ArrayList工作原理及实现

本是古典何须时尚 2022-05-29 05:15 223阅读 0赞

### 1. 概述 ###

[关于Java集合的小抄][Java]中是这样描述的：

> 以数组实现。节约空间，但数组有容量限制。超出限制时会增加50%容量，用System.arraycopy()复制到新的数组，因此最好能给出数组大小的预估值。默认第一次插入元素时创建大小为10的数组。
> 
> 按数组下标访问元素—get(i)/set(i,e) 的性能很高，这是数组的基本优势。
> 
> 直接在数组末尾加入元素—add(e)的性能也高，但如果按下标插入、删除元素—add(i,e), remove(i), remove(e)，则要用System.arraycopy()来移动部分受影响的元素，性能就变差了，这是基本劣势。

然后再来学习一下官方文档：

> Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector, except that it is unsynchronized.)

ArrayList是一个相对来说比较简单的数据结构，最重要的一点就是它的自动扩容，可以认为就是我们常说的“动态数组”。  
来看一段简单的代码：

在执行这四条语句时，是这么变化的：  
[![arraylist][]][arraylist]  
其中，`add`操作可以理解为直接将数组的内容置位，`remove`操作可以理解为删除index为0的节点，并将后面元素移到0处。

### 2. add函数 ###

当我们在ArrayList中增加元素的时候，会使用`add`函数。他会将元素放到末尾。具体实现如下：

我们可以看到他的实现其实最核心的内容就是`ensureCapacityInternal`。这个函数其实就是自动扩容机制的核心。我们依次来看一下他的具体实现

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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">void</code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">int</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">minCapacity) { </code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">if</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { </code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);</code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">}</code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">ensureExplicitCapacity(minCapacity);</code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">}</code> 
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       &nbsp; 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">private</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">void</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">ensureExplicitCapacity(</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">int</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">minCapacity) { </code> 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">modCount++;</code> 
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       &nbsp; 
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      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,130,0);">// overflow-conscious code</code> 
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     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">if</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">(minCapacity - elementData.length &gt; </code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,153,0);">0</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">)</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">grow(minCapacity);</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">}</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;">
       &nbsp; 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">private</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">void</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">grow(</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">int</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">minCapacity) { </code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,130,0);">// overflow-conscious code</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">int</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">oldCapacity = elementData.length;</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,130,0);">// 扩展为原来的1.5倍</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">int</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">newCapacity = oldCapacity + (oldCapacity &gt;&gt; </code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,153,0);">1</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">);</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,130,0);">// 如果扩为1.5倍还不满足需求，直接扩为需求值</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">if</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">(newCapacity - minCapacity &lt; </code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,153,0);">0</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">)</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">newCapacity = minCapacity;</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;font-weight:bold;min-height:auto;color:rgb(0,102,153);">if</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">(newCapacity - MAX_ARRAY_SIZE &gt; </code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,153,0);">0</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">)</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">newCapacity = hugeCapacity(minCapacity);</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:rgb(0,130,0);">// minCapacity is usually close to size, so this is a win:</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">elementData = Arrays.copyOf(elementData, newCapacity);</code> 
     </div> 
     <div style="border:0px;margin:0px;padding:0px 1em;background-image:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;white-space:pre;"> 
      <code style="border:0px;margin:0px;padding:0px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;background:none;float:none;height:auto;line-height:1.1em;vertical-align:baseline;width:auto;min-height:auto;color:#000000;">}</code> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

也就是说，当增加数据的时候，如果ArrayList的大小已经不满足需求时，那么就将数组变为原长度的1.5倍，之后的操作就是把老的数组拷到新的数组里面。例如，默认的数组大小是10，也就是说当我们`add`10个元素之后，再进行一次add时，就会发生自动扩容，数组长度由10变为了15具体情况如下所示：  
[![arraylistadd][]][arraylistadd]

### 3 set和get函数 ###

Array的put和get函数就比较简单了，先做index检查，然后执行赋值或访问操作：

### 4 remove函数 ###

[Java]: http://calvin1978.blogcn.com/articles/collection.html
[arraylist]: /images/20220529/8671ade17c7844ca9d89b3b3cd18f09f.png
[arraylistadd]: /images/20220529/5f11aceb85b644f0b3b7560490c2ce14.png