How to find intersection for two streams with java 8?

debugcn Published at Dev

catch23

I have following csv file:

OF_DEPARTURE_COORDINATE_Y,OF_ARRIVAL_COUNTRY,OF_ARRLV2,OF_ARRLV1,OF_ARRLV0,OF_ARRIVAL_CITY,OF_ARRIVAL_ZIPCODE,OF_ARRIVAL_COORDINATE_X,OF_ARRIVAL_COORDINATE_Y,OF_WEIGHT,OF_VOLUME,OF_LENGTH,OF_GOODS_KND,OF_TAIL_LIFT,OF_PALLETS_EXCHANGE,OF_NB_PALLETS
D,SN,1,,DRESDEN,01067,1372931,5105325,A,3,SB,57,ZELL AM SEE,5700,1279591,4732422,2500,0,36,MG,N,N,0
D,HE,35,,HAIGER,35708,820051,5074357,RO,2,,,ORADEA,410000,2193891,4705371,100,1,0,MG,N,N,0
F,NP,62,,ANVIN,62134,225617,5044640,F,BR,29,,QUIMPER,29000,-410790,4799464,10000,0,50,MG,N,N,0

I need to check how many times arrival and departure countries are mentioned there. And use for this functional approach. CSV file has only country codes. All countries are stored at predefined enum.

My solution works iteratively definitely, I am sure that it can be implemented with streams. I tried to play with collect() & groupingBy() but without success.

Here is iterative solution (result is stored to map with country for key - number of occurrences of values):

public class CountryCounter {
    private static Map<Country, Long> countryMap = Country.getCountryMap();

    public static void main(String[] args) {
        processPath(FileLocation.SEARCHES_REG);
        printMap();
    }

    private static void printMap() {
        Map<Country, Long> reversedMap = new TreeMap<>(countryMap);
        Map<Country, Long> result = new LinkedHashMap<>();

        reversedMap.entrySet().stream()
                .sorted(Map.Entry.<Country, Long>comparingByValue().reversed())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        for (Map.Entry entry : result.entrySet()) {
            System.out.println(entry.getKey() + ", " + entry.getValue());
        }
    }

    private static void processPath(FileLocation filePath) {
        FileLocation.printFileName(filePath);

        Path path = Paths.get(".", filePath.getFilePath());

        List<String> csvLines = null;
        try {
            csvLines = Files.readAllLines(path);
        } catch (IOException e) {
            e.printStackTrace();
        }

        for (String csvLine : csvLines) {
            String[] lineArgs = csvLine.split(",");
            String arrivalCntCode = lineArgs[0];
            String departureCntCode = lineArgs[8];

            if (arrivalCntCode == null || departureCntCode == null) {
                return;
            }

            Country arrCountry = Country.getByCode(arrivalCntCode);
            Country depCountry = Country.getByCode(departureCntCode);

            if (countryMap.containsKey(arrCountry)) {
                countryMap.put(arrCountry, countryMap.get(arrCountry) + 1);
            }
            if (countryMap.containsKey(depCountry)) {
                countryMap.put(depCountry, countryMap.get(depCountry) + 1);
            }
        }
    }
}

FileLocation is enum for storing relative pathes for csv files.

Here you can find Country enum

It works fine:

France, 82109
Germany, 31589
Romania, 27634
Italy, 11652
Netherlands, 9190
...

How to achieve the same with Java 8 features, like streams?

Daniel Rodriguez

We can:

Use Files.lines(path) instead of readAllLines to directly get the lines as a stream.
Use flatMap to turn the lines into a stream of countries.
Group the countries counting occurrences using a downstream collector.

For example:

public Map<Country, Long> count(Path path) throws IOException {
    return Files.lines(path)
            .flatMap(line -> getRelevantCells(line))
            .map(Country::getByCode)
            .filter(Objects::nonNull)
            .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
}

private Stream<String> getRelevantCells(String line) {
    String[] cells = line.split(",");
    return Stream.of(cells[0], cells[8]);
}

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