给定Float值，在HashMap中找到“最近的Float键”

Question

Have a LinkedHashMap<Float, Integer> . The keys of this HashMap are points in time in seconds (like 0.031f or 1.4021f ). There can be hundreds of thousands of entries.

Given a float value, say 0.6102f , I need to find the HashMap key that is closest to it (like 0.6002f maybe, assuming that's in the keyset).

A trivial answer would be to check key by key of the HashMap. After all, the entries happen to be correctly sorted. But I guess that's an O(n) operation which may not be a great idea given that I have to perform this search several times per second.

Is there an efficient way to find the "closest float key" of a HashMap given a float?

Answer 1

There is no efficient way to do this using a HashMap ; ie nothing that is better than O(N) where N is the number of entries in the map.

However, if you switch to using a TreeMap<Float, Integer> you can find the entry with the closest key in O(logN) . You need to use floorEntry and ceilingEntry and then test which of the 2 entry keys is nearest to your given key.

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After all, the entries happen to be correctly sorted.

Actually the keys in a HashMap aren't sorted. The hashCode() implementations for some key types may make appear that the HashMap keys are sorted, but what you are seeing is an artifact.