JDK源码分析－－ArrayList怎么扩容

野性酷女 2022-03-18 10:47 248阅读 0赞

**为何将ArrayList的扩容单独写成一篇博客？**

之前对ArrayList进行过源码分析，对于扩容也有介绍，但考虑到文章写得有点多，可能一些朋友根本没兴趣全部看完；基于此原因，将其扩容规则单独列出。

## 源码： ##

private void grow(int minCapacity) {
 // overflow-conscious code
 int oldCapacity = elementData.length;
 int newCapacity = oldCapacity + (oldCapacity >> 1);//大约增加1.5倍
 if (newCapacity - minCapacity < 0)
 newCapacity = minCapacity;
 if (newCapacity - MAX_ARRAY_SIZE > 0)
 newCapacity = hugeCapacity(minCapacity);
 // minCapacity is usually close to size, so this is a win:
 elementData = Arrays.copyOf(elementData, newCapacity);//将原数据复制到扩容后的数组中，赋值给elementData。
 }

## 简述扩容规则： ##

**        当添加元素后的需要的容量X大于原容量Y时进行扩容，首先将Y扩大约1.5倍（取整）得到Z，取X和Z中的最大值作为此次扩容的实际容量。**

用一个表格来展示（请先弄懂有参和无参构造方法的区别，**下面2行值是数组的容量**）：

<table> 
 <tbody> 
 <tr> 
 <td style="vertical-align:top;width:82.85pt;"> 元素个数（size） </td> 
 <td style="vertical-align:top;width:22.4pt;"> 0 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 1 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 2 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 3 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 4 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 5 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 6 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 7 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 8 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 9 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 11 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 12 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 13 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 14 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 16 </td> 
 </tr> 
 <tr> 
 <td style="vertical-align:top;width:82.85pt;"> new ArrayList() </td> 
 <td style="vertical-align:top;width:22.4pt;"> 0 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 10 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 15 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 22 </td> 
 </tr> 
 <tr> 
 <td style="vertical-align:top;width:82.85pt;"> new ArrayList(0) </td> 
 <td style="vertical-align:top;width:22.4pt;"> 0 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 1 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 2 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 3 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 4 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 6 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 6 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 9 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 9 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 9 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 13 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 13 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 13 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 13 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 19 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 19 </td> 
 <td style="vertical-align:top;width:22.85pt;"> 19 </td> 
 </tr> 
 </tbody> 
</table>

# 看完表格内容，是不是对扩容规则已了如指掌了呢。如有疑问请联系博主，也可详细阅读博主之前写的一篇博客[《JDK源码分析－－ArrayList深入理解》][JDK_ArrayList]。 #

[JDK_ArrayList]: https://blog.csdn.net/u010188178/article/details/87805325