Question

java 中ArrayList集合用indexof方法返回元素的索引 如果元素是重复的 应该返回哪个索引？

Answer 1

以下是indexOf的源代码,可以看出, 是从0往后找,找到就返回
/**
* Returns the index of the first occurrence of the specified element
* in this list, or -1 if this list does not contain the element.
* More formally, returns the lowest index <tt>i</tt> such that
* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>,
* or -1 if there is no such index.
*/
public int indexOf(Object o) {
if (o == null) {
for (int i = 0; i < size; i++)
if (elementData[i]==null)
return i;
} else {
for (int i = 0; i < size; i++)
if (o.equals(elementData[i]))
return i;
}
return -1;
}

Answer 2

返回第一个检索到的元素角标

Answer 3

返回第一个检索到的元素下标