复制ArrayList的前半部分 [英] Copying the first half of an ArrayList

问题描述

有一个ArrayList<Integer> al，我想将其元素的前半部分复制到另一个ArrayList<Integer> firstHalf中. (如果al的元素数量为奇数，则firstHalf应该再有一个元素.)但是，以下代码抛出一个IndexOutOfBoundsException并说Index: 0, Size: 0，尽管我不确定这会是什么问题，因为ArrayLists的索引从0开始.而且，我知道.arraycopy，但是我想通过for循环这样做.

There is an ArrayList<Integer> al, and I want to copy the first half of its elements into another ArrayList<Integer> firstHalf. (If al has an odd number of elements, firstHalf should have one more element.) However, the following code throws an IndexOutOfBoundsException, and says Index: 0, Size: 0, though I'm not sure how that would be a problem, since the indices of ArrayLists start at 0. Also, I am aware of .arraycopy, but I would like to do it this way, with for-loops.

 int x = al.size()/2 + (al.size()%2) - 1;
 for(int i = 0; i < x; i++){
    firstHalf.set(i, al.get(i));
 }

推荐答案

您应使用add而不是set:

int x = al.size()/2 + (al.size()%2) - 1;
for(int i = 0; i < x; i++){
    firstHalf.add(al.get(i));
 }

最好使用 List#subList

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