长度为什么是-2-的幂次方" tabindex="-1"><a class="header-anchor" href="#_09、hashmap数组的长度为什么是-2-的幂次方" aria-hidden="true">#</a> 09、HashMap数组的长度为什么是 2 的幂次方？</h3><p>2 的 N 次幂有助于减少碰撞的几率。如果 length 为2的幂次方，则 length-1 转化为二进制必定是11111……的形式，在与h的二进制与操作效率会非常的快，而且空间不浪费。我们来举个例子，看下图：</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/collection/hashmap-interview-02.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><p>当 length =15时，6 和 7 的结果一样，这样表示他们在 table 存储的位置是相同的，也就是产生了碰撞，6、7就会在一个位置形成链表，4和5的结果也是一样，这样就会导致查询速度降低。</p><p>如果我们进一步分析，还会发现空间浪费非常大，以 length=15 为例，在 1、3、5、7、9、11、13、15 这八处没有存放数据。因为hash值在与14（即 1110）进行&运算时，得到的结果最后一位永远都是0，即 0001、0011、0101、0111、1001、1011、1101、1111位置处是不可能存储数据的。</p><p><strong>再补充数组容量计算的小奥秘。</strong></p><p>HashMap 构造函数允许用户传入的容量不是 2 的 n 次方，因为它可以自动地将传入的容量转换为 2 的 n 次方。会取大于或等于这个数的 且最近的2次幂作为 table 数组的初始容量，使用tableSizeFor(int)方法，如 tableSizeFor(10) = 16（2 的 4 次幂），tableSizeFor(20) = 32（2 的 5 次幂），也就是说 table 数组的长度总是 2 的次幂。JDK 8 源码如下：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">tableSizeFor</span><span class="token punctuation">(</span><span class="token keyword">int</span> cap<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> n <span class="token operator">=</span> cap <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
n <span class="token operator">|=</span> n <span class="token operator">>>></span> <span class="token number">1</span><span class="token punctuation">;</span>
n <span class="token operator">|=</span> n <span class="token operator">>>></span> <span class="token number">2</span><span class="token punctuation">;</span>
n <span class="token operator">|=</span> n <span class="token operator">>>></span> <span class="token number">4</span><span class="token punctuation">;</span>
n <span class="token operator">|=</span> n <span class="token operator">>>></span> <span class="token number">8</span><span class="token punctuation">;</span>
n <span class="token operator">|=</span> n <span class="token operator">>>></span> <span class="token number">16</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token punctuation">(</span>n <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">1</span> <span class="token operator">:</span> <span class="token punctuation">(</span>n <span class="token operator">>=</span> <span class="token constant">MAXIMUM_CAPACITY</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token constant">MAXIMUM_CAPACITY</span> <span class="token operator">:</span> n <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>让cap-1再赋值给n的目的是另找到的目标值大于或等于原值。例如二进制1000，十进制数值为8。如果不对它减1而直接操作，将得到答案10000，即16。显然不是结果。减1后二进制为111，再进行操作则会得到原来的数值1000，即8。</p><h3 id="_10、hashmap-的put方法流程" tabindex="-1"><a class="header-anchor" href="#_10、hashmap-的put方法流程" aria-hidden="true">#</a> 10、HashMap 的put方法流程？</h3><p>以JDK 8为例，简要流程如下：</p><p>1、首先根据 key 的值计算 hash 值，找到该元素在数组中存储的下标；</p><p>2、如果数组是空的，则调用 resize 进行初始化；</p><p>3、如果没有哈希冲突直接放在对应的数组下标里；</p><p>4、如果冲突了，且 key 已经存在，就覆盖掉 value；</p><p>5、如果冲突后，发现该节点是红黑树，就将这个节点挂在树上；</p><p>6、如果冲突后是链表，判断该链表是否大于 8 ，如果大于 8 并且数组容量小于 64，就进行扩容；如果链表节点大于 8 并且数组的容量大于 64，则将这个结构转换为红黑树；否则，链表插入键值对，若 key 存在，就覆盖掉 value。</p><figure><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/collection/hashmap-interview-03.png" alt="" tabindex="0" loading="lazy"><figcaption></figcaption></figure><h3 id="_11、hashmap-的扩容方式" tabindex="-1"><a class="header-anchor" href="#_11、hashmap-的扩容方式" aria-hidden="true">#</a> 11、HashMap 的扩容方式？</h3><p>HashMap 在容量超过负载因子所定义的容量之后，就会扩容。</p><p><a href="https://mp.weixin.qq.com/s/0KSpdBJMfXSVH63XadVdmw" target="_blank" rel="noopener noreferrer">详情参照这篇<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></p><h3 id="_12、一般用什么作为hashmap的key" tabindex="-1"><a class="header-anchor" href="#_12、一般用什么作为hashmap的key" aria-hidden="true">#</a> 12、一般用什么作为HashMap的key?</h3><p>一般用Integer、String 这种不可变类当作 HashMap 的 key，String 最为常见。</p><ul><li>因为字符串是不可变的，所以在它创建的时候 hashcode 就被缓存了，不需要重新计算。</li><li>因为获取对象的时候要用到 equals() 和 hashCode() 方法，那么键对象正确的重写这两个方法是非常重要的。Integer、String 这些类已经很规范的重写了 hashCode() 以及 equals() 方法。</li></ul><h3 id="_13、hashmap为什么线程不安全" tabindex="-1"><a class="header-anchor" href="#_13、hashmap为什么线程不安全" aria-hidden="true">#</a> 13、HashMap为什么线程不安全？</h3><ul><li>JDK 7 时多线程下扩容会造成死循环。</li><li>多线程的put可能导致元素的丢失。</li><li>put和get并发时，可能导致get为null。</li></ul><p><a href="https://mp.weixin.qq.com/s/qk_neCdzM3aB6pVWVTHhNw" target="_blank" rel="noopener noreferrer">详情参照这篇<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></p><blockquote><p>参考链接：<a href="https://zhuanlan.zhihu.com/p/362214327" target="_blank" rel="noopener noreferrer">https://zhuanlan.zhihu.com/p/362214327<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></p></blockquote><hr><p>GitHub 上标星 9300+ 的开源知识库《<a href="https://github.com/itwanger/toBeBetterJavaer" target="_blank" rel="noopener noreferrer">二哥的 Java 进阶之路<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" 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>》第一版 PDF 终于来了！包括Java基础语法、数组&字符串、OOP、集合框架、Java IO、异常处理、Java 新特性、网络编程、NIO、并发编程、JVM等等，共计 32 万余字，500+张手绘图，可以说是通俗易懂、风趣幽默……详情戳：<a href="https://javabetter.cn/overview/" target="_blank" rel="noopener noreferrer">太赞了，GitHub 上标星 9300+ 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