In Java does a data type like this exist?

Question

I found this in redis and was trying to see if there was anything similar in Java. Say I have the following data:

3:1
3:2
3:3
4:1

As you can see non of the data points on their own are unique but the combination is unique. There is a command in redis:

sadd 3 1 2
sadd 3 3
sadd 4 1

That would give me a something like this:

3 -> 1, 2, 3
4 -> 1

By doing something like smembers 3(this would return everything for 3) or smembers 3 2(this would return if the subvalue exists).

I'm wondering what the closest I can get to this functionality within Java?

Answer 1

The Guava MultiMap interface does exactly this. Note that it's up to the particular implementation whether it allows duplicate <K,V> pairs. It sounds like you're after one where the K,V pairs are always unique. If so, have a look at the HashMultimap class.

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However, if you want to roll your own, you're probably looking for a combination of Map and Set: Map<Integer,Set<Integer>>

When you add (key, value) elements to the map:

1. First check to see whether the key is in there. If not, you'll need to add an empty Set<Integer>.
2. Then, do map.get(key).put(value);

When you want to retrieve all elements with a specific key:

1. do map.get(key) and iterate over the result

When you want to see if a specific key/value pair is in there:

1. do if(map.containsKey(key) && map.get(key).contains(value))

For extra credit, you could implement all of this inside a wrapper. The ForwardingMap from guava might be a good place to start.

Answer 2

You can create your own class MultivalueMap like this:

import java.util.Set;
import java.util.Map;
import java.util.HashMap;
import java.util.List;
import java.util.ArrayList;

public class MultiValueMap<T1, T2> {

    public Map<T1, List<T2>> map = null;

    public MultiValueMap(){
        this.map = new HashMap();
    }

    public void putList(T1 key, List<T2> list){

        map.put(key, list);
    }

    public void put(T1 key, T2 value){
        List<T2> list = null;
        if(map.get(key) == null){
            list = new ArrayList<T2>();
            map.put(key, list);
        }
        else {
            list = map.get(key);
        }
        list.add(value);
    }

    public List<T2> get(T1 key){
        return map.get(key);
    }

    public Set<T1> keySet(){

        return map.keySet();
    }

    public Map getMap(){

        return this.map;
    }

    public boolean contains(T1 key, T2 listValue){

        List<T2> list = map.get(key);

        return list.contains(listValue);
    }
}