Java Map with multiple keys

Question

I need to create a map, with 3 columns: 2 keys, and 1 value. So each value will contain 2 keys of different classtypes, and can be fetched by using either one. But my problem is that HashMap/Map supports only 1 key, and 1 value. Is there a way to create something like Map<Key1, Key2, Value> instead of Map<Key, Value>? so Value can be fetched by either using its Key1 or Key2.

I apologize if it is a duplicate or a bad question, but I couldn't find a similar one on Stack Overflow.

P.S: I don't want to create 2 maps: Map<Key1, Value> and Map<Key2, Value> nor creating nested maps I am looking for a multikey table, just one like above.

possible duplicate of Java: Is there a data structure that works like a multimap but accepts duplicate keys? — Adam Arold
– Adam Arold, Commented Sep 29, 2014 at 22:37
I don't believe there is an existing data structure which will do what you want. You would have to write your own. — Carl Manaster
– Carl Manaster, Commented Sep 29, 2014 at 22:43
Guava's Multimap is essentially a Map<T, Map<K, V>> you can also check out the BiMap. — Mr. Polywhirl
– Mr. Polywhirl, Commented Sep 29, 2014 at 22:43
@Mr.Polywhirl: I think you're referring to Guava's Table, not Multimap. — Louis Wasserman
– Louis Wasserman, Commented Sep 29, 2014 at 22:44
You might need to elaborate why two maps is not an acceptable solution. Without further details, encapsulating two such maps in a new class is how I'd naively solve that problem. — William Price
– William Price, Commented Sep 29, 2014 at 22:45

cogitos · Accepted Answer · 2014-09-29 22:54:15Z

2

You're probably going to have to write a custom implementation of a map-like class to implement this. I agree with @William Price above, the easiest implementation will be to simply encapsulate two Map instances. Be careful using the Map interface, as they rely on equals() and hashCode() for key identity, which you intend to break in your contract.

answered Sep 29, 2014 at 22:54

cogitos

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Comments

Ruslan · Accepted Answer · 2014-09-29 23:33:06Z

Write class with your requirements by yourself:

import java.util.HashMap;
import java.util.Map;

public class MMap<Key, OtherKey, Value> {

    private final Map<Key, Value> map = new HashMap<>();

    private final Map<OtherKey, Value> otherMap = new HashMap<>();

    public void put(Key key, OtherKey otherKey, Value value) {
        if (key != null) { // you can change this, if you want accept null.
            map.put(key, value);
        }
        if (otherKey != null) {
            otherMap.put(otherKey, value);
        }
    }

    public Value get(Key key, OtherKey otherKey) {
        if (map.containsKey(key) && otherMap.containsKey(otherKey)) {
            if (map.get(key).equals(otherMap.get(otherKey))) {
                return map.get(key);
            } else {
                throw new AssertionError("Collision. Implement your logic.");
            }
        } else if (map.containsKey(key)) {
            return map.get(key);
        } else if (otherMap.containsKey(otherKey)) {
            return otherMap.get(otherKey);
        } else {
            return null; // or some optional.
        }
    }

    public Value getByKey(Key key) {
        return get(key, null);
    }

    public Value getByOtherKey(OtherKey otherKey) {
        return get(null, otherKey);
    }
}

Bohemian · Accepted Answer · 2014-09-29 23:48:23Z

1

Just store the value twice:

Map<Object, Value> map = new HashMap<>();
map.put(key1, someValue);
map.put(key2, someValue);

The thing is, it doesn't really matter what type the key is, so use a generic bound that allows both key types - Object is fine.

Note that the parameter type of Map#get() method is just Object anyway, so from a look-up perspective there's no value in having separate maps (the type of the key is only relevant for put()).

edited Sep 29, 2014 at 23:48

answered Sep 29, 2014 at 23:03

Bohemian♦

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4 Comments

Victor2748 Over a year ago

Ok, I'll just do 2 maps. Thanks

William Price Over a year ago

As long as one doesn't need type safety of the specific Key1 and Key2 generic type variables, this is a good solution.

Bohemian Over a year ago

@victor I would not use two maps. Having to look up both maps is a waste of your time, code and CPU cycles, AFAICT without benefit. It's the value type of a map that matters. Note that the parameter type of the get method of Map is Object!

William Price Over a year ago

The type of the map key might matter if one expects to use some APIs like keySet() or entrySet(), but without a stated use case it's hard to say.

Robin Varghese · Accepted Answer · 2015-01-16 15:33:28Z

0

Have you looked into Apache Commons Collection multi map interface? https://commons.apache.org/proper/commons-collections/javadocs/api-3.2.1/org/apache/commons/collections/MultiMap.html

answered Jan 16, 2015 at 15:33

Robin Varghese

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Comments

Jey Ganesh · Accepted Answer · 2016-04-15 05:49:35Z

0

Take a look at guava's Table collection that can be used in your context

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Table.html

 Table<String,String,String> ==> Map<String,Map<String,String>>

answered Apr 15, 2016 at 5:49

Jey Ganesh

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