HashMap transformation using streams

Question

I have Map<Long, Map<String, String>> map , and I have to filter that by key and further get only value. I'm trying to do some like that:

Map<Object, Object> resultMap = map.entrySet().stream()
  .filter(x -> x.getKey().equals(filterValue))
  .map(Map.Entry::getValue).collect(Collectors.toMap(k -> k,v -> v));

But I got Map<Object, Object> map instead of Map<String, String> .

Maybe, there is some better way to do it.

Answer 1

You should set values of map to specific types in collector

.collect(Collectors.toMap(Object::toString, Object::toString))

Answer 2

The following should work as you need:

Map<String, String> resultMap = map.entrySet().stream()
    .filter(x -> x.getKey().equals(filterValue))
    .flatMap(entry -> entry.getValue().entrySet().stream())
    .collect(Collectors.toMap(Entry::getKey, Entry::getValue));

Answer 3

From map.entrySet().stream().filter(x -> x.getKey().equals(filterValue)) , it could be understood that filterValue is a long . There is no need to stream a map to filter out the values matching a particular key. Because map keys are unique, and there will be only one matching key. You could have just used:

Map<String, String> result = map.get(filterValue);