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javajava-stream

Java stream Collection Map

I have a map which has as key string, and as a value a list of objects Names. In this map, I saved information some years about a Names. Each object of the `` class has a name, gender and rank atributes.

Map <String, ArrayList<Names>> myMap = new TreeMap<>();

I tried to do this but it doesn't work:

List<Names> filterList = myMap.filter((k, v) -> v.name == name).collect(Collectors.toList);
Solution

  • Assuming that the input map is like this:

    Map<String, List<BabyName>> inputMap;

    The list of BabyName should be retrieved using flatMap and filter operations of Stream API:

    List<BabyName> result = inputMap.values() // Collection<List<BabyName>>
        .stream() // Stream<List<BabyName>>
        .flatMap(List::stream) // Stream<BabyName>
        .filter(bn -> bn.getRank() == 3) // filtered Stream<BabyName>
        .collect(Collectors.toList());