关于 Java 的库映射类的问题？

Question

The answers are (2) and (4) but not sure why. I don't have much foundation on these topics. Could someone please explain why these are the correct answers and why the others are incorrect. Thank you

Answer 1

A HashMap is a data structure that consists of keys and values. Values are stored in the HashMap with an associated key. The values can then be retrieved by recalling from the HashMap with the same key you used when putting the value in.

1

TreeMaps and LinkedHashMaps are different versions of a Map. A HashMap uses hashing to store its keys, whereas a TreeMap uses a binary search tree to store its keys and a LinkedHashMap uses a LinkedList to store keys. If you iterate over a HashMap, the keys will be returned in hash-sorted order (unpredictable in most cases), because that's how they were stored. The TreeMap, however, has a tree of all the values, so when you iterate over the tree, you'll get all the keys in actual sorted order. A LinkedHashMap has the keys in an ordered list, so an iterator will return the keys in the same order in which you inserted them.

2, 3, and 5

In a HashMap, values are looked up using their keys. If you had duplicate keys, the HashMap couldn't know which value to return. Therefore, every key in a HashMap must be unique, but the values do not have to be.

4

In a normal HashMap, the key is hashed and then inserted in the appropriate spot. With a TreeMap and a LinkedHashMap, you have the additional overhead of inserting the key into the tree or linked list which will take up additional time and memory.