Java stream find value by key if exists

Question

I'm having simple DataStructure

public class DataStructure {
    private String key;
    private String value;
    //get, set
}

And I need to return value from `List' based on key and I want to do it Java8 way, with streams. I think code speaks for himself:

public class Main {
    public static void main(String args[]) {
      List<DataStructure> dataList = new ArrayList<>();
      dataList.add(new DataStructure("first", "123"));
      dataList.add(new DataStructure("second", "456"));

        System.out.println(findValueOldSchool(dataList, "third")); //works ok
        System.out.println(findValueStream(dataList, "third")); //throws NoSuchElementException
    }

    static String findValueOldSchool(List<DataStructure> list, String key) {
        for (DataStructure ds : list) {
            if (key.equals(ds.getKey())) {
                return ds.getValue();
            }
        }
        return null;
    }

    static String findValueStream(List<DataStructure> list, String key) {
        return list.stream()
                .filter(ds -> key.equals(ds.getKey()))
                .findFirst()
                .get().getValue();
    }
}

How can I modify findValueStream() to not throw NoSuchValueException while I search for non existing key? I don't want to return Optional<String> because this method is already used in a lot of places in project. And, ofcourse I'v tried map, ifPresent, anyMatch, just can't find the right way to do it.

Answer 1

You shall use Stream.findFirst with an Optional.orElse like :

static String findValueStream(List<DataStructure> list, String key) {
    return list.stream() // initial Stream<DataStructure>
            .filter(ds -> key.equals(ds.getKey())) // filtered Stream<DataStructure>
            .map(DataStructure::getValue) // mapped Stream<String>
            .findFirst() // first Optional<String>
            .orElse(null); // or else return 'null'
}

Note: The above uses the Stream.map to map the stream of DataStructure to a corresponding stream of value.

Answer 2

use orElse to return a default value if the Optional has an empty state:

This also means you'll need to map to DataStructure::getValue first like so:

return list.stream()
           .filter(ds -> key.equals(ds.getKey()))
           .findFirst()
           .map(DataStructure::getValue)
           .orElse(null);

just replacing get with orElse will not suffice:

 return list.stream()
            .filter(ds -> key.equals(ds.getKey()))
            .findFirst()
            .orElse(null)
            .getValue();

As this time you'll get a NullPointerException instead of a NoSuchElementException in the case of an empty optional.