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

Some of the other answers contain code that does a lot of linear searching through lists. I think the Java 8 Stream solution should not be much slower than the classical variant. So here is a solution that takes advantage of Streams without sacrificing much performance.

List people = ...
List regions = ...

Map> zipToRegions =
    regions.stream().collect(
        () -> new HashMap<>(),
        (map, region) -> {
            for(String zipCode: region.getZipCodes()) {
                List list = map.get(zipCode);
                if(list == null) list = new ArrayList<>();
                list.add(region);
                map.put(zipCode, list);
            }
        },
        (m1, m2) -> m1.putAll(m2)
    );
Map> personToRegions =
  people.stream().collect(
    Collectors.toMap(person -> person,
                     person -> zipToRegions.get(person.getZip()))
  );