dictionary - Implement java map accessed by multiple threads that is only updated once for each key -

i need create map accessed multiple threads , ever has 1 (not null) value inserted in it.

to clarify i'm trying :

object getvalue(key)       {           if(map.get(key) != null)              return map.get(key)           else           {               object obj = new object();              map.put(key, obj);              return obj;            }        } 

the easiest way make getvalue method synchronised, want make method efficient possible, requirement put 'else' part in synchronised block , locked somehow on 'key' value. best way implement this?

i suspect want computeifabsent method.

// support map allows concurrent access. concurrentmap<key,value> map = ....  value oneperkey = map.computeifabsent(key, value::new); 

or

value oneperkey = map.computeifabsent(key, key -> new value()); 

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from javadoc computeifabsent

the default implementation equivalent following steps map, returning current value or null if absent:

if (map.get(key) == null) {   v newvalue = mappingfunction.apply(key);   if (newvalue != null)      return map.putifabsent(key, newvalue); }    

the default implementation may retry these steps when multiple threads attempt updates including potentially calling mapping function multiple times.