Streaming over a Java list to populating a Map

December 8, 2021

Java 11 here. I have the following POJO:

@Data // Lombok; adds getters, setters, all-args constructor and equals and hashCode
public class Fliflam {
    private String merf;
    private String tarf;
    private Boolean isFlerf;
}

I have a method that validates a Flimflam and returns a List<String> of any errors encountered while validating the Flimflam. I can change this to return Optional<List<String>> if anyone thinks thats helpful for some reason, especially when dealing with the Stream API:

public List<String> validateFlimflam(Flimflam flimflam) {
    List<String> errors = new ArrayList<>();

    // ... validation code omitted for brevity
    // 'errors' list is populated with any errors; otherwise it returns empty

    return errors;
}

I want to stream (Stream API) through a List<Flimflam> and populate a Map<Flimflam,List<String>> errors map, where the key of the map is a Flimflam that failed validation, and its corresponding value is the list of validation error strings.

I can achieve this the "old fashioned" way like so:

List<Flimflam> flimflams = getSomehow();
Map<Flimflam,List<String>> errorsMap = new HashMap<>();
for (Flimflam ff : flimflams) {
    List<String> errors = validateFlimflam(ff);
    if (!errors.isEmpty() {
        errorsMap.put(ff, errors);
    }
}

How can I accomplish this via the Stream API?

>Solution :

Like this

Map<Flimflam,List<String>> errorsMap = flimflams.stream().collect(Collectors.toMap(f -> f, f-> f::validateFlimflam));

toMap takes 2 parameters (keyMapper,valueMapper)
In your case key mapper is object from stream itself, and value is calling validateFlimflam on that object