@Override
  public TypeV   replace    ( TypeK  key, TypeV val ) { return putIfMatch( key,      val,MATCH_ANY   ); }
Atomically do a <code>put(key,val)</code> if-and-only-if the key is
 mapped to some value already.
 @throws NullPointerException if the specified key or value is null
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMap.java