Extract values from map with stream

Question

Assume that I have this map :

String str = ResourceUtils.getResourceAsString("/myjson.json");
ObjectMapper mapper = new ObjectMapper();

TypeReference rootType = new TypeReference<List<Map<String, Object>>>() {
            };
List<Map<String, Object>> root = mapper.readValue(str, rootType);
Map<String, Object> map = root.stream().reduce(new LinkedHashMap<>(), (dest, m2) -> {
        dest.put(m2.get("map").toString(), m2.get("values"));
        return dest;
});

Where str is a json file that contains "map" and "values" fields, map is a string and values is an array of string.

How can I retrieve a list (or array) of Strings that contains values of filtered keys? I am at this point for the moment :

List<String> startWords = map.entrySet()
                .stream()
                .filter(x -> x.getKey().equalsIgnoreCase("x") || x.getKey().equalsIgnoreCase("y"))

I know that there will be a .collect(Collectors::toList()) at the end, but if I try a flatMap after my filter, I have an Stream inferred error.

Thanks!

EDIT: The output intended is a list like this:

List<String> list = new ArrayList<>();
list.put("hello");
list.put("world");

Answer 1

You don't want to use flatMap but simply map . It should look like this:

List<String> startWords = map.entrySet()
                .stream()
                .filter(x -> x.getKey().equalsIgnoreCase("x") || x.getKey().equalsIgnoreCase("y"))
                .map(x -> x.getValue())
                .collect(Collectors.toList())

You use flatMap when you want to "flatten" some data structure. For example if you can a List of Lists then you will use flatMap to get a single stream of all elements.

Answer 2

you must add a map before collect, like this :

List<Object> startWords = map.entrySet()
        .stream()
        .filter(x -> x.getKey().equalsIgnoreCase("x") || x.getKey().equalsIgnoreCase("y"))
        .map(x -> x.getValue())
        .collect(Collectors.toList());

and the map depend to what you need : getKey or getValue