@Override public CompletableFuture<String> join(final CharSequence delimiter,final CharSequence prefix,final CharSequence suffix){
  return CompletableFuture.supplyAsync(null,exec);
}

@ Override public CompletableFuture < String > join ( final CharSequence delimiter , final CharSequence prefix , final CharSequence suffix ) { return CompletableFuture . supplyAsync ( null , exec ) ; }
perform an asynchronous join operation
