Java ArrayList是否支持稀疏索引? [英] Does Java ArrayList support sparse indicies?

问题描述

标题几乎说明了一切.我很难找到有关此方面的信息，所以做了一些反复试验，我想在这里分享我的结果.

Title pretty much says it all. I had a hard time finding information on this, so I did some trial and error, and I thought I would share my results here.

来自PHP背景，我期望Java的 ArrayList 类型支持稀疏索引.例如，在PHP中，我可以这样做:

Coming from a PHP background, I expected Java's ArrayList type to support sparse indicies. For example, in PHP I can do:

$test = array(
    "Item 1",
    "Item 2",
    "Item 3"
);

unset($test[1]);

echo $test[2];

然后返回项目3".但是，当我尝试使用Java进行类似操作时:

And get "Item 3" back. But, when I try something similar in Java:

ArrayList<String> test = new ArrayList<>();

test.add("Item 1");
test.add("Item 2");
test.add("Item 3");

test.remove(1);

System.out.println(test.get(2));

我得到一个 IndexOutOfBoundsException .看起来当您删除该元素时，该数组将被重新索引.也许我缺少了一些东西(我对Java很陌生)，但是在我看来，如果您依赖于知道删除元素后索引不会改变，则应该使用 HashMap 而不是 ArrayList .

I get an IndexOutOfBoundsException. It would appear that when you remove the element, the array gets re-indexed. Maybe I'm missing something (I'm pretty new to Java), but it seems to me that if you depend on knowing an index won't change if elements are removed, you should using something like HashMap instead of ArrayList.

推荐答案

否.Java ArrayList (根据Javadoc)

No. Java ArrayList (per the Javadoc)

此类提供了一些方法来操纵内部用于存储列表的数组的大小.

this class provides methods to manipulate the size of the array that is used internally to store the list.

并且根据 JLS-10.3.数组创建

数组创建表达式为至少一个嵌套级别指定元素类型，嵌套数组的级别数以及数组的长度.数组的长度作为 final 实例变量 length 可用.

在Java中，通常会使用

In Java, one would generally implement a sparse Collection with a Map (like HashMap or possibly LinkedHashMap).

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