How to group elements of a List by elements of another in Java 8

Question

I have the following problem: Given these classes,

class Person {
    private String zip;
    ...
    public String getZip(){
        return zip;
    }
}

class          


        

Answer 1

It's still pretty ugly, and I think it would be improved by changing how you model things a bit, but I've only managed to come up with the following so far:

public static void main(String[] args) {
    Person[] people = {new Person("00001"), new Person("00002"), new Person("00005")};
    Region[] regions = {
            new Region("Region 1", Arrays.asList("00001", "00002", "00003")),
            new Region("Region 2", Arrays.asList("00002", "00003", "00004")),
            new Region("Region 3", Arrays.asList("00001", "00002", "00005"))
    };

    Map> result = Stream.of(regions)
            .flatMap(region -> region.getZipCodes().stream()
                    .map(zip -> new SimpleEntry<>(zip, region)))
            .flatMap(entry -> Stream.of(people)
                    .filter(person -> person.getZip().equals(entry.getKey()))
                    .map(person -> new SimpleEntry<>(person, entry.getValue())))
            .collect(Collectors.groupingBy(Entry::getKey, Collectors.mapping(Entry::getValue, Collectors.toList())));

    result.entrySet().forEach(entry -> System.out.printf("[%s]: {%s}\n", entry.getKey(), entry.getValue()));

    //      Output:
    //      [Person: 0]: {[name: Region 1, name: Region 3]}
    //      [Person: 1]: {[name: Region 1, name: Region 2, name: Region 3]}
    //      [Person: 2]: {[name: Region 3]}
}

Having a ZipCode class that contained the mapping and could be keyed on would make things cleaner:

public static void main(String[] args) {
        Region r1 = new Region("Region 1");
        Region r2 = new Region("Region 2");
        Region r3 = new Region("Region 3");

        ZipCode zipCode1 = new ZipCode("00001", Arrays.asList(r1, r3));
        ZipCode zipCode2 = new ZipCode("00002", Arrays.asList(r1, r2, r3));
        ZipCode zipCode3 = new ZipCode("00003", Arrays.asList());
        ZipCode zipCode4 = new ZipCode("00004", Arrays.asList());
        ZipCode zipCode5 = new ZipCode("00005", Arrays.asList(r3));

        Person[] people = {
                new Person(zipCode1),
                new Person(zipCode2),
                new Person(zipCode5)
        };

        Map> result = Stream.of(people)
            .collect(Collectors.toMap(person -> person,
                    person -> person.getZip().getRegions()));

        result.entrySet().forEach(entry -> System.out.printf("[%s]: {%s}\n", entry.getKey(), entry.getValue()));

//      Output:
//      [Person: 0]: {[name: Region 1, name: Region 3]}
//      [Person: 1]: {[name: Region 1, name: Region 2, name: Region 3]}
//      [Person: 2]: {[name: Region 3]}
}