需要一个地图，以便通过键更快地检索以及获取键在某个范围内的条目的列表

Question

I need a Map<Integer,String> with a major need to do fast retrievals of values by key . However I also have the need to retrieve List of all entries ( key , value pairs) whose keys are in range (n1 to n2). However, No sorting required in the list. The map would hold atleast 10,000 such entries.

I initially thought of using TreeMap but that doesn't help with faster retrievals(O(log n) for get() operations). Is it possible to get a list of entries from HashMap whose keys are in range n1 to n2 ?

What would be my best bet to go with ?

Answer 1

The two implementations of NavigableMap (which allow you to retrieve sub-maps or subsets based on key ranges) are TreeMap and ConcurrentSkipListMap , both of which offer O(log n) access time.

Assuming you require O(1) access time as per a regular HashMap , I suggest to implement your own (inefficient) "key range" methods. In other words, sacrifice the performance of the key-range operation for the improved access time you achieve with a regular HashMap . There isn't really another way around this: NavigableMap methods are inherently dependent on the data being stored in a sorted fashion which means you will never be able to achieve O(1) access time.

Answer 2

How close are the keys distributed? For 10000 elements, equally distributed over 20 000 possibilities like 0 to 19999, I could imagine a search for elements from 4 to 14 could be fine. You would miss at a 50% rate.

I wonder why TreeMap doesn't help with faster retrievals (O(log n) for get() operations)?

If you have Tree, with smaller values Left, and bigger ones right, you could return big parts of subtrees. Need it be Map and List?