HashMap to TreeMap

Question

I am using TreeMap to sort the keys in the Map.

Map<Byte, List<TagEntity>> hashMap = list.stream().collect(Collectors.groupingBy(TagEntity::getTagType));

Map<Byte, List<TagEntity>> treeMap = new TreeMap<>(Comparator.reverseOrder());

But how to convert HashMap to TreeMap?

Eran · Accepted Answer · 2018-07-12T07:30:59.383

3

You can create the TreeMap directly by passing a map supplier to groupingBy:

Map<Byte, List<TagEntity>> treeMap = 
    list.stream()
        .collect(Collectors.groupingBy(TagEntity::getTagType,
                                      () -> new TreeMap<Byte, List<TagEntity>>(Comparator.reverseOrder()),
                                      Collectors.toList()));

edited Jul 12 '18 at 07:30

answered Jul 12 '18 at 07:23

Eran

387,369
54
702
768

score 2 · Answer 2 · answered Jul 12 '18 at 07:28

2

This is how you can create hashmap to treemap:

HashMap<Integer, String> hashMap = new HashMap<Integer, String>();
TreeMap<Integer, String> treeMap = new TreeMap<Integer, String>();
treeMap.putAll(hashMap);

Refer here java-putting-hashmap-into-treemap

answered Jul 12 '18 at 07:28

Abhijith S

526
5
17

score 0 · Answer 3 · answered Jul 12 '18 at 07:23

0

This should work:

TreeMap treeMap = new TreeMap<>(hashMap);

Or:

treeMap.putAll(hashMap);

answered Jul 12 '18 at 07:23

Thiru

2,541
4
25
39

Using `new TreeMap<>(hashMap)` doesn't allow you to pass a `Comparator`. – Eran Jul 12 '18 at 07:29

HashMap to TreeMap

3 Answers3