Re: Concurrent bidirectional one-to-many map?

markspace <-@.>
Fri, 06 May 2011 13:45:53 -0700
On 5/6/2011 1:07 PM, Sebastian wrote:

Does anyone know of a concurrent bidirectional one-to-many
map implementation?

By bidirectional I mean that I can lookup keys by values, by
one-to-many I mean that the value end of the map is a list or
set, and by concurrent I mean that I do not need to synchronize
externally and get performance comparable to that of
java.util.concurrent.ConcurrentHashMap in both directions.

If this beast doesn't exist, how would I go about inventing it?
I must say that I am by no means any sort of concurrency guru...

-- Sebastian

Can you just put both keys and values in as the key? Something like:

Map<Key,Key> myMap = new ConcurrentHashMap();
// add key k and value v
Key holder = new Key( k, v, KEY );
myMap.put( holder, holder );
holder = new Key( v, k, VALUE )
myMap.put( holder, holder );

public class Key<K,V> {
   public enum KeyValue {KEY,VALUE};
   private k key;
   private V value;
   private KeyValue keyOrValue;

   public Key( K key, V value, KeyValue keyOrValue ) {
     this.key = key;
     this.value = value;
     this.keyOrValue = keyOrValue;
// must also override hashcode and equals...

This way when you add something, it gets added as both key and value.
You might want to override "put" to do that automatically. When you
look up either a key or a value, you'll find both.

There may be better ways of doing this, it's just the first thing that I
thought of. Also the code above is meant more as a sketch than a
rigorous code example.

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