How to sort a Map by key in Java

Question

I’m trying to use a loop to show the grouped information of a Map.

for(Map.Entry<Date, List<Exam>> entry : groupedList.entrySet()) {
 setNewList(entry.getKey(), entry.getValue());
}

But it ain’t ordered, baby key is the type Date and my values are a List<Classe>.

I wonder how I can order mine Map for his key?

Obs: my minSdkVersion is 23.

Answer 1

If you just want to iterate through map, can use a TreeSet to keep the keys, for he will already order them:

Map<Date, List<Exam>> map = ...

// cria um TreeSet, com as chaves ordenadas
SortedSet<Date> keys = new TreeSet<>(map.keySet());
for (Date date : keys) { // percorre as chaves ordenadas e obtém o respectivo valor
    setNewList(date, map.get(date));
}

As the class Date already implements the interface Comparable, the keys are already inserted in the TreeSet orderly.

---

But if you want, you can also create another map, already with the sorted keys. For this, use a TreeMap:

Map<Date, List<Exam>> map = ... // map com as chaves não-ordenadas

TreeMap<Date, List<Exam>> mapOrdenadoPorChaves = new TreeMap<>(map);
for (Date date : mapOrdenadoPorChaves.keySet()) {
    setNewList(date, mapOrdenadoPorChaves.get(date));
}

---

Another option is to use a List to store keys, so you can even go through in reverse order, for example:

Map<Date, List<Exam>> map = ... // map com as chaves não-ordenadas

List<Date> chaves = new ArrayList<>(map.keySet());
// ordenar as chaves na ordem inversa
Collections.sort(chaves, Collections.reverseOrder());
for (Date date : chaves) {
    setNewList(date, map.get(date));
}

Answer 2

Using Java 8 this could be solved as follows:

public static void main(String[] args) {
    Map<Date, Integer> map = new HashMap<>();

    map.put(LocalDate.now().toDate(), 1);
    map.put(LocalDate.now().plusDays(5).toDate(), 2);
    map.put(LocalDate.now().minusDays(10).toDate(), 3);
    map.put(LocalDate.now().plusDays(1).toDate(), 4);

    LinkedHashMap<Date, Integer> orderedMap = map.entrySet() //
            .stream() //
            .sorted(Map.Entry.comparingByKey()) //
            .collect(Collectors.toMap(Map.Entry::getKey, //
                    Map.Entry::getValue, //
                    (key, content) -> content, //
                    LinkedHashMap::new)); //

    System.out.println(orderedMap);
}

The output would be sort by date:

{Mon Jul 01 00:00:00 BRT 2019=3, Thu Jul 11 00:00:00 BRT 2019=1, Fri Jul 12 00:00:00 BRT 2019=4, Tue Jul 16 00:00:00 BRT 2019=2}