Bi-directional Map in Java?

Question

I have a simple integer-to-string mapping in Java, but I need to be able to easily retrieve string from integer, and also integer from string. I've tried Map, but it can retrieve only string from integer, it's one way:

private static final Map<Integer, String> myMap = new HashMap<Integer, String>();
// This works one way:
String myString = myMap.get(myInteger);

// I would need something like:
Integer myInteger = myMap.getKey(myString);

Is there a right way to do it to have it both directions?

Another problem is that I only have a few constant values that don't change (1->"low", 2->"mid", 3->"high", so it wouldn't be worth to go for a complicated solution.

https://stackoverflow.com/questions/72487163/bidirectional-mapping-in-java-witch-rejects-entries-of-key-or-value-already-exis demonstrates a solution based on [this][1] tutorial. [1]: https://www.tutorialspoint.com/commons_collections/commons_collections_bidimap.htm — Simeon, Jun 03 '22 at 09:23

epoch · Accepted Answer · 2020-07-20T17:00:52.210

98

You can use the Google Collections API for that, ~~recently~~ renamed to Guava, specifically a BiMap

A bimap (or "bidirectional map") is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an "inverse view", which is another bimap containing the same entries as this bimap but with reversed keys and values.

edited Jul 20 '20 at 17:00

answered May 22 '12 at 09:44

epoch

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See http://stackoverflow.com/a/39329395/5466401 – Sibin John Mattappallil Sep 05 '16 at 11:15
3

Google probably needs BiMap for specific use cases. But, why does Java not provide BiMap ? Its nice to have options in data structures & algorithms instead of having to get them from libraries or coding from scratch. – MasterJoe Jul 11 '20 at 19:31

score 54 · Answer 2 · edited Sep 18 '22 at 08:55

54

Creating a Guava BiMap and getting its inverted values is trivial.

A simple example:

import com.google.common.collect.BiMap;
import com.google.common.collect.HashBiMap;

public class BiMapTest {

  public static void main(String[] args) {

    BiMap<String, String> biMap = HashBiMap.create();

    biMap.put("k1", "v1");
    biMap.put("k2", "v2");

    System.out.println("k1 = " + biMap.get("k1"));
    System.out.println("v2 = " + biMap.inverse().get("v2"));
  }
}

edited Sep 18 '22 at 08:55

Ali Faradjpour

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answered Mar 21 '13 at 07:15

Michal Zmuda

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How does this example illustrate that it is “not so trivial”? – Guildenstern Sep 25 '20 at 06:50

score 31 · Answer 3 · edited Dec 20 '13 at 11:10

31

There is no bidirectional map in the Java Standard API. Either you can maintain two maps yourself or use the BidiMap from Apache Collections.

edited Dec 20 '13 at 11:10

gawi

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answered May 22 '12 at 09:45

Mathias Schwarz

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Like `BidiMap bidiMap = new DualHashBidiMap();` – Kanagavelu Sugumar Aug 08 '18 at 06:04
But this dont have generic support :( – Kanagavelu Sugumar Sep 24 '19 at 09:38
4

For future readers -- this does have generics support at this point – Richard Rast Dec 10 '19 at 14:44
It has generic support now – Bala Jan 18 '23 at 13:37

Chicowitz · Answer 4 · 2017-08-22T19:50:17.427

16

You could insert both the key,value pair and its inverse into your map structure, but would have to convert the Integer to a string:

map.put("theKey", "theValue");
map.put("theValue", "theKey");

Using map.get("theValue") will then return "theKey".

It's a quick and dirty way that I've made constant maps, which will only work for a select few datasets:

Contains only 1 to 1 pairs
Set of values is disjoint from the set of keys (1->2, 2->3 breaks it)

If you want to keep <Integer, String> you could maintain a second <String, Integer> map to "put" the value -> key pairs.

edited Aug 22 '17 at 19:50

answered Nov 19 '13 at 17:12

Chicowitz

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Furthermore, all keys and values should be different. – Mert Mertce Apr 15 '14 at 13:32
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This answer is pure blasphemy. thats why i like it :) – Gewure Apr 24 '17 at 13:56
This a great hack – Ben Aug 02 '19 at 23:55

score 11 · Answer 5 · edited Dec 20 '13 at 11:12

11

Apache commons collections has a BidiMap

edited Dec 20 '13 at 11:12

gawi

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answered May 22 '12 at 09:45

hage

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score 5 · Answer 6 · edited Oct 05 '20 at 13:48

5

Use Google's BiMap

It is more convenient.

edited Oct 05 '20 at 13:48

Andrii Abramov

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answered May 22 '12 at 09:51

BOSS

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Bi-directional Map in Java?

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