Thanks @Sweeper It's always obvious once you've figured out the answer.

    int size = ...
    var a = myHashMap.entrySet()
      .stream()
      .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
      .map(e -> String.format("%s %.3f", e.getKey(), 100.0 * e.getValue() / size))
      .toList();