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<!DOCTYPE html><html style="display:none" lang="zh"><head><meta charset="utf-8"><script>window.materialVersion="1.5.6",window.oldVersion=["codestartv1","1.3.4","1.4.0","1.4.0b1","1.5.0","1.5.2","1.5.5"]</script><meta http-equiv="x-dns-prefetch-control" content="on"><link rel="dns-prefetch" href="https://cdn1.lncld.net"><link rel="dns-prefetch" href="https://busuanzi.ibruce.info"><link rel="dns-prefetch" href="https://cdn1.lncld.net"><link rel="dns-prefetch" href="https://cdn.jsdelivr.net"><link rel="dns-prefetch" href="https://hm.baidu.com"><link rel="dns-prefetch" href="https://www.google-analytics.com"><link rel="dns-prefetch" href="https://fonts.googleapis.com"><meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1"><meta name="renderer" content="webkit"><meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no"><title>ArrayList | FREELEE'S WORLD</title><link rel="icon shortcut" type="image/ico" href="/img/favicon.png"><link rel="icon" href="/img/favicon.png"><meta name="format-detection" content="telephone=no"><meta name="description" itemprop="description" content="Do not follow crowd,be yourself"><meta name="keywords" content=",array,Java容器,List"><meta name="theme-color" content="#0097A7"><meta http-equiv="Cache-Control" content="no-transform"><meta http-equiv="Cache-Control" content="no-siteapp"><!--[if lte IE 9]>
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$('.post_thumbnail-random').addClass('lazy');</script><p class="article-headline-p">ArrayList</p></div><div class="mdl-color-text--grey-700 mdl-card__supporting-text meta"><div id="author-avatar"><img src="/img/avatar.jpg" width="44px" height="44px" alt="Author Avatar"></div><div><strong>freelee</strong> <span>创建于5月 26, 2018</span> <span>更新于4月 07, 2019</span></div><div class="section-spacer"></div><button id="article-functions-viewtags-button" class="mdl-button mdl-js-button mdl-js-ripple-effect mdl-button--icon"><i class="material-icons" role="presentation">bookmark</i> <span class="visuallyhidden">bookmark</span></button><ul class="mdl-menu mdl-menu--bottom-right mdl-js-menu mdl-js-ripple-effect" for="article-functions-viewtags-button"><li class="mdl-menu__item"><a class="post_tag-link" href="/tags/Java容器/">Java容器</a></li><li class="mdl-menu__item"><a class="post_tag-link" href="/tags/List/">List</a></li><li class="mdl-menu__item"><a class="post_tag-link" href="/tags/array/">array</a></li></ul><button id="article-fuctions-share-button" class="mdl-button mdl-js-button mdl-js-ripple-effect mdl-button--icon"><i class="material-icons" role="presentation">share</i> <span class="visuallyhidden">share</span></button><ul class="mdl-menu mdl-menu--bottom-right mdl-js-menu mdl-js-ripple-effect" for="article-fuctions-share-button"><a class="post_share-link" href="#"><li class="mdl-menu__item"><span id="/ArrayList.html" class="leancloud-views_num" data-flag-title="ArrayList"> 浏览量</span></li></a><a class="post_share-link" href="http://service.weibo.com/share/share.php?appkey=&title=ArrayList&url=https://freelee.me/ArrayList.html&pic=https://freelee.me/img/favicon.png&searchPic=false&style=simple" target="_blank"><li class="mdl-menu__item">分享到微博</li></a><a class="post_share-link" href="http://connect.qq.com/widget/shareqq/index.html?site=FREELEE'S WORLD&title=ArrayList&summary=Do not follow crowd,be yourself&pics=https://freelee.me/img/favicon.png&url=https://freelee.me/ArrayList.html" target="_blank"><li class="mdl-menu__item">分享到 QQ</li></a></ul></div><head><link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.css"><script src="https://cdn.jsdelivr.net/npm/aplayer/dist/APlayer.min.js"></script><script src="https://cdn.jsdelivr.net/npm/meting@2/dist/Meting.min.js"></script><script src="https://unpkg.com/[email protected]"></script><link rel="stylesheet" href="/css/prism-atom-dark.css" type="text/css"></head><div id="post-content" class="mdl-color-text--grey-700 mdl-card__supporting-text fade out"><p>今天写Java集合框架里的其中一种容器——ArrarList。听上去跟数组差不多，其实，它们很多理念上是真差不多。先放一张集合框架图。<br><img src="https://f002.backblazeb2.com/file/freelee/Java/Collections/ArrayList/20180402190247303.png" alt="这里写图片描述"><br>乍一看，感觉贼复杂，头要炸。其实只要小小分析一波，就不会觉得难了。要不，我再放一张简化图？！<br><img src="https://f002.backblazeb2.com/file/freelee/Java/Collections/ArrayList/20180402190417789.png" alt="这里写图片描述"><br>简化图里把重点都给留下了。从上面那张框架图中可以看出ArrayList是一个实现类，继承了AbstarctList抽象类，实现了List接口，我们可以从源码中看到这些，除了这些，ArrayList还实现了RandomAccess，Cloneable，Serializable这三个接口。<br>简单介绍一下这三个接口（等学深了，再来给详细的）。</p><ul><li><p>RandomAccess<br>翻译一下，就是随机访问，这就像数组那样可以通过下标直接访问那个元素。</p></li><li><p>Cloneable<br>克隆接口，其可以将一个实例拷贝到一个新实例。实际上只拷贝其中的各变量的值，是浅拷贝。</p></li><li><p>Serializable<br>可序列化接口，通过此接口可以实现实例的存储，使用时可以通过反序列化得到那个实例。</p></li></ul><p>ArrayList的本质就是一个Object对象类型的数组，在这个数组上进行一系列操作，因为Object是所有类的基类，所以可以接收任意同类型的对象。并且用到了泛型，可以自己指定对象的类型。</p><h3 id="ArrayList的初始化"><a href="#ArrayList的初始化" class="headerlink" title="ArrayList的初始化"></a>ArrayList的初始化</h3><p>1、<strong>静态初始化</strong>(在声明变量时，赋予初始值。)</p><pre><code>//第一种
public static void main(String[] args) {
ArrayList<String> arraylist=new ArrayList<String>(){
{
add("a");
add("b");
}
};
}
//第二种
public static void main(String[] args) {
ArrayList<String> arraylist=new ArrayList<String>(Arrays.asList("a","b"));
System.out.print(arraylist);
}
//第三种
public static void main(String[] args) {
String[] str={"a","b"};
List arraylist=Arrays.asList(str);
}
</code></pre><p>要注意的是，第三种的arraylist是无法改变其长度的，即无法进行add(),remove()等操作，因为数组一旦给定，长度就无法改变，初始化时将数组泛型化赋给它。<br>2、<strong>动态初始化</strong></p><pre><code>public static void main(String[] args) {
ArrayList<String> arraylist=new ArrayList<String>();
arraylist.add("a");
arraylist.add("b");
System.out.println(arraylist);
}
</code></pre><h3 id="ArrayList-常用方法"><a href="#ArrayList-常用方法" class="headerlink" title="ArrayList 常用方法"></a>ArrayList 常用方法</h3><p>讲完了初始化，来看一下ArrayList的一些常用方法。</p><h5 id="1、add"><a href="#1、add" class="headerlink" title="1、add()"></a>1、add()</h5><p>我们先来看看add()方法的源码</p><pre><code> public boolean add(E e) {
modCount++;
add(e, elementData, size);
return true;
}
</code></pre><p>可以看到这里的add()又调用了与它互为重载的方法，如下。</p><pre><code> /**
* This helper method split out from add(E) to keep method
* bytecode size under 35 (the -XX:MaxInlineSize default value),
* which helps when add(E) is called in a C1-compiled loop.
*/
private void add(E e, Object[] elementData, int s) {
if (s == elementData.length)
elementData = grow();
elementData[s] = e;
size = s + 1;
}
</code></pre><p>可以看到如果当前数组的长度等于当前设定的size就会调用grow()方法。size不用多讲，就是ArrayList包含的元素个数，来看一下elementData和grow()的定义。</p><blockquote><p>先看elementDate，其本质是一个对象数组。如果创建一个空的ArrayList时，系统会将其capacity置 为10,也就是默认的大小为10。</p><pre><code> transient Object[] elementData; // non-private to simplify nested class access
</code></pre><p>然后在看grow()。它也去调用了与它重载的方法，可以看到它将size+1值传给重载函数，带参函数将其扩展成当前数组大小的两倍。</p><pre><code>private Object[] grow(int minCapacity) {
return elementData = Arrays.copyOf(elementData,
newCapacity(minCapacity));
}
private Object[] grow() {
return grow(size + 1);
}
</code></pre></blockquote><p>再回到add()，如果数组长度与当前数组容量相同，则先扩增一倍大小，然后赋值。<br>还没讲完呢，add()还有另外一个重载方法（我的天哪，一个add()的源码比我想象的要多）。</p><pre><code> public void add(int index, E element) {
rangeCheckForAdd(index);
modCount++;
final int s;
Object[] elementData;
if ((s = size) == (elementData = this.elementData).length)
elementData = grow();
System.arraycopy(elementData, index,
elementData, index + 1,
s - index);
elementData[index] = element;
size = s + 1;
}
</code></pre><p>这个add()方法接受外部两个参数，index就是要添加的位置的下标。首先rangeCheckForAdd(),是判断index是否合法的（大于等于0&&小于等于size，注意add时，不能跨长度给它赋值）,modCount是记录此数组修改的次数，（这里又可以展开很多知识，我直接抛一个链接<a href="https://www.cnblogs.com/zuochengsi-9/p/7050351.html），其主要就是防止线程安全的ArrayList一边用iterator遍历时一边对ArrayList进行增删操作时发生异常。然后是arraycopy()，查看源码时发现它是native（C++真厉害）,那就看声明，其意思就是将从src里的srcPos位置开始的length长度赋给从destPos位置的dest。" target="_blank" rel="noopener">https://www.cnblogs.com/zuochengsi-9/p/7050351.html），其主要就是防止线程安全的ArrayList一边用iterator遍历时一边对ArrayList进行增删操作时发生异常。然后是arraycopy()，查看源码时发现它是native（C++真厉害）,那就看声明，其意思就是将从src里的srcPos位置开始的length长度赋给从destPos位置的dest。</a></p><pre><code>public static native void arraycopy(Object src, int srcPos, Object dest,int destPos,int length);
</code></pre><p>再来看看具体使用。</p><pre><code> ArrayList <String>arraylist = new ArrayList<String>();
arraylist.add("b");
arraylist.add(0,"a");
arraylist.add(2,"c")
//下标为3的地方还为null
//arraylist.add(4,"d"); //error
System.out.println(arraylist);
Output：
[a, b, c]
</code></pre><p>这里a和b的位置是因为arraycopy。</p><h5 id="2、remove"><a href="#2、remove" class="headerlink" title="2、remove()"></a>2、remove()</h5><p>remove()也有两个重载方法。就不贴源码了，源码哪都可以查到。</p><pre><code>//按照下标来移除
public E remove(int index);
//按照元素来移除
public boolean remove(Object o);
//clear()方法是删除所有元素
public void clear()
//删除指定下标范围的元素
protected void removeRange(int fromIndex, int toIndex)
</code></pre><p>如果使用remove删除某个下标是ArrayList的中间一个元素，那么ArrayList会将所删元素后面的元素前移一个位置。</p><h3 id="ArrayList与Array"><a href="#ArrayList与Array" class="headerlink" title="ArrayList与Array"></a>ArrayList与Array</h3><p>那不是有数组就可以了吗？为什么还要ArrayList呢？<br>| | ArrayList | Array |<br>| ——– | :—–: | :—-: |<br>| 可扩展性 | 可以动态改变其长度 | 数组是静态的，一旦长度给定，就无法改变 |<br>| 方法支持 | 有很多的方法可以方便的对ArrayList进行操，但是牺牲了效率(判定数组是否outBounds…) | 数组没有 |<br>总之，各有各的优点，选用哪种也是视具体情况而定的，并无绝对的说法。</p><p><code>`</code></p></div><script language="javascript">function siteTime(){window.setTimeout("siteTime()",1e3);var e=36e5,t=24*e,o=new Date,i=o.getFullYear(),a=o.getMonth()+1,n=o.getDate(),r=o.getHours(),M=o.getMinutes(),l=o.getSeconds(),s=Date.UTC(2019,4,6,22,30,0),g=Date.UTC(i,a,n,r,M,l)-s,m=Math.floor(g/31536e6),u=Math.floor(g/t-365*m),T=Math.floor((g-(365*m+u)*t)/e),f=Math.floor((g-(365*m+u)*t-T*e)/6e4),h=Math.floor((g-(365*m+u)*t-T*e-6e4*f)/1e3);document.getElementById("sitetime").innerHTML=" "+m+" Year "+u+" Day "+T+" Hour "+f+" Min "+h+" 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