MultiValuedMap to TreeMap with sorted Keys

I am using OpenCsv to get a table of values with dates as column headers, which looks something like this:

I am new to OpenCsv and functional programming in general, and after reading the OpenCsv user guide and trying for a few days, I managed:

    @CsvBindByName(column="Country/Region",required=true)
    private String country;
    @CsvBindAndJoinByName(column="[0-9]{1,2}/[0-9]{1,2}/[0-9]{1,4}", elementType = String.class, mapType = ArrayListValuedHashMap.class)
    private MultiValuedMap<String,String> cases;
    private TreeMap<LocalDate, List<String>> sortedCases = new TreeMap<>();

    public String getCountry() {return country;}
    public MultiValuedMap<String, String> getCases() {return cases;}

    public void addToSortedCases(MultiValuedMap<String,String> map) {
        DateTimeFormatter dateFormat = DateTimeFormatter.ofPattern("M/d/yy");
        for(String key:map.keys()){
            sortedCases.put(LocalDate.parse(key,dateFormat),map.get(key).stream().toList());
        }
    }

    //attempt on converting addToSortedCases to Functional Interface.
    public BiConsumer<MultiValuedMap<String,String>, List<String>> addToSorted = (map, keys) ->
            keys.forEach(key ->
                    sortedCases.put(
                            LocalDate.parse(key,DateTimeFormatter
                                    .ofPattern("M/d/yy")),
                            Integer.valueOf(map.get(key)
                                    .toString()
                                    .replaceAll("[\\[\\]]",""))
                    ));
    @Override
    public String toString() {
        return "Confirmed{" +
                "country='" + country + '\'' +
                ", cases=" + sortedCases.toString() +
                '}';
    }

the addToSorted functional interface doesn't work when I use it in place of the addToSortedCases method in main, with the error being "Method call expected". I think I wrote the functional interface correctly but I'm not sure.

...
public static List<Recovered> recoveredCases;

    public static void main(String[] args) {
        ...

        recoveredCases = (List<Recovered>) readFile.apply(Recovered.path,Deaths.class);
        recoveredCases.forEach(recovered -> recovered.addToSortedCases(recovered.getCases()));//works
        recoveredCases.forEach(recovered -> recovered.addToSorted(recovered.getCases(),recovered.getCases().keys()));//doesn't work
    }

    public static BiFunction<String,Class<?>, List<?>> readFile = (path, classType) -> {
        try {
            return new CsvToBeanBuilder(new FileReader(path))
                    .withType(classType)
                    .build()
                    .parse();
        } catch (FileNotFoundException e) {
            throw new RuntimeException(e);
        }
    };

I'd appreciate it very much if I could get a solution to this problem, or even better, have OpenCsv sort the MultiValuedMap or use a TreeMap directly.