Java HashMap reverse way

Question

I need to have a map between 2 kind of objects, lets say, for demonstrarion purposes, String and Long.

I know a HashMap has an O(1) search performance, which is great. I am using something like this:

HashMap<String, Long> a = new HashMap<String, Long>();

This works great when I want to search by the key. However, now I do need to also find the key searching by value.

If I start iterating over keys, then I will lose the performance.

The first option I have thought about is to create another HashMap the other way around.

HashMap<Long, String> reverseA = new HashMap<Long, String>();

And whenever I write in one, write in the other one.

Isn't there an specific class to be able to search by keys and by values, with that great performance?

Should I create my own class with both HashMaps?

Any other solution?

But of course they key is unique and the value is maybe not so — Scary Wombat
– Scary Wombat, Commented Sep 29, 2017 at 6:51
@ScaryWombat: Well, if the reverseA map is an option, then the values better be unique or there will be a bit of inconsistency. — Henrik Aasted Sørensen
– Henrik Aasted Sørensen, Commented Sep 29, 2017 at 6:55
If you don't want to use third API, why not filter the map using new Streaming API which does not have overhead, unless you do a terminal call? — Optional
– Optional, Commented Sep 29, 2017 at 7:00
Why not use 2 HashMaps? It would be better than adding a new dependency to Guava just for this one case. — Kaspars Rinkevics
– Kaspars Rinkevics, Commented Sep 29, 2017 at 7:08

Junior Dussouillez · Accepted Answer · 2017-09-29 06:54:02Z

6

You can use Guava BiMap :

BiMap<String, Long> map = HashBiMap.create();
map.put("a", 1L);
map.put("b", 2L);
map.put("c", 3L);

System.out.println(map.get("b")); // 2L
System.out.println(map.inverse().get(2L)); // "b"

An other alternative is Apache commons BidiMap :

BidiMap<String, Long> map = new DualHashBidiMap<>();
map.put("a", 1L);
map.put("b", 2L);
map.put("c", 3L);

System.out.println(map.get("b")); // 2L
System.out.println(map.inverseBidiMap().get(2L)); // "b"

answered Sep 29, 2017 at 6:54

Junior Dussouillez

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James Roper · Accepted Answer · 2017-09-29 06:51:40Z

0

Java itself doesn't offer anything, but Guava has BiMap, which has an inverse view that supports efficient reverse lookups.

answered Sep 29, 2017 at 6:51

James Roper

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