public Set<String> keyNames(String prefix){
  Set<String> keys=keyNames();
  HashSet<String> subset=new HashSet<String>();
  for (  String key : keys) {
    if (key != null && key.startsWith(prefix))     subset.add(key);
  }
  return subset;
}

public Set < String > keyNames ( String prefix ) { Set < String > keys = keyNames ( ) ; HashSet < String > subset = new HashSet < String > ( ) ; for ( String key : keys ) { if ( key != null && key . startsWith ( prefix ) ) subset . add ( key ) ; } return subset ; }
returns keys of all properties where the key name matches the provided prefix .
