Java的ArrayList中的ensureCapacity不工作 [英] java arraylist ensureCapacity not working

问题描述

无论我在做这个错误的或者说我不理解这种方法是如何工作的。

Either I'm doing this wrong or i'm not understanding how this method works.

ArrayList<String> a = new ArrayList<String>();
a.ensureCapacity(200);
a.add(190,"test");
System.out.println(a.get(190).toString());

我本来以为的ensureCapacity会让我插入一条记录与指数上升到该值。是否有不同的方式来做到这一点？

I would have thought that ensureCapacity would let me insert a record with an index up to that value. Is there a different way to do this?

我得到第三行的IndexOutOfBounds错误。

I get an IndexOutOfBounds error on the third line.

推荐答案

没有，的ensureCapacity 不会改变的逻辑大小的的的ArrayList - 它改变了的容量的，这是清单可以达到前下一个需要复制值的大小

No, ensureCapacity doesn't change the logical size of an ArrayList - it changes the capacity, which is the size the list can reach before it next needs to copy values.

您必须非常清楚一个逻辑大小之间的差异（范围，即所有的值 [0，大小）是可访问的，并增加了新的元素将索引添加尺寸），并且更多的是一种实现细节的真正的能力 - 这是用于存储文件的支持数组的大小

You need to be very aware of the difference between a logical size (i.e. all the values in the range [0, size) are accessible, and adding a new element will add it at index size) and the capacity which is more of an implementation detail really - it's the size of the backing array used for storage.

调用的ensureCapacity 只能永远做在性能方面的差异（以避免过度复制） - 这并不影响什么是在列表中的逻辑模型，如果你明白我的意思。

Calling ensureCapacity should only ever make any difference in terms of performance (by avoiding excessive copying) - it doesn't affect the logical model of what's in the list, if you see what I mean.

编辑：这听起来像你想有一个排序 ensureSize（）的方法，这可能会是这个样子：

It sounds like you want a sort of ensureSize() method, which might look something like this:

public static void ensureSize(ArrayList<?> list, int size) {
    // Prevent excessive copying while we're adding
    list.ensureCapacity(size);
    while (list.size() < size) {
        list.add(null);
    }
}

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