How to implement a Map with multiple keys? [duplic

Question

This question already has an answer here:

• Multiple indexes for a Java Collection - most basic solution? 14 answers

I need a data structure which behaves like a Map, but uses multiple (differently-typed) keys to access its values.
(Let's not be too general, let's say two keys)

Keys are guaranteed to be unique.

Something like:

MyMap<K1,K2,V> ...

With methods like:

getByKey1(K1 key)...
getByKey2(K2 key)...
containsKey1(K1 key)...
containsKey2(K2 key)...

Do you have any suggestions?

The only thing I can think of is:
Write a class which uses two Maps internally.

EDIT Some people suggest me to use a tuple, a pair, or similar as a key for Java's Map, but this would not work for me:
I have to be able, as written above, to search values by only one of the two keys specified.
Maps use hash codes of keys and check for their equality.

Answer 1

Proposal, as suggested by some answerers:

public interface IDualMap<K1, K2, V> {

    /**
    * @return Unmodifiable version of underlying map1
    */
    Map<K1, V> getMap1();

    /**
    * @return Unmodifiable version of underlying map2
    */
    Map<K2, V> getMap2();

    void put(K1 key1, K2 key2, V value);

}

public final class DualMap<K1, K2, V>
        implements IDualMap<K1, K2, V> {

    private final Map<K1, V> map1 = new HashMap<K1, V>();

    private final Map<K2, V> map2 = new HashMap<K2, V>();

    @Override
    public Map<K1, V> getMap1() {
        return Collections.unmodifiableMap(map1);
    }

    @Override
    public Map<K2, V> getMap2() {
        return Collections.unmodifiableMap(map2);
    }

    @Override
    public void put(K1 key1, K2 key2, V value) {
        map1.put(key1, value);
        map2.put(key2, value);
    }
}

Answer 2

See Google Collections. Or, as you suggest, use a map internally, and have that map use a Pair. You'll have to write or find Pair<>; it's pretty easy but not part of the standard Collections.

Answer 3

If keys are unique then there is no need for 2 maps, map of maps, mapOfWhateverThereIs. There needs to be only 1 single map and just a simple wrapper method that would put your keys and value into that map. Example:

Map<String, String> map = new HashMap<>();

public void addKeysAndValue(String key1, String key2, String value){
    map.put(key1, value);
    map.put(key2, value);
}

public void testIt(){
    addKeysAndValue("behemoth", "hipopotam", "hornless rhino");
}

Then use your map as you normally would. You don't even need those fancy getByKeyN and containsKeyN.

Answer 4

Sounds like your solution is quite plausible for this need, I honestly don't see a problem with it if your two key types are really distinct. Just makes ure you write your own implementation for this and deal with synchronization issues if needed.

Answer 5

A dirty and a simple solution, if you use the maps just for sorting lets say, is to add a very small value to a key until the value does not exist, but do not add the minimum (for example Double.MIN_VALUE) because it will cause a bug. Like I said, this is a very dirty solution but it makes the code simpler.

Answer 6

What about you declare the following "Key" class:

public class Key {
   public Object key1, key2, ..., keyN;

   public Key(Object key1, Object key2, ..., Object keyN) {
      this.key1 = key1;
      this.key2 = key2;
      ...
      this.keyN = keyN;
   }

   @Override   
   public boolean equals(Object obj) {
      if (!(obj instanceof Key))
        return false;
      Key ref = (Key) obj;
      return this.key1.equals(ref.key1) && 
          this.key2.equals(ref.key2) &&
          ...
          this.keyN.equals(ref.keyN)
   }

    @Override
    public int hashCode() {
        return key1.hashCode() ^ key2.hashCode() ^ 
            ... ^ keyN.hashCode();
    }

}

Declaring the Map

Map<Key, Double> map = new HashMap<Key,Double>();

Declaring the key object

Key key = new Key(key1, key2, ..., keyN)

Filling the map

map.put(key, new Double(0))

Getting the object from the map

Double result = map.get(key);