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class="title">2.5 集合框架（容器）</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><a href="/collection/gailan.html" class="nav-link sidebar-link sidebar-page" aria-label="概述">概述</a><ul class="sidebar-sub-headers"></ul></li><li><a aria-current="page" href="/collection/arraylist.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="ArrayList">ArrayList</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/collection/linkedlist.html" class="nav-link sidebar-link sidebar-page" aria-label="LinkedList">LinkedList</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/collection/list-war-2.html" class="nav-link sidebar-link sidebar-page" aria-label="ArrayList和LinkedList的区别">ArrayList和LinkedList的区别</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/collection/iterator-iterable.html" class="nav-link sidebar-link sidebar-page" aria-label="Iterator和Iterable">Iterator和Iterable</a><ul 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Java新特性</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.11 并发编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.12 JVM</span><span class="arrow right"></span></button></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习资源</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">十、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>Java集合ArrayList详解</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon 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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 clickable" role="navigation">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 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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 11 分钟</span><meta property="timeRequired" content="PT11M"></span></div><hr></div><div class="theme-hope-content"><h1 id="java集合arraylist详解" tabindex="-1"><a class="header-anchor" href="#java集合arraylist详解" aria-hidden="true">#</a> Java集合ArrayList详解</h1><p>“二哥，听说今天我们开讲 ArrayList 了？好期待哦！”三妹明知故问，这个托配合得依然天衣无缝。</p><p>“是的呀，三妹。”我肯定地点了点头，继续说道，“ArrayList 可以称得上是集合框架方面最常用的类了，可以和 HashMap 一较高下。”</p><p>从名字就可以看得出来，ArrayList 实现了 List 接口，并且是基于数组实现的。</p><p>数组的大小是固定的，一旦创建的时候指定了大小，就不能再调整了。也就是说，如果数组满了，就不能再添加任何元素了。ArrayList 在数组的基础上实现了自动扩容，并且提供了比数组更丰富的预定义方法（各种增删改查），非常灵活。</p><p>Java 这门编程语言和 C语言的不同之处就在这里，如果是 C语言的话，就必须动手实现自己的 ArrayList，原生的库函数里面是没有的。</p><p>“二哥，<strong>如何创建一个 ArrayList 啊</strong>？”三妹问。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> alist <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></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>可以通过上面的语句来创建一个字符串类型的 ArrayList（通过尖括号来限定 ArrayList 中元素的类型，如果尝试添加其他类型的元素，将会产生编译错误），更简化的写法如下：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> alist <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</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>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>由于 ArrayList 实现了 List 接口，所以 alist 变量的类型可以是 List 类型；new 关键字声明后的尖括号中可以不再指定元素的类型，因为编译器可以通过前面尖括号中的类型进行智能推断。</p><p>如果非常确定 ArrayList 中元素的个数，在创建的时候还可以指定初始大小。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> alist <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token number">20</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>这样做的好处是，可以有效地避免在添加新的元素时进行不必要的扩容。但通常情况下，我们很难确定 ArrayList 中元素的个数，因此一般不指定初始大小。</p><p>“二哥，<strong>那怎么向 ArrayList 中添加一个元素呢</strong>？”三妹继续问。</p><p>可以通过 <code>add()</code> 方法向 ArrayList 中添加一个元素，如果不指定下标的话，就默认添加在末尾。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>alist<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>“三妹，你可以研究一下 <code>add()</code> 方法的源码（基于 JDK 8 会好一点），它在添加元素的时候会判断需不需要进行扩容，如果需要的话，会执行 <code>grow()</code> 方法进行扩容，这个也是面试官特别喜欢考察的一个重点。”我叮嘱道。</p><p>下面是 <code>add(E e)</code> 方法的源码：</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">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>size <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">// Increments modCount!!</span>

elementData<span class="token punctuation">[</span>size<span class="token operator">++</span><span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>

<span class="token keyword">return</span> <span class="token boolean">true</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>调用了私有的 <code>ensureCapacityInternal</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>elementData <span class="token operator">==</span> DEFAULTCAPACITY_EMPTY_ELEMENTDATA<span class="token punctuation">)</span> <span class="token punctuation">{</span>

minCapacity <span class="token operator">=</span> <span class="token class-name">Math</span><span class="token punctuation">.</span><span class="token function">max</span><span class="token punctuation">(</span>DEFAULT_CAPACITY<span class="token punctuation">,</span> minCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span>minCapacity<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>假如一开始创建 ArrayList 的时候没有指定大小，elementData 就会被初始化成一个空的数组，也就是 DEFAULTCAPACITY_EMPTY_ELEMENTDATA。</p><p>进入到 if 分支后，minCapacity 的值就会等于 DEFAULT_CAPACITY，可以看一下 DEFAULT_CAPACITY 的初始值：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> DEFAULT_CAPACITY <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>也就是说，如果 ArrayList 在创建的时候没有指定大小，默认可以容纳 10 个元素。</p><p>接下来会进入 <code>ensureExplicitCapacity</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>

modCount<span class="token operator">++</span><span class="token punctuation">;</span>

<span class="token comment">// overflow-conscious code</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>minCapacity <span class="token operator">-</span> elementData<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 function">grow</span><span class="token punctuation">(</span>minCapacity<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>接着进入 <code>grow(int minCapacity)</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">grow</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment">// overflow-conscious code</span>

<span class="token keyword">int</span> oldCapacity <span class="token operator">=</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">;</span>

<span class="token keyword">int</span> newCapacity <span class="token operator">=</span> oldCapacity <span class="token operator">+</span> <span class="token punctuation">(</span>oldCapacity <span class="token operator">>></span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> minCapacity <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span>

newCapacity <span class="token operator">=</span> minCapacity<span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> MAX_ARRAY_SIZE <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>

newCapacity <span class="token operator">=</span> <span class="token function">hugeCapacity</span><span class="token punctuation">(</span>minCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// minCapacity is usually close to size, so this is a win:</span>

elementData <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> newCapacity<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>然后对数组进行第一次扩容 <code>Arrays.copyOf(elementData, newCapacity)</code>，由原来的 DEFAULTCAPACITY_EMPTY_ELEMENTDATA 扩容为容量为 10 的数组。</p><p>“那假如向 ArrayList 添加第 11 个元素呢？”三妹看到了问题的关键。</p><p>此时，minCapacity 等于 11，elementData.length 为 10，<code>ensureExplicitCapacity()</code> 方法中 if 条件分支就起效了：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureExplicitCapacity</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>

modCount<span class="token operator">++</span><span class="token punctuation">;</span>

<span class="token comment">// overflow-conscious code</span>

<span class="token function">grow</span><span class="token punctuation">(</span>minCapacity<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>会再次进入到 <code>grow()</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">grow</span><span class="token punctuation">(</span><span class="token keyword">int</span> minCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment">// overflow-conscious code</span>

<span class="token keyword">int</span> oldCapacity <span class="token operator">=</span> elementData<span class="token punctuation">.</span>length<span class="token punctuation">;</span>

newCapacity <span class="token operator">=</span> minCapacity<span class="token punctuation">;</span>

<span class="token comment">// minCapacity is usually close to size, so this is a win:</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>“oldCapacity 等于 10，<code>oldCapacity >> 1</code> 这个表达式等于多少呢？三妹你知道吗？”我问三妹。</p><p>“不知道啊，<code>>></code> 是什么意思呢？”三妹很疑惑。</p><p>“<code>>></code> 是右移运算符，<code>oldCapacity >> 1</code> 相当于 oldCapacity 除以 2。”我给三妹解释道，“在计算机内部，都是按照二进制存储的，10 的二进制就是 1010，也就是 <code>0*2^0 + 1*2^1 + 0*2^2 + 1*2^3</code>=0+2+0+8=10 。。。。。。”</p><p>还没等我解释完，三妹就打断了我，“二哥，能再详细解释一下到底为什么吗？”</p><p>“当然可以啊。”我拍着胸脯对三妹说。</p><p>先从位全的含义说起吧。</p><p>平常我们使用的是十进制数，比如说 39，并不是简单的 3 和 9，3 表示的是 <code>3*10 = 30</code>，9 表示的是 <code>9*1 = 9</code>，和 3 相乘的 10，和 9 相乘的 1，就是<strong>位权</strong>。位数不同，位权就不同，第 1 位是 10 的 0 次方（也就是 <code>10^0=1</code>），第 2 位是 10 的 1 次方（<code>10^1=10</code>），第 3 位是 10 的 2 次方（<code>10^2=100</code>），最右边的是第一位，依次类推。</p><p>位权这个概念同样适用于二进制，第 1 位是 2 的 0 次方（也就是 <code>2^0=1</code>），第 2 位是 2 的 1 次方（<code>2^1=2</code>），第 3 位是 2 的 2 次方（<code>2^2=4</code>），第 34 位是 2 的 3 次方（<code>2^3=8</code>）。</p><p>十进制的情况下，10 是基数，二进制的情况下，2 是基数。</p><p>10 在十进制的表示法是 <code>0*10^0+1*10^1</code>=0+10=10。</p><p>10 的二进制数是 1010，也就是 <code>0*2^0 + 1*2^1 + 0*2^2 + 1*2^3</code>=0+2+0+8=10。</p><p>然后是<strong>移位运算</strong>，移位分为左移和右移，在 Java 中，左移的运算符是 <code><<</code>，右移的运算符 <code>>></code>。</p><p>拿 <code>oldCapacity >> 1</code> 来说吧，<code>>></code> 左边的是被移位的值，此时是 10，也就是二进制 <code>1010</code>；<code>>></code> 右边的是要移位的位数，此时是 1。</p><p>1010 向右移一位就是 101，空出来的最高位此时要补 0，也就是 0101。</p><p>“那为什么不补 1 呢？”三妹这个问题很尖锐。</p><p>“因为是算术右移，并且是正数，所以最高位补 0；如果表示的是负数，就需要补 1。”我慢吞吞地回答道，“0101 的十进制就刚好是 <code>1*2^0 + 0*2^1 + 1*2^2 + 0*2^3</code>=1+0+4+0=5，如果多移几个数来找规律的话，就会发现，右移 1 位是原来的 1/2，右移 2 位是原来的 1/4，诸如此类。”</p><p>也就是说，ArrayList 的大小会扩容为原来的大小+原来大小/2，也就是差不多 1.5 倍。</p><p>除了 <code>add(E e)</code> 方法，还可以通过 <code>add(int index, E element)</code> 方法把元素添加到指定的位置：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>alist<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token string">"沉默王三"</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><code>add(int index, E element)</code> 方法的源码如下：</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">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">E</span> element<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">rangeCheckForAdd</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">,</span>

size <span class="token operator">-</span> index<span class="token punctuation">)</span><span class="token punctuation">;</span>

elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> element<span class="token punctuation">;</span>

size<span class="token operator">++</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>该方法会调用到一个非常重要的本地方法 <code>System.arraycopy()</code>，它会对数组进行复制（要插入位置上的元素往后复制）。</p><p>“三妹，注意看，我画幅图来表示下。”我认真地做起了图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/collection/arraylist-01.png" alt=""></p><p>“二哥，那怎么<strong>更新 ArrayList 中的元素</strong>呢？”三妹继续问。</p><p>可以使用 <code>set()</code> 方法来更改 ArrayList 中的元素，需要提供下标和新元素。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>alist<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token string">"沉默王四"</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>假设原来 0 位置上的元素为“沉默王三”，现在可以将其更新为“沉默王四”。</p><p>来看一下 <code>set()</code> 方法的源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">E</span> element<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">E</span> oldValue <span class="token operator">=</span> <span class="token function">elementData</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">=</span> element<span class="token punctuation">;</span>

<span class="token keyword">return</span> oldValue<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>该方法会先对指定的下标进行检查，看是否越界，然后替换新值并返回旧值。</p><p>“二哥，那怎么<strong>删除 ArrayList 中的元素</strong>呢？”三妹继续问。</p><p><code>remove(int index)</code> 方法用于删除指定下标位置上的元素，<code>remove(Object o)</code> 方法用于删除指定值的元素。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>alist<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

alist<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token string">"沉默王四"</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>先来看 <code>remove(int index)</code> 方法的源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

modCount<span class="token operator">++</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> numMoved <span class="token operator">=</span> size <span class="token operator">-</span> index <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>numMoved <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>

<span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span>

numMoved<span class="token punctuation">)</span><span class="token punctuation">;</span>

elementData<span class="token punctuation">[</span><span class="token operator">--</span>size<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> <span class="token comment">// clear to let GC do its work</span>

<span class="token keyword">return</span> oldValue<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><p>该方法会调用 <code> System.arraycopy()</code> 对数组进行复制移动，然后把要删除的元素位置清空 <code>elementData[--size] = null</code>。</p><p>再来看 <code>remove(Object o)</code> 方法的源码：</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">boolean</span> <span class="token function">remove</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 keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> index <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> index <span class="token operator"><</span> size<span class="token punctuation">;</span> index<span class="token operator">++</span><span class="token punctuation">)</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">fastRemove</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

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

<span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>index<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">fastRemove</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

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

<span class="token keyword">return</span> <span class="token boolean">false</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></div><p>该方法通过遍历的方式找到要删除的元素，null 的时候使用 == 操作符判断，非 null 的时候使用 <code>equals()</code> 方法，然后调用 <code>fastRemove()</code> 方法；有相同元素时，只会删除第一个。</p><p>既然都调用了 <code>fastRemove()</code> 方法，那就继续来跟踪一下源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">fastRemove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>

modCount<span class="token operator">++</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">arraycopy</span><span class="token punctuation">(</span>elementData<span class="token punctuation">,</span> index<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">,</span> elementData<span class="token punctuation">,</span> index<span class="token punctuation">,</span>

numMoved<span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>同样是调用 <code>System.arraycopy()</code> 方法对数组进行复制和移动。</p><p>“三妹，注意看，我画幅图来表示下。”我认真地做起了图。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/collection/arraylist-02.png" alt=""></p><p>“二哥，那怎么<strong>查找 ArrayList 中的元素</strong>呢？”三妹继续问。</p><p>如果要正序查找一个元素，可以使用 <code>indexOf()</code> 方法；如果要倒序查找一个元素，可以使用 <code>lastIndexOf()</code> 方法。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>alist<span class="token punctuation">.</span><span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token string">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

alist<span class="token punctuation">.</span><span class="token function">lastIndexOf</span><span class="token punctuation">(</span><span class="token string">"沉默王二"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div></div></div><p>来看一下 <code>indexOf()</code> 方法的源码：</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">int</span> <span class="token function">indexOf</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 keyword">if</span> <span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">==</span><span class="token keyword">null</span><span class="token punctuation">)</span>

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

<span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>

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

<span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</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>如果元素为 null 的时候使用“==”操作符，否则使用 <code>equals()</code> 方法。</p><p><code>lastIndexOf()</code> 方法和 <code>indexOf()</code> 方法类似，不过遍历的时候从最后开始。</p><p><code>contains()</code> 方法可以判断 ArrayList 中是否包含某个元素，其内部调用了 <code>indexOf()</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">boolean</span> <span class="token function">contains</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 keyword">return</span> <span class="token function">indexOf</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span> <span class="token operator">>=</span> <span class="token number">0</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>如果 ArrayList 中的元素是经过排序的，就可以使用二分查找法，效率更快。</p><p><code>Collections</code> 类的 <code>sort()</code> 方法可以对 ArrayList 进行排序，该方法会按照字母顺序对 String 类型的列表进行排序。如果是自定义类型的列表，还可以指定 Comparator 进行排序。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> copy <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span>alist<span class="token punctuation">)</span><span class="token punctuation">;</span>

copy<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"a"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

copy<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"c"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

copy<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"b"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

copy<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"d"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span>copy<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>copy<span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>输出结果如下所示：</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>[a, b, c, d]

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>排序后就可以使用二分查找法了：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">int</span> index <span class="token operator">=</span> <span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">binarySearch</span><span class="token punctuation">(</span>copy<span class="token punctuation">,</span> <span class="token string">"b"</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>“最后，三妹，我来简单总结一下 ArrayList 的时间复杂度吧，方便后面学习 LinkedList 时对比。”我喝了一口水后补充道。</p><p>1）通过下标（也就是 <code>get(int index)</code>）访问一个元素的时间复杂度为 O(1)，因为是直达的，无论数据增大多少倍，耗时都不变。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token function">rangeCheck</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> <span class="token function">elementData</span><span class="token punctuation">(</span>index<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>2）默认添加一个元素（调用 <code>add()</code> 方法时）的时间复杂度为 O(1)，因为是直接添加到数组末尾的，但需要考虑到数组扩容时消耗的时间。</p><p>3）删除一个元素（调用 <code>remove(Object)</code> 方法时）的时间复杂度为 O(n)，因为要遍历列表，数据量增大几倍，耗时也增大几倍；如果是通过下标删除元素时，要考虑到数组的移动和复制所消耗的时间。</p><p>4）查找一个未排序的列表时间复杂度为 O(n)（调用 <code>indexOf()</code> 或者 <code>lastIndexOf()</code> 方法时），因为要遍历列表；查找排序过的列表时间复杂度为 O(log n)，因为可以使用二分查找法，当数据增大 n 倍时，耗时增大 logn 倍（这里的 log 是以 2 为底的，每找一次排除一半的可能）。</p><hr><p>ArrayList，如果有个中文名的话，应该叫动态数组，也就是可增长的数组，可调整大小的数组。动态数组克服了静态数组的限制，静态数组的容量是固定的，只能在首次创建的时候指定。而动态数组会随着元素的增加自动调整大小，更符合实际的开发需求。</p><p>学习集合框架，ArrayList 是第一课，也是新手进阶的重要一课。要想完全掌握 ArrayList，扩容这个机制是必须得掌握，也是面试中经常考察的一个点。</p><p>要想掌握扩容机制，就必须得读源码，也就肯定会遇到 <code>oldCapacity >> 1</code>，有些初学者会选择跳过，虽然不影响整体上的学习，但也错过了一个精进的机会。</p><p>计算机内部是如何表示十进制数的，右移时又发生了什么，静下心来去研究一下，你就会发现，哦，原来这么有趣呢？</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/collection/arraylist.md" rel="noopener noreferrer" target="_blank" aria-label="在 GitHub 上编辑此页" class="nav-link label"><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 190.883-192.61h267.754a60.46 60.46 0 0 1 0 120.92H252.891a69.962 69.962 0 0 0-69.098 69.099V771.98a69.962 69.962 0 0 0 69.098 69.098h475.911A69.962 69.962 0 0 0 797.9 771.98V503.363a60.46 60.46 0 1 1 120.922 0V771.98A190.883 190.883 0 0 1 728.802 962z"></path></svg>在 GitHub 上编辑此页<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a></div><div class="meta-item update-time"><span class="label">上次编辑于: </span><span class="info">2022/5/14 上午7:14:28</span></div><div class="meta-item contributors"><span class="label">贡献者: </span><span class="contributor" title="email: [email protected]">itwanger</span>,<span class="contributor" title="email: [email protected]">沉默王二</span></div></footer><nav class="page-nav"><a href="/collection/gailan.html" class="nav-link prev" aria-label="概述"><div class="hint"><span class="arrow left"></span>上一页</div><div class="link">概述</div></a><a href="/collection/linkedlist.html" class="nav-link next" aria-label="LinkedList"><div class="hint">下一页<span class="arrow right"></span></div><div class="link">LinkedList</div></a></nav><div class="giscus-wrapper input-top" style="display:block;">Loading...</div></main><footer class="footer-wrapper"><div class="footer"><a href="https://beian.miit.gov.cn/" target="_blank">豫ICP备2021038026号-1</a><img src="https://cdn.tobebetterjavaer.com/tobebetterjavaer/images/beian.png" height="15px" width="15px" /><a target="_blank" href="http://www.beian.gov.cn/portal/registerSystemInfo?recordcode=41030502000411"><span>豫公网安备 41030502000411号</span></a></div><div class="copyright">Copyright © 2022 沉默王二</div></footer></div></div>

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