java - 访问 HashMap 的元素

Question

我可以分别访问HashMap的第一个和第二个元素吗？我的意思是如果我有一个 HashMap

Map<Integer, Integer> testMap = new HashMap<Integer, Integer>();
int f = testMap.first();

我知道没有这样的功能，但是我如何访问该元素？

Answer 1

HashMap 不维护顺序。

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

Reference

您可以使用维护插入顺序的LinkedHashMap或TreeMap。

LinkedHashMap

This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)

TreeMap

map 根据其键的自然顺序进行排序，或者根据 map 创建时提供的Comparator进行排序，具体取决于使用的构造函数。