HashMap到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 ?

Answer 1

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()));

Answer 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

Answer 3

This should work:

TreeMap treeMap = new TreeMap<>(hashMap);

Or:

treeMap.putAll(hashMap);