java中的意义arraylist参数 [英] Significance arraylist argument in java

问题描述

<pre>List<Integer> l=new ArrayList<>();
       l.add(1);
       l.add(2);
       l.add(3);
       l.add(4);
       System.out.println("Element at 0th index: "+l.get(0));
       Iterator i=l.iterator();
        while(i.hasNext())
        {
            Integer obj=(Integer)(i.next());
            int o=obj;
            System.out.println(o);
        }

输出：

ouput:

Element at 0th index: 1
1
2
3
4

我尝试了什么：

What I have tried:

<pre lang="java"><pre>List<Integer> l=new ArrayList<>();

将以上行更改为以下

Upon changing the above line to the following

<pre><pre lang="java"><pre>List<Integer> l=new ArrayList<>(2);

获得相同的输出。任何人都可以解释使用ArrayList中传递的参数作为ArrayList（2）吗？

the same output is obtained. Could anyone explain the use of argument passed in ArrayList as ArrayList(2) ?

推荐答案

它是; ArrayList的初始容量：ArrayList（Java Platform SE 7） [ ^ ] - 如果未指定，则以容量为10.当您将其指定为2时，它仅分配两个要素。



这是为了提高效率 - 如果您知道要使用多少元素，那么您可以从容量开始并保存不断的重新分配，直到达到它为止。

见这里：列表与LT; T> - 它是否真的像您想象的那样高效？ [ ^ ] - 它基于C＃，但Java ArrrayList完全相同，原因相同。

It;s the initial capacity of the ArrayList: ArrayList (Java Platform SE 7 )[^] - if it isn't specified then it starts with a capacity of 10. When you specify it as 2 it allocates just two elements to start with.

It's for efficiency - if you know how many elements you are going to use, then you can start with that as the capacity and save the constant reallocation until you reach it.
See here: List<T> - Is it really as efficient as you probably think?[^] - it's C# based but the Java ArrrayList does exactly the same thing, for the same reasons.

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