How to sort a Map by key in Java

Question

How to sort a Map by key in Java

I am 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 is not ordered, my key is of type Date and my values are a List<Classe>.

Would you like to know how I can sort my Map by your key?

Obs: my minSdkVersion is 23.

2

android java hashmap

Author: hkotsubo, 2019-07-11

Source

2 answers

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 sorting 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}

2

Author: nullptr, 2019-07-11 20:16:47

score 3 · Accepted Answer

If you just want to iterate through the map, you can use a TreeSet to save the Keys, as 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 into TreeSet in an orderly manner.

But if you want, you can also create another map, already with the ordered 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 save the keys, you can even scroll 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));
}