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ArrayList自动扩容解析

分类：Java作者：编程之家

转自：nofollow">http://blog.csdn.net/huangfan322/article/details/52685922

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<code class="java plain" style="font-family:Monaco,monospace; border:0px!important; bottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; margin:0px!important; padding:0px!important; color:rgb(0,0)!important">探索ArrayList自动改变size真相

一个引用型数组里的，有人认为该数组有“自动增长机制”可以自动改变size大小。正式地说，该数组是无法改变
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生成一个长度为
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copyOf方法覆盖原数组，是实现自动改变ArrayList的大小(size)的关键。有人说ArrayList是复杂的数组，我
方法组合。
方法源码如下：
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属性指向新建长度为初始容量的临时数组
一个空列表
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  c) {
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ottom:auto!important; float:none!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; margin:0px!important; padding:0px!important">            copyOf(elementData, size, Object[].
自动改变size机制
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用户来讲，它是无意义的。而size存储着列表
数量，才是最终用户所需要的。为了防止用户错误修改，这一属性被设置为privae的，不过可以通过size()获取。
增加和删除等三种情况下的size自动改变机制进行分析。
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方法源码中，我们不难看出Capacity初始值（initialCapacity）可以由用户直接指定或由用户指定的Collection集合存
默认为默认为用户指定Collection创建ArrayList时，size值等于
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方法
方法的源码如下：
  
添加对象时，自增size
 
方法中调用的ensureCapacityInternal主要用来调整容量，修改elementData数组的指向。其中涉及到方法的调用，其核心在于grow方法：
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父类AbstractList，用于存储结构修改次数
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IoUs code
> 
 
 
  MAX_ARRAY_SIZE) ?
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代码，我们可知java自动增加ArrayList大小的思路是：向ArrayList添加对象时，原对象数目加
一个原数组的拷贝，并修改原数组，指向这个新建数组。原数组自动抛弃（java垃圾回收机制会自动回收）。size则在向数组添加对象，自增
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方法其一源码如下（其它的就不累述了）：
 
stem.arraycopy(elementData, index+
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 = 
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获取指定index所在位置的对象
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方法相似，都是同数组拷贝。
用户也可以指定ArrayList实例的容量，可以有效的降低时间成本。它是通过调用ensureCapacityInternal来实现的，源代码
 
方法来访问它：
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用户向ArrayList追加对象时，Java总是要先计算容量（Capacity）是否适当，若容量不足则把原数组拷贝到以指定容量为长度创建的
在这同时，size进行自增删除对象时，先使用拷贝方法把指定index后面的对象前移

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