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From: markspace <-@.>
Newsgroups: comp.lang.java.programmer
Subject: Re: Concurrent bidirectional one-to-many map?
Date: Fri, 06 May 2011 13:45:53 -0700
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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.