code: public static <U> CompletableFuture<U> completedFuture(U value) {
      return new CompletableFuture<U>((value == null) ? NIL : value);
    }
docs: Returns a new CompletableFuture that is already completed with the given value.
  
  @param value the value
  @param <U> the type of the value
  @return the completed CompletableFuture