Used Multikey(1,2,3) to store entries in Map. Now only have (1) . Can I retrieve all the entries that have (1) in the Multikeys?

Question

I've inserted values into a Map using Multikey with 3 params. Now I would like to retrieve all the entries that have a specific key in their Multikey -and I've no idea as to values of the other 2 ..

map.put(new MultiKey(valueA,valueB,valueC), value);

Now only have valueA , I need to retrieve value

If clarification is required, please ask and I'll elaborate rather than voting to close.. Thanks :)

..

would you please post the code (so far what have you done) ? — 1000111, Mar 11 '15 at 06:56
I think you're misunderstanding how Multikey works. Using it as a key does not mean that **any** of the keys you've given point to a value, but that the **combination of all of them** does. — Mureinik, Mar 11 '15 at 07:07
Iterate over the entries and filter them. Maybe what you need is a tree, or multiple maps. — JB Nizet, Mar 11 '15 at 07:13
@Mureinik nope, I understand the principle. Just hoped there is a way to code over it :) — akapulko2020, Mar 11 '15 at 07:14
@JBNizet can't change the type of container, it has to be a HashMap... How do you mean -iterate ? Just go over all the entries in Map? Hmm. Thanks! — akapulko2020, Mar 11 '15 at 07:15

RDM · Answer 1 · 2015-03-11T09:23:54.480

I've actually created a class just for this.

public class MultiMap<K, V> {

    private final HashMap<KeySet<K>, V> model = new HashMap<>();

    public MultiMap() {}

    public V add(V value, K first, K... keys) {
        return model.put(new KeySet<>(first, keys));
    }

    public V add(V value, Set<K> keys){
        return model.get(new KeySet<>(keys));
    }

    public Set<V> getIncludingSubsets(K... keys){
        HashSet<V> all = new HashSet<>();
        for (Entry<KeySet<K>, V> entry : model.entrySet()) {
            if (entry.getKey().containsPartially(keys)) {
                all.add(entry.getValue());
            }
        }
        return all;
    }    

    public Set<V> getIncludingSubsets(Set<K> keys){
        HashSet<V> all = new HashSet<>();
        for (Entry<KeySet<K>, V> entry : model.entrySet()) {
            if (entry.getKey().containsPartially(keys)) {
                all.add(entry.getValue());
            }
        }
        return all;
    }

    public void clear(){
        model.clear();
    }

    private class KeySet<T> extends HashSet<T>{

        private KeySet(T first, T ... rest){
            super();
            add(first);
            for (T object : rest) {
                add(object);
            }
        }

        private KeySet(Collection<T> data){
            super(data);
        }

        @Override
        public int hashCode() {
            int hash = 5;
            for (T value : this) {
                hash = 41 * hash + Objects.hashCode(value);
            }
            return hash;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj == null) {
                return false;
            }
            if (getClass() != obj.getClass()) {
                return false;
            }
            final KeySet<?> other = (KeySet<?>) obj;
            return hashCode() == other.hashCode();
        }

        public boolean containsPartially(T... values){
            for (T value : values) {
                if (!contains(value)) {
                    return false;
                }
            }
            return true;
        }

        public boolean containsPartially(Set<T> values){
            for (T value : values) {
                if (!contains(value)) {
                    return false;
                }
            }
            return true;
        }
    }
}

Here is some sample code illustrating how it works:

MultiMap<String, String> mm = new MultiMap<>();

// "A","B","C" -> v1
// "A","B" -> v2
// "A","C" -> v3

mm.add("v1", "A", "B", "C");
mm.add("v2", "A", "B");
mm.add("v3", "A", "C");

System.out.println(mm.getIncludingSubsets("A", "C"));
System.out.println(mm.getIncludingSubsets("B"));
System.out.println(mm.getIncludingSubsets("C"));
System.out.println(mm.getIncludingSubsets("C", "B", "A"));
System.out.println(mm.getIncludingSubsets("B", "A"));
System.out.println(mm.getIncludingSubsets("B", "C"));

This will produce:

[v1, v3]
[v1, v2]
[v1, v3]
[v1]
[v1, v2]
[v1]

I'm sure with a little creativity you can change this implementation so it extends a HashMap rather than having one as a field.

score 1 · Accepted Answer · answered Mar 11 '15 at 09:46

Something as simple as

static <V> List<V> getValues(Map<MultiKey, V> map, Object key1) {
  List<V> values = new ArrayList<>();
  for (Map.Entry<MultiKey, V> entry : map.entrySet()) {
    MultiKey key = entry.getKey();
    if (key.getKey(0).equals(key1)) {
      values.add(entry.getValue());
    }
  }
  return values;
}

could work just fine. Extending that to use more than one (key1, key2, ...) is left as an exercise to the reader.

Used Multikey(1,2,3) to store entries in Map. Now only have (1) . Can I retrieve all the entries that have (1) in the Multikeys?

2 Answers2